3.0 BASELINE ASSESSMENT RESULTS - slcdocs.com Study website/Emigration/Final/6... · FINAL EMIGRATION CREEK MANAGEMENT PLAN Figure 3.1. Emigration Creek watershed. (Map from SLCO

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FINAL EMIGRATION CREEK MANAGEMENT PLAN

Figure 3.1. Emigration Creek watershed. (Map from SLCO 2009).

3.0 BASELINE ASSESSMENT RESULTS

Watershed Conditions

Size and Land Use

Emigration Creek lies betweenRed Butte Creek to the north andParleys Creek to the south(Figure 3.1). The uppersubwatershed, located inEmigration Canyon above the

debris basin, drains 11,635 acresof mountainous areas withmoderate slopes. The uppersubwatershed contains about 9miles of stream from theheadwaters to the EmigrationCreek debris basin (SLCO 2009). The majority of the uppersubwatershed is private landwithin unincorporated areas of

the County. Existing land use isprimarily residential andundeveloped forest, with anestimated impervious cover of12.5%.

The lower Emigration Creeksubwatershed is much smaller,draining 3,742 acres of primarilybench area below the canyon

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SALT LAKE CITY RIPARIAN CORRIDOR STUDY

Figure 3.2. Relationship between impervious cover and surfacerunoff. Impervious cover in a watershed results in increased surface runoff. (Diagram and caption textfrom FISRWG 1998).

mouth. The open channel lengthof Emigration Creek totalsapproximately 3.1 miles in thelower subwatershed. About 2.1total miles of lower EmigrationCreek have been piped inunderground culverts, includingthe 1300 South conduit thatconveys Emigration Creek from1100 East to the Jordan River(Figure 1.1). Most of the lowersubwatershed is located withinthe City’s municipal boundaries(Figure 3.1). Existing land use isprimarily residential, with somesmaller areas of park land andcommercial development. Estimated impervious cover is23.2% within the lowersubwatershed.

Hydrology

Because of natural alluvialdeposition patterns, WasatchMountain streams—includingEmigration Creek—naturally losesome surfaceflow to groundwaterwhere the canyons transition tothe valley. Within the RCS studyarea, Emigration Creek flowsthrough areas mapped asprimary and secondarygroundwater recharge zones, andstudies have estimated losses togroundwater to be around 11%of runoff (SLCO 2009). Urbanization and developmentthroughout the watershed havealtered surface water-groundwater patterns. As moreof the watershed has beenconverted to impervious surfaces,a greater proportion of stormwater runs off as surfaceflowrather than infiltrating into theground, leaving less groundwater

available to supply baseflow tothe creek during the summer dryperiod (Figure 3.2).

The headwater portion ofEmigration Creek is classified ashaving intermittent flow; belowPerkins Hollow the creek isclassified as perennial (Figure3.1, SLCO 2009). The Countyclassifies flows in EmigrationCreek from reach LEM_R02Adownstream as “perennial-reduced,” indicating that flowsare artificially reduced by streamdiversions. Within the studyarea, recorded points ofdiversion from Emigration Creek

include the downstream end ofreach LEM_R02A within HogleZoo and a point near the middleof reach LEM_R10 in theWasatch Hollow area (UDWRT2009). Water rights holdersinclude a mix of local and Stategovernment entities, Mt. OlivetCemetery, and other privateindividuals and companies.

Although the stream is classifiedas perennial above reachLEM_R02A, the creek was dry inreach UEM_R16 during RCSfield work conducted in October2008. Various diversions fromEmigration Creek and its

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Figure 3.4. A comparison of hydrographs before and afterurbanization. The discharge curve is higher and steeper for urban streams than for natural streams. (Diagram and caption text from FISRWG 1998).

Figure 3.3. Monthly flows at Salt Lake County’s gage at Rotary GlenPark.

tributary springs occur withinEmigration Canyon, and somereturn flows from septic systemsalso occur in this area. At theRCS public workshops, residentsof the lower portions of the creekrepeatedly indicated concernsabout summertime reduced flowsand flow fluctuations apparentlyassociated with diversionoperations. Reducedsummertime baseflows on lowerEmigration Creek are most likelythe result of a combination offactors including urbanizationeffects on hydrology, flowdiversions, and natural surfacewater losses to alluvial deposits.

Emigration Creek’s hydrology ischaracterized by a distinctspringtime peak typical ofsnowmelt-driven systems. Basedon analysis of flow data recordedat the County gage in RotaryGlen Park from 1980–2005,average monthly flow is highestin May (Figure 3.3), and peakdaily flow occurs on May 1 onaverage (SLCO 2009). Averageannual high flow is 55 cubic feetper second (cfs), while typicalbase flows are on the order of2.5 cfs. No streamflow gageexists at the downstream end ofthe study area, so no quantitativedata are available to characterizehydrology after the creek flowsthrough the most urbanizedportion of watersheddownstream from the RotaryGlen Park gage. However, fieldobservations during storm eventssuggest that flows in the lowerreaches of the creek are quite“flashy,” with rapid, brief rises inflow during storms. This is a

common hydrologic pattern inurbanized systems (Figure 3.4). Flow patterns at the Rotary GlenPark gage show a moremoderated response duringstorm events.

No significant water storagereservoirs are present on

Emigration Creek, but sedimentsupply to the lower reaches isaltered by the Emigration Creekdebris basin, an on-stream facilitylocated in Rotary Glen Park(Figure 2.1). The debris basinwas originally constructed in1985, and it is maintained by theCounty. The facility traps the

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Below: Emigration Creek debris basin in Rotary Glen Park.

majority of coarse sediment loadsand typically requires dredging toremove accumulated materialabout every 2 years.

Water Quality

Between Foothill Drive and itsheadwaters, Emigration Creek’sbeneficial use classifications (asdesignated by the Utah Divisionof Water Quality [DWQ]) include2B (secondary contactrecreation) and 3A (cold waterfishery). Below Foothill Drive,the creek is designated with thedefault classifications of 2B and3D (waterfowl/shorebirdprotection). The DWQ has listeda segment of Emigration Creekabove Foothill Drive as impairedfor E. coli bacterialcontamination (DWQ 2006). The number of residential septicsystems in the uppersubwatershed is most likely asignificant contributing source ofE. coli to the stream (SLCO2009). Other potential nonpoint

sources of pollution in the lowersubwatershed include urbanrunoff, hydrologic modificationand habitat alteration, managedpark and golf course areas, andconstruction runoff (SLCO2009).

Emigration Creek was originallylisted as impaired for high fecalcoliform levels in 2000; however,with a change in state standards,E. coli is now the targetparameter. The County andDWQ have collaborated toconduct a Total Maximum DailyLoad (TMDL) study for E. coli inEmigration Creek and have beencollecting monthly samples sinceJanuary 2007. Watertemperature, dissolved oxygen,and pH data are collected inconjunction with the monthly E.coli samples, and more intensiveweekly samples are collected for5 weeks in August/September. Six DWQ water qualitymonitoring stations are beingused for the TMDL studies (H.

Arens 2009, pers. comm.). Fourof the monitoring stations arelocated upstream of the RCSstudy area, one is located inreach UEM_R17 (STORET4992145), and one is locatednear the Rotary Glen Parkstreamflow gage in reachLEM_R01 (STORET 4992140). In summer 2008, a separate butrelated microbial source-trackingstudy to identify fecalcontamination sources wasperformed by Dr. Ramesh Goel,who was contracted through theCounty. The final report for thatstudy is expected in summer2009. The Emigration CreekTMDL study is ongoing andexpected to be complete in April2010. Once complete, theTMDL plan will include a list ofimplementation measures toachieve water quality targets andre-attain full support of waterquality standards.

No established DWQ waterquality monitoring stations arepresent on Emigration Creekdownstream from the RotaryGlen Park station. However, aspart of ongoing TMDL studies onthe Jordan River, data are beingcollected at a station on the 1300South conduit (STORET4992070). Water in the conduitoriginates from Emigration,Parleys, and Red Butte Creeks,so the data collected at thismonitoring station will providean indication of water qualityconditions and storm watereffects within the lower,urbanized portions of thesecreeks.

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Geology and Soils

The uppermost reaches ofEmigration Creek flow throughmembers of the CretaceousKelvin formation, which includeslimestone, conglomerate, andsandstone components. Thecreek then flows past PreussSandstone and Twin CreekLimestone, both Jurassicformations (Bryant 1990). Approximately 50–86.2% of thesoils in the upper EmigrationCreek subwatershed have severeto very severe soil-erosionpotential. Once it exits thecanyon, Emigration Creek flowsthrough a series of PleistoceneLake Bonneville deposits rangingfrom finer-grained silt and claydeposits to coarser sand andgravel deposits (Bryant 1990). Inthe lower subwatershed, 20–35%of the soils have severe to verysevere erosion potential (SLCO2009).

After Lake Bonneville recededapproximately 16,000 years ago,it left a series of old shorelinedeposits that now formprominent “benches” along theedges of Salt Lake Valley. Toreach its modern base level at theJordan River, Emigration Creekhad to carve through thesedeposits. In part because of thisnatural geologic history, streamgradient is relatively steep, andthe creek is entrenched betweentall slopes that extend up to theBonneville bench levels. Varioushuman-induced alterations to thecreek—including channelstraightening, installation of road-crossing culverts, fill placement,

and bank hardening—havefurther contributed to the steepgrade and entrenched shape ofthe channel.

Fish, Birds, and Wildlife

Quantitative data on fish andwildlife populations within theurban portion of EmigrationCreek are limited. However,known populations of nativeBonneville cutthroat trout(Oncorhynchus clarki utah) andintroduced rainbow trout(Oncorhynchus mykiss) exist inupper Emigration Creek, andBonneville cutthroat troutpopulations exist in lowerEmigration Creek (SLCO 2009). During the RCS fieldassessments, trout were observedin several scour pools at theoutlets of stream-crossingculverts. At the RCS publicworkshops, residents reportedobservations of fish, deer, fox,and a wide variety of bird specieswithin the riparian corridor. Nuisance species, includingracoons and skunks, are alsoregularly seen.

Reach LEM_R02D, just upstreamof the Bonneville Golf Course, isused as a site for the AudubonSociety’s annual Christmas birdcount. During the 2005Christmas bird count, 24 differentbird species were observedwithin the East Bench surveyarea, which includes portions ofthe Emigration Creek ripariancorridor (Carr 2009). The RCSworkshop attendees expressedan interest in knowing more

about the existing populations ofwildlife within the corridor.

Historical Conditionsand Current Trends

Emigration Creek History

Emigration Canyon holds aprominent place in Utah’shistory, as it was the routethrough which Mormon pioneersfirst entered Salt Lake Valley in1847. Although detaileddescriptions, drawings, andphotographs specific to theriparian corridor are limited,available information suggeststhat the corridor at that time wasa dense thicket of willows, scruboak, and tall grasses (Figure 3.5). The Donner-Reed Party traveledthrough Emigration Canyon in1846 on their way to California. They found the brush along thecreek bottom so impenetrablethat near the mouth of thecanyon they opted to climb upand over the steep hill to thesouth, a decision that exhaustedtheir livestock and likelycontributed to their tragic fate inthe California mountains (USHD2009). Pioneer accounts alsodescribe a marsh near the currentHogle Zoo location that hinderedtravel (Dixon 1997).

These available historicaldescriptions suggest that theoverall vegetation density, grasscover, and shrub cover alongEmigration Creek wassubstantially greater 160 yearsago than it is today. Thedescription of a marsh near

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Figure 3.5. Emigration Creek historical timeline.

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Hogle Zoo also suggests that thecreek was not as deeplyentrenched during that timeperiod as it is today. Historicalaccounts of Emigration Creekdescribe diverse and abundantfish, bird, and wildlifepopulations within the ripariancorridor (Figure 3.5). Fish,including native cutthroat trout,were plentiful, and fish werestocked into the creek during the1930s and 1940s (Carlstrom andFurse 2003).

Alterations to the RiparianCorridor

Over the last 160 years, thevarious activities associated withdevelopment and populationgrowth in Salt Lake Valley haveresulted in significant alterationsto the stream channel andriparian conditions of EmigrationCreek. Among other factors,systematic programs to cleardebris from channels and removebeaver populations have likelycontributed to the currentlyreduced vegetation densityrelative to historical conditions. When beaver were morecommon, their dams increasedinundated streamside habitatarea, elevated the water table,reduced flood velocities anderosion, and trapped sedimentand nutrients (Gardner et al.1999). As beaver populationsdecreased, the “checks” onsediment and water created bybeaver dams also decreased,resulting in greater flow velocitiesand streambed down-cutting. The control and reduction ofbeaver populations throughout

the west has profoundly alteredstream channel, floodplain, andriparian vegetation conditions(Wohl 2000).

Many of the direct alterations toEmigration Creek have occurredin order to address floodingconcerns and accommodateurban development andpopulation growth. A 1902newspaper report describes awashout on Emigration Creekthat blocked 1100 South for 10years, illustrating the conflictbetween efficient transportationand natural flooding processes(Deseret News 1902). One ofthe most significant directchanges to the creek was theconstruction of the 1300 Southconduit, which converted thewestern, open-channel portionsof Emigration, Red Butte, andParleys Creeks to anunderground pipe system. Theexact date of conduitconstruction is not known, buthousing stock located over theconduit system dates to the late1920s, suggesting thatconstruction was complete priorto that time. No creek channelcan be seen west of 1100 East in1938 air photos of Salt Lake City(Bowman and Beisner 2008).

In general, the channel alignmentof Emigration Creek does notappear to have changeddramatically since 1938. Somestraightening and bendrealignment are evident near thepresent site of Clayton MiddleSchool (in reach LEM_R09B)and in the Wasatch Hollow area(reach LEM_R10). A more

Historical activities that have alteredriparian corridorconditions:

• mining and quarrying forlimestone and sandstone

• beaver trapping andremoval

• channel clearing anddebris removal

• flow diversion forirrigation and drinkingwater

• development and piping of springs

• road and stream crossingconstruction

• sheep grazing

• residential andcommercialdevelopment

• introduction of invasive,nonnative plants

• piping of the creek inunderground conduits

• channel relocation andstraightening

• bank armoring

• placement of fill withinfloodplain areas

• debris basin construction

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Figure 3.7. 1938 aerial photograph of Emigration Creek from 1700South to 1300 East. The Photograph is overlaidwith 2006 channel alignment in red and yellow; gapsin the line indicate underground culverts.

Figure 3.6. 1938 aerial photograph of Emigration Creek from 2100East to 1700 East. The Photograph is overlaid with 2006channel alignment in red; gaps in the line indicateunderground culverts.

significant change is the increasein length and number of culvertpipes since 1938 (Figures 3.6and 3.7). The 1938 photo showsa continuous open streamchannel between 2100 East andwhat is now Hogle Zoo(Bowman and Beisner 2008);today this stretch of stream isinterrupted by six different culvertcrossings. The construction ofculvert crossings and piping ofportions of Emigration Creekfacilitated urban growth but alsoreduced total channel length,resulting in greater channel slopeand higher stream velocities. The culverts have also disruptedthe connectivity of the ripariancorridor by creating barriers tofish and wildlife migration. Insome residential areas along thecreek, it appears that tree canopydensity has increased since 1938. This is most likely the result oflandscaping and tree planting asdense residential neighborhoodswere built along the creek. Based on the ages of homes inthe corridor, much of theresidential development withinthe RCS study area occurredbetween about 1940 and 1960.

Urban ChannelAdjustments

Urbanized streams have beenfound to undergo a sequence oftypical channel adjustments inresponse to changes in hydrologyand sediment supply (Wolman1967, Riley 1998, Colosimo andWilcock 2007). Studies of urbanchannel adjustment generallyidentify two main stages ofadjustment: an early depositional

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Figure 3.8. Illustration of streambed lowering (incision) processcommon on urbanized streams. Following initial incision(B), the channel may continue to incise and widen until a new equilibrium channel/floodplain geometry isreached, posing a potential risk to urban development onterrace surfaces adjacent to the channel. (Diagram fromFISRWG 1998).

phase and a later, fully urbanizedphase. The early phase occursduring initial development, whenactive construction leads toincreased fine sediment supply,increased bar deposits, andreduced channel size. Thelate/fully urbanized phase occursafter construction activities areessentially complete and thewatershed has become stablewith a high percentage ofimpervious surface area, andrunoff magnitudes and volumeshave correspondingly increased. Channels in the “late urbanized”phase are typically enlargedrelative to their original form dueto an over supply of water

relative to sediment supply. These channels have few bardeposits and are commonlydowncut (incised) with reducedfloodplain access (Figure 3.8). Many of the reaches ofEmigration Creek that wereassessed exhibit characteristics ofthe late urbanized phase, such asevidence of down-cutting andlow bank erosion/root scour. During the RCS publicworkshops, a number ofresidents who live alongEmigration Creek indicate thatthey have observed lowering ofthe streambed adjacent to theirproperty over the last severaldecades.

Other influences, such aslocalized sediment inputs fromeroding storm drain outfalls orsediment deposition near culvertinlets, modify conditions fromthis generalized late urbanizedchannel condition. Existingchannel conditions within theEmigration Creek corridor reflecta complex response to a varietyof historical and ongoingalterations throughout thewatershed. This complexitymakes it difficult to distinguishwhether channel loweringobserved in a specific location isdue to a corridor-scale streambedlowering trend, a localized culverteffect, or a combination ofseveral factors.

Recent and AnticipatedFuture Trends

The residential population of theupper Emigration Creeksubwatershed grew dramaticallyfrom 1980–2005, with a currentpopulation estimate of about1,200 year-round residents(WRPRP 2009). Developmenthas often involved directalterations to the stream channelsuch as construction of newbridge or culvert crossings, bankarmoring projects, and channelrelocation efforts (UDWRT2009).

Within the lower EmigrationCreek subwatershed, land usepredictions for 2030 indicate aincrease in impervious coverfrom 23.2% to 27.7% and a 6%loss of open space. Most of thischange is associated with anexpected increase in the amount

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Figure 3.9. Plot of temporal trends in annual stream flowat Emigration Creek gage. Plot provided by the Salt LakeCity Department of Public Utilities.

of commercial land use. Noimpervious surface coverincrease is predicted for theupper subwatershed. However,more than 600 currentlyundeveloped residential lots arepresent in Emigration Canyon,suggesting a potential forsignificant further development. There are currently no plans todevelop a sewer system in theUpper Emigration Creeksubwatershed (SLCO 2009).

Climate change is another factorthat can be anticipated to affectthe Emigration Creek ripariancorridor. Climate projections forthe southwest region of theUnited States show increasedtemperatures, reduced mountainsnowpack, a 10–20% decrease inannual runoff volume, reducedspringtime precipitation amounts,and anticipated water supplyshortages (Karl et al. 2009). Therisk of drought, as well as the riskof flooding, are expected toincrease. The changes intemperature will likely result in ashift in vegetation communities,and altered precipitation patternswill influence stream hydrologyand channel conditions. Thetiming of snowmelt runoff isexpected to occur earlier in thespring, with an anticipatedreduction in summertimebaseflows (Karl et al. 2009). It isdifficult to predict specificchanges to Emigration Creekwith certainty, but recordedannual stream flow volumes onEmigration Creek show adeclining trend between 1970and 2006 (L. Alserda 2009, pers.comm., Figure 3.9). This trend is

anticipated to continue into thefuture (Karl et al. 2009).

Stream and VegetationConditions

Stream ChannelCharacteristics

Salt Lake County has classifiedthe majority of stream lengthwithin the lower EmigrationCreek subwatershed asmoderately entrenched, meaningthe channel is somewhatvertically confined. Over 90% ofthe channel in the lowerwatershed received a fair to poorstream stability rating duringCounty stream studies (SLCO2009). During field assessmentsin 2008, the County classifiedlower Emigration Creek asRosgen (1996) stream type B5between Rotary Glen Park andFoothill Drive (reaches LEM_R01

through LEM_R05) and streamtype B4 below Foothill Drive.The assigned stream type forreaches UEM_R16 andUEM_R17 was also B4 (K.Collins 2009, pers. comm.). County bankfull width estimatesfor the stream reaches in lowerEmigration Creek ranged from12–37 feet, with an averagevalue of 18 feet. Estimates forreaches UEM_R16 andUEM_R17 were 10 feet and 11feet, respectively (K. Collins 2009, pers. comm.).

Results of RCS field surveys andGIS analyses further illustrate thefact that the Emigration Creekchannel is moderatelyentrenched and typically insetbetween tall, steep slopes (Figure3.10). Because of thischaracteristic, residents along thecreek corridor who attended theRCS public workshops often referto the channel as a “gully” or

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Figure 3.10. Cross-section plots extrapolated from digital elevation data. Plots on left (in red) exhibit lessvertical confinement and include relatively well-developed active floodplain surfaces. Plots onright (in blue) exhibit a greater degree of vertical confinement between tall, steep side slopes.

“canyon.” The steep side slopesalso make access to the creekchallenging in many areas. However, the extent of verticalconfinement varies, and in somelocations the channel shape iswider and includes larger areasof flat, active floodplain surfaces(Figure 3.10). These surfaces are

important because they allowwater to spread out horizontallyduring flood events, dissipatingvelocity and reducing erosionpotential.

Surveyed channel width valuesare quite variable, ranging fromabout 4–14 feet at low flow, with

an average value of 10 feet(Table 3.1). In about half of thereaches, field surveys wereconducted at a streamflow of 66cfs, which is close to the averageannual high flow value of 55 cfs. Width at this high flow valuevaries from about 13–26 feet,with an average of 17 feet. In

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Table 3.1. Summary of streambed material, channel geometry, and slope data.

REACHNUMBER

MEASURED VALUES AT RIFFLE CROSS SECTIONREACH DATASTREAMBED MATERIAL SIZE

DATACHANNEL GEOMETRY DATA

D16(mm) a

D50(mm) a

D84(mm) a

PercentEmbedded

Low FlowWetted

Width (ft) b

WettedWidth (ft) b

at 21 cfs c

WettedWidth (ft) b

at 66 cfs c

LocalSlope

(ft/ft) d

Reach Slope(ft/ft) d

Reach Length(ft) b

UEM_R16 5 17 92 21 N/A e 9.9 13.8 0.020 0.021 2864

UEM_R17 7 22 49 3 9.1 9.9 - 0.013 0.021 681

LEM_R01 <2 28 59 9 10.9 13.9 18.0 0.007 0.018 1284

LEM_R02A 4 20 70 22 11.9 14.1 - 0.014 0.007 290

LEM_R02B 8 68 181 47 14.8 17.9 - 0.036 0.033 734

LEM_R02C - - - - - - - - 0.025 1120

LEM_R02D 5 27 85 10 9.3 12.6 15.5 0.013 0.015 277

LEM_R03A 5 19 46 0 12.2 15.3 - 0.013 0.018 341

LEM_R03B <2 17 72 15 4.2 8.9 - 0.052 0.023 451

LEM_R04 <2 36 179 27 7.5 11.1 - 0.038 0.029 768

LEM_R05A <2 37 121 22 13.9 14.7 15.1 0.024 0.016 615

LEM_R05B <2 34 64 10 5.9 10.5 - N/A e 0.027 317

LEM_R06 12 34 91 16 8.8 16.6 17.6 0.025 0.029 155

LEM_R07_XS1 18 45 104 17 10.6 14.6 15.3 0.027 0.023 674

LEM_R07_XS2 - - - - 5.3 8.7 - 0.045 0.023 674

LEM_R08A 8 43 109 24 12.1 13.7 16.5 0.024 0.025 988

LEM_R08B 20 51 116 14 11.5 13.0 - 0.022 0.020 677

LEM_R09A <2 32 99 8 11.1 18.6 - 0.037 0.024 579

LEM_R09B - - - - - - - - 0.050 264

LEM_R09C 4 15 33 3 10.0 25.9 26.1 0.011 0.021 1248

LEM_R10 6 43 102 14 7.4 11.2 - 0.028 0.012 1121

LEM_R11A - - - - - - - - 0.025 520

LEM_R11B 4 37 128 15 10.7 12.6 13.2 0.056 0.020 614

LEM_R12 - - - - - - - - 0.028 1666

LEM_R13A 13 61 171 28 10.9 14.8 17.3 0.029 0.024 1304

a The 16th, 50th, and 84th percentile values of the particle size distribution in millimeters.b Feet.c Cubic feet per second.d Feet per feet.e Not applicable.

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Figure 3.11. Longitudinal profile plot of Emigration Creek streambed. Circles indicate culvert inlets oroutlets; red and blue lines indicate open channel stream sections.

some locations, such asLEM_R07 cross-section 2,channel width has been directlyaffected by installed bankhardening measures.

Channel slope, as determined foreach stream reach from digitalelevation data, varies from0.7%–5.0% within the RCS studyarea, with an average value of2.0%. (Figure 3.11, Table 3.1).

Emigration Creek does not showany consistent spatial trends ingradient through the study areabecause the valley slope remainssteep throughout the study area,which traverses the LakeBonneville bench deposits. Thevalley becomes significantlyflatter west of 1100 East, and

historically Emigration Creekwould have shifted to a flatter,less confined, more sinuouschannel type in this area. However, this portion of thecreek is now piped undergroundin the 1300 South conduit.

Median (D50) streambed particlesize at the measured crosssections ranges from 15–68millimeters (mm), indicating thatmedium- and large-sized gravelare the dominant substrate sizesin riffle areas of EmigrationCreek (Table 3.1). At most of thecross-section riffles, sand or finegravel comprises the D16 particlesize, and cobble-sized materialcomprises the D84 particle size(Table 3.1). Embeddednessvalues are highly variable. In

flatter-gradient portions of thechannel, such as run and poolareas, particle sizes are smaller,with sand and silt oftendominant. No consistentupstream-to-downstream trendsare evident in the pebble countresults; rather, bed material sizeand embeddedness appear to belargely a function of local factorssuch as sediment inputs fromerosion areas, composition ofbank material, and proximity toculvert outlets or inlets.

Vegetation Characteristics

Table 3.2 lists all dominant plantspecies noted on the data formsduring the study area mappingeffort. Species are identified bytheir common and scientific

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Table 3.2. Dominant species noted during Emigration Creek mapping work.SCIENTIFIC NAME COMMON NAME WETLAND INDICATOR STATUS NATIVE TO UTAH OR INTRODUCEDAcer grandidentatum bigtooth maple obligate upland nativeAchillea millefolium common yarrow facultative upland nativeAcer negundo box elder facultative wetland native Aegilops cylindrica jointed goatgrass obligate upland introducedAilanthus altissima tree of heaven no indicator introducedAmbrosia artemisiifolia annual ragweed facultative upland nativeArctium minus lesser burdock obligate upland introducedArtemisia tridentata big sagebrush obligate upland nativeBromus arvensis field brome obligate upland introducedBromus inermis smooth brome obligate upland introduced/naturalizedBromus tectorum cheatgrass obligate upland introducedCatabrosa aquatica water whorlgrass obligate wetland nativeCardaria draba whitetop (hoary cress) obligate upland introducedCampanula rapunculoides rampion bellflower obligate upland introducedCichorium intybus chicory obligate upland introducedConvolvulus arvensis field bindweed obligate upland introducedCornus sericea redosier dogwood facultative wetland nativeCrataegus spp. hawthorn obligate upland unknownCynoglossum officinale gypsyflower (houndstongue) no occurrence introducedElaeagnus angustifolia Russian olive facultative introducedElymus repens quackgrass facultative upland introducedEuphorbia myrsinites myrtle spurge obligate upland introducedFraxinus pennsylvanica green ash facultative wetland nativeGleditsia triacanthos honeylocust facultative nativeHedera helix English ivy obligate upland introducedLeymus cinereus basin wildrye no indicator nativeLepidium latifolium broadleaved pepperweed facultative introducedLinaria dalmatica dalmatian toadflax obligate upland introducedLonicera involucrata twinberry honeysuckle facultative nativeMaianthemum racemosum feathery false lily of the valley obligate upland nativeMahonia repens creeping barberry obligate upland nativeMentha arvensis wild mint facultative wetland nativeMelilotus officinalis yellow sweetclover facultative upland introducedMedicago sativa alfalfa obligate upland introducedOnopordum acanthium scotch cottonthistle obligate upland introducedParthenocissus quinquefolia Virginia creeper no occurrence nativePhalaris arundinacea reed canarygrass obligate wetland nativePopulus alba white poplar obligate upland introducedPopulus angustifolia narrowleaf cottonwood facultative nativePoa bulbosa bulbous bluegrass obligate upland introducedPopulus deltoides eastern cottonwood facultative wetland nativePopulus fremontii fremont cottonwood facultative wetland nativePoa pratensis Kentucky bluegrass facultative upland introducedPrunus virginiana chokecherry facultative upland nativePseudoroegneria spicata bluebunch wheatgrass obligate upland nativeQuercus gambelii Gambel oak obligate upland nativeRhus trilobata skunkbush sumac no indicator nativeRosa woodsii Woods' rose facultative nativeRumex crispus curly dock facultative wetland introduced

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Table 3.2. Dominant species noted during Emigration Creek mapping work (cont.).SCIENTIFIC NAME COMMON NAME WETLAND INDICATOR STATUS NATIVE TO UTAH OR INTRODUCEDSalix exigua narrowleaf willow obligate wetland nativeSalix fragilis crack willow facultative introducedSecale cereale cereal rye obligate upland introducedSolanum dulcamara climbing nightshade facultative introducedTaraxacum officinale common dandelion facultative upland introduced/ naturalizedToxicodendron rydbergii western poison ivy facultative upland nativeTribulus terrestris puncture vine obligate upland introducedUlmus pumila Siberian elm obligate upland introducedVeronica americana American speedwell obligate wetland nativeVinca major bigleaf periwinkle obligate upland introducedVinca minor common periwinkle obligate upland introduced

names, mapping code, wetlandindicator status (USFWS 1988,USACE 2008), and whether thespecies is native to Utah orintroduced (NRCS 2009). Sixtydifferent species were notedduring the Emigration Creekmapping work, about half ofwhich are native to Utah. Asseen in Table 3.3, most of thenonnative species within thecorridor occur in the canopy andunderstory vegetation layers,while the shrub layer isdominated entirely by nativespecies. Box elder (Acernegundo) and cottonwood(Populus sp.) are the mostcommon trees along the stream-side areas of Emigration Creek,with Gambel oak (Quercusgambelii) common onundeveloped upper slope areas. Siberian elm (Ulmus pumila)andRussian olive (Elaeagnusangustifolia), which areintroduced invasive trees, arealso quite common in the studyarea (Table 3.3). Commonshrub species include narrowleafwillow (Salix exigua), twinberryhoneysuckle (Lonicerainvolucrata), and redosier do

gwood (Cornus sericea), withWood’s rose (Rosa woodsii)common on upper portions ofslopes. The understoryvegetation layer shows thegreatest species variety. Creeping barberry (Mahoniarepens) is the most commonnative understory species instream-side areas, withbluebunch wheatgrass andragweed (Ambrosia sp.) commonon upper slopes. Introducedornamental English ivy (Hederahelix) and periwinkle (Vinca sp.)dominate the understory cover ina number of the urbanizedresidential stream reaches. Othercommon nonnative understoryspecies include lesser burdock(Arctium minus) and climbingnightshade (Solanum dulcamara)(Table 3.3).

Canopy (tree) cover is generallyhigh throughout the study area. All but ten of the mapped near-stream vegetation polygons havecover values greater than 75%. Because of the high-quality treecover within the EmigrationCreek riparian corridor, theriparian functions of shading and

Vegetation associationspresent in the study area:

• Big Sagebrush Shrubland/ Introduced Herbaceous

• Bigtooth Maple /Narrowleaf Cottonwood /Redosier DogwoodWoodland

• Box Elder Forest

• Box Elder / NarrowleafCottonwood / RedosierDogwood Forest

• Box Elder / NarrowleafWillow Woodland

• Box Elder / Gambel OakWoodland

• Box Elder / DisturbedUnderstory Woodland

• Box Elder / RedosierDogwood Forest

• Box Elder Semi-naturalWoodland

• Chokecherry Shrubland

• Eastern CottonwoodSemi-natural Woodland

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Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed streamreach.

PLANT SPECIES

UEM

_R16

UEM

_R17

LEM

_R0

1

LEM

_R0

2A

LEM

_R0

2B

LEM

_R0

2D

LEM

_R0

3A

LEM

_R0

3B

LEM

_R0

4

LEM

_R0

5A

LEM

_R0

5B

LEM

_R0

6

LEM

_R0

7

LEM

_R0

8A

LEM

_R0

8B

LEM

_R0

9A

LEM

_R0

9B

LEM

_R0

9C

LEM

_R10

LEM

_R11B

LEM

_R13

A

Common Name Scientific Name

CANO

PY

Bigtooth maple Acer grandidentatum X X X XBox elder Acer negundo X X X X X X X X X X X X X X X X X X X X XChokecherry Prunus virginiana X X XCrack willow a Salix fragilis a X X XEasterncottonwood

Populus deltoides X X X X X X X X X X X

Fremontcottonwood

Populus fremontii X X X

Gambel oak Quercus gambelii X X X X X X X X X XGreen ash Fraxinus pennsylvanica XHawthorn a Crataegus spp. a X XHoneylocust a Gleditsia triacanthos a XNarrowleafcottonwood

Populus angustifolia X X X X X X X X

Russian olive b Elaeagnusangustifolia b

X X X X X X X

Tree of Heaven b Ailanthus altissima b XSiberian elm b Ulmus pumila b X X X X X X X X X X X XWhite poplar a Populus alba a X X X

SH

RUB

Big sagebrush Artemisia tridentata XNarrowleaf willow Salix exigua X X X X X X XRedosier dogwood Cornus sericea X X X X X X X X X X XSkunkbush sumac Rhus trilobata X X X XTwinberryhoneysuckle

Lonicera involucrata X X X X X X X X X X X X

Woods’ rose Rosa woodsii X X X X X X X X

UNDE

RSTO

RY

Alfalfa a Medicago sativa a XAmericanspeedwell

Veronica americana X X

Annual ragweed Ambrosia artemisiifolia X X X X XBasin wildrye Leymus cinereus XBigleaf periwinkle c Vinca major c X X XBluebunchwheatgrass

Pseudoroegneriaspicata

X X X X X X

Broadleavedpepperweed b

Lepidium latifolium b X

Bulbousbluegrass a

Poa bulbosa a X X

Cereal rye a Secale cereale a X XCheatgrass c Bromus tectorum c XChicory a Cichorium intybus a XClimbingnightshade a

Solanum dulcamara a X X X X

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Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed stream reach(cont.).

PLANT SPECIES

UEM

_R16

UEM

_R17

LEM

_R0

1

LEM

_R0

2A

LEM

_R0

2B

LEM

_R0

2D

LEM

_R0

3A

LEM

_R0

3B

LEM

_R0

4

LEM

_R0

5A

LEM

_R0

5B

LEM

_R0

6

LEM

_R0

7

LEM

_R0

8A

LEM

_R0

8B

LEM

_R0

9A

LEM

_R0

9B

LEM

_R0

9C

LEM

_R10

LEM

_R11B

LEM

_R13

A

Common Name Scientific Name

UNDE

RSTO

RY

Commonperiwinkle c

Vinca minor c X X X X

Creeping barberry Mahonia repens X X X X X X X X

Curly dock a Rumex crispus a X

Dalmatiantoadflax b

Linaria dalmatica b X X

English ivy c Hedera helix c X X X

Feathery false lilyof the valley

Maianthemumracemosum

X X

Field bindweed b Convolvulus arvensis b X X

Field brome a Bromus arvensis a X

Houndstongue bCynoglossum

officinale bX X

Jointedgoatgrass b

Aegilops cylindrica b X X X

Kentuckybluegrass a

Poa pratensis a X X X X X

Lesser burdock b Arctium minus b X X X X X X X X

Myrtle spurge b Euphorbia myrsinites b X X X

Puncture vine b Tribulus terrestris b X

Quackgrass b Elymus repens b X

Rampionbellflower c

Campanularapunculoides c

X X

Reed canarygrass Phalaris arundinacea X X

Scotchcottonthistle b

Onopordumacanthium b

X X X

Smooth brome a Bromus inermis a X

Virginia creeperParthenocissus

quinquefoliaX

Water whorlgrass Catabrosa aquatica X

Western poison ivy Toxicodendron rydbergii X X X

Whitetop(hoary cress) b Cardaria draba b X X

Wild mint Mentha arvensis X

Yellowsweetclover a

Melilotus officinalis a X

a Species not native to Utah.b State- or city-listed, nonnative, noxious weed species.c Nonnative, invasive species.

water-temperature control aremet to a high degree. Incontrast, plant cover within thelower structural layers is typically

much lower, with 31 and 28 ofthe mapped near-streampolygons having cover of 50% orless in the shrub and understory

communities, respectively (Table3.4). Invasive species cover wasvariable throughout the studyarea, with about half of the near-

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Table 3.4 Percent cover and invasive species class for mapped vegetation polygons.

REACH POLYGONNUMBER

PERCENTCANOPY COVER

PERCENTSHRUB COVER

PERCENTUNDERSTORY COVER

INVASIVE SPECIES CLASS

LEM_R02D 1 76—100+ 6—25 1—5 moderateLEM_R03A/R03B 2 76—100+ 0 0 highLEM_R03A 3 0 76—100+ 0 noneLEM_R03B 4 76—100+ 6—25 26—50 highLEM_R03B 5 76—100+ 0 76—100+ highLEM_R03B/R04 6 76—100+ 76—100+ 26—50 moderateLEM_R04 7 76—100+ 6—25 0 noneLEM_R04 8 51—75 0 6—25 moderateLEM_R04/R05A 9 51—75 0 6—25 moderateLEM_R04 10 26—50 51—75 0 noneLEM_R04/R05A 11 76—100+ 26—50 0 highLEM_R05A 12 76—100+ 26—50 0 highLEM_R05B 13 76—100+ 51—75 0 moderateUEM_R16 14 76—100+ 26—50 26—50 moderateUEM_R17 15 51—75 26—50 6—25 noneUEM_R17 16 76—100+ 26—50 6—25 lowLEM_R01 17 76—100+ 26—50 26—50 moderateLEM_R01 18 76—100+ 51—75 26—50 moderateLEM_R01 19 76—100+ 26—50 26—50 highLEM_R06 20 76—100+ 6—25 0 noneLEM_R09A 21 76—100+ 26—50 51—75 lowLEM_R09A/R09B 22 76—100+ 26—50 51—75 lowLEM_R09C/R10 23 76—100+ 26—50 51—75 lowLEM_R09C 24 76—100+ 26—50 51—75 lowLEM_R09C 25 76—100+ 26—50 51—75 lowLEM_R09A/R09B/R09C 26 76—100+ 76—100+ 0 moderateLEM_R09C 27 76—100+ 51—75 6—25 moderateLEM_R10 28 76—100+ 51—75 0 highLEM_R10 29 76—100+ 6—25 0 noneLEM_R13A 30 76—100+ 6—25 6—25 moderateLEM_R13A 31 76—100+ 6—25 26—50 highLEM_R02A 32 51—75 26—50 51—75 moderateLEM_R02B 33 76—100+ 26—50 6—25 moderateLEM_R11B 34 76—100+ 0 6—25 moderateLEM_R08B 37 76—100+ 6—25 51—75 majorityLEM_R08A 38 26—50 51—75 26—50 highLEM_R08B 39 76—100+ 26—50 51—75 majorityLEM_R08A 40 76—100+ 26—50 51—75 majorityLEM_R07 41 6—25 6—25 76—100+ highLEM_R07 42 51—75 6—25 76—100+ majorityLEM_R10 201 26—50 6—26 76—100+ majorityLEM_R10 202 76—100+ 0 51—75 majorityLEM_R10 203 6—26 0 76—100+ highLEM_R10 204 76—100+ 0 51—75 majorityLEM_R09C 205 76—100+ 0 26—50 majorityLEM_R09C 206 76—100+ 0 76—100+ majorityLEM_R09C 207 6—26 0 76—100+ majorityLEM_R09C 208 76—100+ 0 76—100+ majorityLEM_R09C 209 0 0 76—100+ majority

3-20


Table 3.4 Percent cover and invasive species class for mapped vegetation polygons (cont.).

REACH POLYGONNUMBER

PERCENTCANOPY COVER

PERCENTSHRUB COVER

PERCENTUNDERSTORY COVER

INVASIVE SPECIES CLASS

LEM_R09C 210 76—100+ 0 26—50 highLEM_R09C 211 76—100+ 6—25 76—100+ moderateLEM_R09C/R09B/R09A

212 0 0 76—100+ majority

LEM_R09A 213 0 6—25 76—100+ majorityLEM_R09B 214 76—100+ 26—50 0 moderateLEM_R09B 215 6—25 0 76—100+ noneLEM_R09B 216 76—100+ 6—25 0 noneLEM_R09C 217 76—100+ 6—25 6—25 moderateLEM_R10 218 76—100+ 6—25 51—75 highLEM_R10 219 0 0 76—100+ majorityLEM_R10 220 26—50 6—25 76—100+ majority

stream vegetation polygonshaving an invasive species classof “low” or “none” (i.e., 5%cover or less), with the other halfclassified as moderate, high, ormajority invasive cover (Table3.4). In reaches LEM_R09 andLEM_R10, the channel issurrounded by a wider area ofnatural upland vegetation. Polygons mapped in theseupland areas farther away fromthe stream (polygons 201–220)

generally included high amountsof invasive plants (Table 3.4).

Issues AffectingRiparian Functions

During the baseline assessmentwork, several common issueswere observed to be affectingand limiting riparian functions inthe Emigration Creek corridor. These issues are discussed byfunction below.

Aesthetics

Although many visuallyappealing portions of EmigrationCreek exist, the presence of trashand debris degrades corridoraesthetics in a number oflocations. Common types oftrash include miscellaneous smallitems such as bottles, cans, foodwrappers, ropes, tarps, etc. These items are common in themore accessible stream reachesthat are used for recreation. Another common category of

trash is remnant/obsoleteinfrastructure such as pieces ofconcrete and asphalt, old pipesand barrels, broken fencing,obsolete erosion-control devicessuch as failing silt fence, etc. Inmany instances the concretepieces are associated with priorbank stabilization efforts thathave failed due to the concretebeing undermined by scour orstreambed lowering. Twenty-seven individual, significant litterareas were mapped in the studyarea during the RCS baselineassessment work.

Wildlife Habitat and Connectivity

A wide range of native bird andmammal species rely on nativeinsects as a key food source(Tallamy 2009). These insectsmust share an evolutionaryhistory with plants in order torecognize them and use them asa food source. Therefore,healthy native plant communitiesare necessary for a riparian

3-21


corridor to function to itsmaximum potential in terms ofwildlife habitat. As discussedabove, invasive nonnative plantspecies are a concern in abouthalf of the study reaches withinthe Emigration Creek corridor,and they affect the compositionof the understory and canopyvegetation layers. In some areasinvasive species comprise themajority plant cover within avegetative layer, limiting theability of native plants to thriveand support native insects, birds,and wildlife. The lack ofunderstory and shrub cover inmany reaches also limits habitatquality in terms of structuraldiversity, which is particularlyimportant for bird populations.

Another issue affecting wildlifehabitat, as well as ripariancorridor connectivity, is thepresence of stream-crossingculverts. Fourteen culvertcrossings are present within thestudy area (Figure 3.11). Most ofthese culverts impede or blockfish passage due to steep verticaldrops at their outlets and high-flow velocities within the smoothconcrete pipes. This limits theability of fish populations to useEmigration Creek as acontinuous travel corridor. Thesmall diameter of the culvertsalso blocks passage by mammalspecies such as deer. Within thestudy area, a total length of 1mile of stream is contained inculvert pipes, limiting the overalllength of open-channel streamavailable as aquatic habitat. Thelongest continuous segments ofstream in the study area include

a 3,200-foot-long segmentbetween 1900 East and 1700South, and study reachUEM_R16, which is 2,800 feetlong (Figure 3.11).

Nutrient Filtration and Sediment Trapping

As discussed above, many areasof the Emigration Creek corridorlack the dense understory andshrub cover that are needed tomaximize the ability of theriparian corridor to filtersediment, nutrients, andpollutants from storm runoff. Insome areas, understory cover ishigh but the community isdominated by invasive periwinkleor English ivy vines. Becausethese vines have shallow, low-density root and stem systems,they do not serve the filtrationfunction as well as native grassand forb communities would.

Stream Stability

A number of different issues werenoted as affecting stream stabilitywithin the Emigration Creekriparian corridor. Specific issuesare discussed in the subsectionsbelow.

Stream-Crossing CulvertsLocalized erosion and depositionproblems were noted at many ofthe stream-crossing culvertswithin the study area. Most ofthe culverts have diameters of4–5.5 feet (Table 3.5), which issignificantly smaller than the 17-foot average channel width athigh flow. Because of this width

Vegetation associationspresent in the study area(cont.):

• Gambel Oak Forest

• Gambel Oak / SparseUnderstory Shrubland

• Introduced OrnamentalSemi-natural Woodland

• Introduced Herbaceous

• Mixed Semi-naturalWoodland

• Mixed Semi-natural Treesand Shrubs / IntroducedHerbaceous

• Mixed Semi-naturalWoodland / IntroducedHerbaceous

• Narrowleaf Cottonwood /Narrowleaf WillowWoodland

• Narrowleaf Willow BarrenShrubland

• Narrowleaf Cottonwood /Redosier DogwoodWoodland

• Russian Olive Semi-naturalWoodland

• Siberian Elm / Russian OliveIntroduced OrnamentalWoodland

• Siberian Elm Semi-naturalWoodland

• Siberian Elm IntroducedOrnamental Woodland /Kentucky BluegrassIntroduced Herbaceous

• White Poplar Semi-naturalWoodland

3-22


Table 3.5 Size and condition of stream crossing culverts in the study area.CROSSINGLOCATIONANDDESCRIPTION

REACH NUMBER(S)

CULVERT LENGTH

(ft) a

VERTICAL DROPFROM INLET TO OUTLET b

(ft) a

CULVERTTYPE

CULVERTDIAMETER

(ft) a

INLETCONDITION

OUTLET CONDITION

EmigrationCanyon

betweenUEM_R16

and UEM_R171029 53

roundconcrete pipe

4 not assessedfair; crack in concrete headwall;obsolete silt fence; rock outletprotection functioning well

Debris BasinOutlet

LEM_R01 187 3 round pipe 4.6 not assessed fair; partially filled with silt

Crestview Drivebetween

LEM_R01 and LEM_R02A

242 9round

corrugatedmetal pipe

5.5 goodpoor; bank erosion; 4-5 ft.scour depth

Hogle Zoobetween

LEM_R02A andLEM_R02B

1174 57round

corrugatedmetal pipe

5.5 goodpoor; scour around concreteapron (2.8' depth); trash/concrete chunks

Path in middle of LEM_R02D

LEM_R02D 51 <1metal three-sided arch

8.5 H x 13.5 Wc good good

Bonneville GolfCourse-easterngolf path crossing

betweenLEM_R02D and

LEM_R03A181 8

roundconcrete pipe

5good; some debrisat grate

poor; scour around concreteapron (2.5' depth); rebar/concrete chunks

Bonneville GolfCourse-centralgolf path crossing

betweenLEM_R03A and

LEM_R03B85 5

roundconcrete pipe

5poor: significantdebris and sedimentaccumulation

poor; scour around concreteapron (3' depth); steep/barebanks

Bonneville GolfCourse-westerngolf path crossing

betweenLEM_R05A and

LEM_R05B73 6

roundconcrete pipe

5poor; debris jamdiverting flow; treeleaning on headwall

poor; crack in concreteheadwall; scour/undercuttingaround apron; bare slopes

Foothill Drivebetween

LEM_R05B andLEM_R06

223 2round

concrete pipe5

fair; some debrisaccumulation and erosion aroundconcrete wing wall

fair; tailwater pool present;steep/bare banks; some bankerosion

2100 Eastbetween

LEM_R06 andLEM_R07

280 20

concrete box(inlet); roundconcrete pipe

(outlet)

3 H x 4 W(inlet) c; 4.75

(outlet)

fair; some trash/sediment depositionat grate

poor; severe bank erosion;scour around concrete apron;broken concrete wall

1300 Southbetween


200 6round

concrete pipe4

fair; some concretechunks and minorerosion aroundgrouted rockheadwall

fair; large concrete apron andconcrete walls; some erosion atlip of apron and at edge ofheadwall

1900 Eastbetween

LEM_R08B andLEM_R09A

358 10round

concrete pipe4

fair; appears to clogoccasionally;

good; tailwater pool present

1700 Southbetween


559 35

two verticallystackedconcrete

boxes

each3 H x 3 W c

poor; appears toclog regularly andcause siltdeposition for ~200ft upstream

not assessed

1500 Eastbetween

LEM_R11B and LEM_R12

499 12square

concrete pipe3 H x 3W c

fair; clogsperiodically; asphaltand concretechunks around/above headwall

not assessed

1300 Eastbetween


361 13round

concrete pipe3.8 not assessed

poor; bed, and bank erosionevident; scour around brokenconcrete apron; rill at stormoutfall

a Feet.b Elevation change between inlet and outlet based on digital elevation data.c H = height, W = width.

3-23


discrepancy, a hydraulicconstriction occurs at culvertinlets, slowing flow velocities andleading to deposition andaccumulation of sediment anddebris. During the springtimehigh-flow period, regularmaintenance is needed to keepculvert inlets from becoming

blocked by debris and creating astability and flooding concern. Inbetween maintenance visits,inlets often become partiallyclogged, creating backwaterpools and causing silt to drop outonto the affected streambanks. This silt buries existing plants andlimits future vegetationestablishment, leaving bare bankareas that are susceptible toerosion during storm events.

The size and design of thestream-crossing culverts alsocontribute to stability concerns atthe culvert outlets. During highflows velocities at the culvertoutlets are very high because ofwidth constriction and a lack ofbed roughness within the smooth

concrete pipe material. Most ofthe culverts have wingwalls andaprons made of concrete, whichfurther accelerates velocities. This leads to scour and bankerosion where the outlet structure meets the natural channel andbanks. Scour holes between2.5–5.0 feet deep were measuredat culvert outlets within the studyarea.

The stream-crossing culverts onEmigration Creek createsignificant interruptions in thetransport of sediment and woodydebris through the corridor. Because they create “hardpoints” in the streambed, theyalso control and alter localstream gradient and affect

Invasive plants of concern in the studyarea:

• Russian olive

• Siberian elm

• lesser burdock

• cheatgrass

• myrtle spurge

• periwinkle vine

• English ivy

• broadleavedpepperweed

• whitetop

• dalmatian toadflax

• houndstongue

• quackgrass

• puncture vine

• Scotch thistle

• tree of heaven

• rampion bellflower

• jointed goatgrass

• field bindweed

Top left: Invasive vines on streambank. Top right: Debris and sedimentaccumulation at culvert inlet. Bottom left: Scour at culvert outlet.Bottom right: Erosion at storm drain outfall.

3-24


channel characteristics. Within agiven stream reach bounded byculverts, there is commonly anupstream “zone” near the culvertoutlet that is characterized byscour and evidence of streambedlowering and a downstream zonenear the culvert inlet that is flatterand shows evidence of sedimentdeposition. The length of theseculvert-influenced zones varies,but it can be as great as 200 feetin some reaches.

Storm Drain OutfallsErosion was commonly observedat storm drain pipe outfalls withinthe study area. These outfalls

deliver storm water runoff to thecreek from streets, gutters, androoftops. The outfalls often lackadequate outlet protection todissipate runoff velocities andprotect against erosion. Evenwhere outlet protection isprovided, stabilized conveyancechannels are typically lackingbetween the protected outlet andthe main Emigration Creekchannel, and evidence of rill erosion in these areas iscommon. Of the 23 mappedoutfall locations, 16 were rankedas medium- or high-priority areasfor stability improvements.

Streambank ErosionLateral erosion of streambanks isa natural process in streamchannels, which are dynamicsystems. Erosion and sedimenttransport are necessary for thecreation and maintenance ofimportant habitat features suchas scour pools, undercut banks,and spawning gravels. Deposition of sediment ontofloodplain areas is alsoimportant, as it provides freshsubstrate for the growth of willowand cottonwood seedlings thatare needed to maintain nativeriparian forests. However,excessive amounts of erosion or

deposition can degrade habitatand water quality, and threatenmunicipal infrastructure andresidential homes.

Several types of bank erosionwere observed in the study area. Low bank erosion/root zonescour are evident in nearly allstudy reaches and are associatedwith the flashy urban hydrologythat produces frequent, erosiverunoff events during storms. Insome areas, it appears thatstreambed lowering is alsocontributing to low-bank erosionby causing the toe of the slope tobecome undermined. In somereaches tall, vertical, bare banksare present where the creek hasmigrated laterally into a fine-grained Bonneville terracedeposit. This type of terraceerosion at the outside of bends isa natural process, but it is aconcern where it poses a risk toinfrastructure. Localized bankerosion caused by direct channelalterations is another type oferosion problem observed in thestudy area. Several RCSworkshop attendees describedbank erosion problems on theirproperty that began after fill-placement or bank-hardeningprojects on adjacent propertiesaltered streamflow patterns. Thistype of problem can occur whenbank-stabilization efforts are notimplemented comprehensivelythroughout a reach, becausemeasures taken to fix erosion inone location may alter channelshape and flow hydraulics andinadvertently create erosion in adifferent location.

Below: (left) Terrace erosion; (right) Low bank/root zone erosion.

Factors limiting shruband understory cover:

! oversteepened slopes

! inadequate revegetationefforts followingconstruction

! soil compaction fromheavy foot traffic

! bank siltation at culvertinlets

! uncontrolled runoff fromupland areas

Documents

3.0 BASELINE ASSESSMENT RESULTS - slcdocs.com Study website/Emigration/Final/6... · FINAL EMIGRATION CREEK MANAGEMENT PLAN Figure 3.1. Emigration Creek watershed. (Map from SLCO