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V 372
ents
dori
dain
N 445
eomer
bilbo
bifur
beorn
balin
arwen
eomund
elrond
dwalin
deagol
cotton
cirdan
bombur
barrow
balrog
entings
dunadan
dernhem
crebain
aragorn
denethor
bregalad
butterbur
bracegirdle
U 372
bofur
elessar
boromer
entwives
beachbone
dunlending
black riders
1
2
5
4
3
6
7
89
10
1112
13
14
15
17
17.01
16
18
17.02
17.03
17.04
19
20
2122
23
24
25
26
27
2829
3031
32
33
34
35
36
37
38
39
MAP SEGMENT 1
N
A
A'
200
200
200
Flathead
River
Dixon0.125 mile
Agen
cy0.
25m
ile
12388200
JockoRiver
Jocko
River
Railroad bridge
Railroad bridge
Highway bridge
Railroadbridge
S-212
Looking downstream from levee on left bank near cross section 21.
Looking downstream from left bank near cross section 13.
20
27
R.21W.T.18N.
T.17N.T.16N.
1 6
36
6
36 31
DIAGRAM SHOWING RELATIVEPOSITION OF MAP SEGMENTS
AND MATCHLINES ALONGTHE JOCKO RIVER
A
A'
B
B'
EXPLANATION
JOCKO RIVER STUDY AREA
A A' MATCHLINE--Line separatingMap segments 1-3
1 MAP SEGMENT NUMBER--Redsegment number representssegments shown on this sheet
R.20W.
T.17N.T.18N.
R.20W.
R.20W. R.19W.
7 8 9
18 16
22 23
29 28 25 30 29
34 35 32 33
45
97
18 17 16 15
20 22
28 26
34
3
11 12 7
2
1
3
11'
47 15'
114 15'o 14'
13' 12' 11'
114 10'o
16'17'18'
09'
08'
07'
06'
05' 04'
19'
18'
17'
16'
o
14'
13'
12'
21
17
26
8
21
27
35
2
Figure 3. Cross section 32, which is typical of channel and flood-plain conditions in Reach 5 along the Jocko River, western Montana.
ELEV
ATIO
N,I
NFE
ET(N
AVD
88)
0 200 400 600 800 1,000100 300 500 700 900
DISTANCE FROM LEFT EDGE, IN FEET
Ground surface
0 200 400 600 800 1,000100 300 500 700 900
DISTANCE FROM LEFT EDGE, IN FEET
2,630
ELEV
ATIO
N,I
NFE
ET(N
AVD
88)
Ground surface
2,650
For this simulation, the left bank is assumed to extend vertically to 500-yearwater-surface elevation in HEC-RAS model (flow actually spills from mainchannel to left-flood plain)
Mai
nch
anne
l
Mai
nch
anne
l
Flood plain
Flood plain
Main channel (existing condition)
Left-flood plain (existing condition)
Hypothetical (no-levee) condition
CALCULATED WATER SURFACE FOR 500-YEAR FLOOD DISCHARGE
Main channel (existing condition)
Left-flood plain (existing condition)
Hypothetical (no-levee) condition
CALCULATED WATER SURFACE FOR 100-YEAR FLOOD DISCHARGE
Leve
e
Leve
e
Ground surface used forhypothetical (no-levee) condition
Ground surface used forhypothetical (no-levee) condition
2,644
2,648
2,642
2,646
2,632
2,634
2,640
2,636
2,638
2,630
2,650
2,644
2,648
2,642
2,646
2,632
2,634
2,640
2,636
2,638
35
EXPLANATION
34
AREA INUNDATED BY THE 100-YEAR ESTIMATED FLOOD DISCHARGE--Includesfloodway where floodway data were available
AREA INUNDATED BY THE 500-YEAR ESTIMATED FLOOD DISCHARGE--Includesthe area inundated by the 100-year flood
FLOODWAY--Existing conditions (1999-2001)
CHANNEL CENTERLINE
CROSS SECTION AND NUMBER--Red cross-sectionsymbol represents cross section in split-flow reach
NATIONAL GEODETIC SURVEY BENCH MARK AND NAME
APPROXIMATE LOCATION OF LEVEE OR BANKLINE--Shownonly in split-flow reaches
N 445
ELEVATION-CONTROL POINT AND NAME--Data from Confederated Salish andKootenai Tribes of the Flathead Nation
butterbur
Aerial-photographic base (scanned) from Water Consulting, Inc.,for the Confederated Salish and Kootenai Tribes of the FlatheadNation, 1:3,600, 2002. Imagery flown by MAP, Inc. Lambert ConformalConic Projection State Plane Coordinate System (SPCS) Standardparallels 45 00' and 49 00'; central meridian -109 30'. North AmericanDatum of 1983 (NAD 83)
00 0
WATER-SURFACE ELEVATIONS AND BOUNDARIES OF THE 100-YEAR AND 500-YEAR FLOODS AND FLOODWAY FOR THE JOCKO RIVER, WESTERN MONTANA
U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY
SCIENTIFIC INVESTIGATIONS MAP 2912(Sheet 1 of 3)
Prepared in cooperation withTHE CONFEDERATED SALISH AND KOOTENAI TRIBES
OF THE FLATHEAD NATION Chase and Parrett, 2006, Water-surface elevations and boundaries of the100-year and 500-year floods and floodway for the Jocko River, western Montana
ByKatherine J. Chase and Charles Parrett
93
200
20093
0
0 1
1 2 3 4 5 MILES
5 KILOMETERS432
Base modified from U. S. Geological SurveyPlains and Seeley Lake, Montana, 30' x 60'quadrangles, 1979 and 1980, 1:100,000.North American Datum of 1927 (NAD 27)
47 11'o
47 15'o
114 15'o 114oR.19W.R.20W.R.21W.
T.18N.
T.17N.
T.16N.
1 6
36 31
1 6
36 31
1 6
36
31
1 6
36 31
LAKE
CO
Arlee
Dixon
FLATHEAD
INDIAN
RESERVATION
MISSION
RANGE
St.Ignatius
Flathead
Rive
r
CSKT12384500
FishHatchery
Jocko
River
Jocko
River
Spring
Creek
K
Canal
Jocko
River KnifeCreek
BigFinley
Creek
Vall
ey
Creek
Fork
E.
RESERVATION
DIVIDE
NATIONAL
BISON
RANGEMAP SEGMENT 1
MAP SEGMENT 2
MAP SEGMENT 3
Figure 1. Location of the Jocko River study area, western Montana.
A
A'
B
Agency
S-212
Ravalli
B'
SAN
DERS
CO
Mission ReservoirInset
below
12388000
12388200
R 1
R 4
R 6
R 7
R 8
R 5
R 2, 3
61
3136
1R.21W.R.22W.
R.23W.R.19W.
T.18N.
Flathead
River
200Dixon
Perma
12388700
R.24W. Inset0
0 1 KILOMETER
1 MILE
114 30'o
47 20'o
JOCKO RIVER STUDY AREA
REACH AND NUMBER
STREAMFLOW-GAGING STATIONAND NUMBER
U.S. Geological Survey
Confederated Salishand Kootenai Tribesof the Flathead Nation
CSKT
12384500
R 6R 5
MATCHLINE--Line separating Mapsegments 1-3
A A'
0 1 MILE0.25 0.50 0.75
SCALE 1:12,000
0 1 KILOMETER0.25 0.50 0.75
0 1,000 METERS250 500 750
0 5,000 FEET1,000 2,000 3,000 4,000
Printed on recycled paper
Reach 1
Reach 2, 3
Reach 4
Reach 5
Reach 6
Figure 2. Cross section 18, which is typical of split-flow conditions in Reach 3 along the Jocko River, western Montana.
0 200 400 600 800 1,000100 300 500 700 900 1,100
DISTANCE FROM LEFT EDGE, IN FEET
Main channel (existing condition)
Left-flood plain (existing condition)
Hypothetical (no-levee) condition
Ground surface used forhypothetical (no-levee) condition
Ground surface
0 200 400 600 800 1,000100 300 500 700 900 1,100
DISTANCE FROM LEFT EDGE, IN FEET
ELEV
ATIO
N,I
NFE
ET(N
AVD
88)
CALCULATED WATER SURFACE FOR 100-YEAR FLOOD DISCHARGE
Main channel (existing condition)
Left-flood plain (existing condition)
Hypothetical (no-levee) condition
Ground surface
No effective conveyence of floodwatersbeyond left edge of surveyed section
Mai
nch
anne
l
Mea
nder
chan
nel
Mea
nder
chan
nel
Mai
nch
anne
l
No effective conveyence of floodwatersin abandoned meander channel (ponded)
No effective conveyence of floodwatersin abandoned meander channel (ponded)
No effective conveyence of floodwatersbeyond left edge of surveyed section
Flood plain Flood plain
Leve
e
Leve
e
CALCULATED WATER SURFACE FOR 500-YEAR FLOOD DISCHARGE
Ground surface used forhypothetical (no-levee) condition
2,570
2,590
2,584
2,588
2,582
2,586
2,572
2,574
2,580
2,576
2,578
2,566
2,568
ELEV
ATIO
N,I
NFE
ET(N
AVD
88)
2,570
2,590
2,584
2,588
2,582
2,586
2,572
2,574
2,580
2,576
2,578
2,566
2,568
Looking upstream from railroad bridge at cross section 6.
122 DS 2
2 US 2
34566 DS 2
6 US 2
7899 DS 2
9 US 2
1011121314151616 DS 2
16 US 2
1717.01
0.00.42.43.45.63
1.682.052.442.452.472.693.013.293.293.323.594.886.737.768.91
10.0010.3510.3510.3710.9511.36
2,513.22,511.02,511.02,511.02,514.72,514.52,519.42,520.42,520.42,520.42,519.92,523.02,524.32,524.32,524.32,525.62,533.52,545.92,550.62,553.02,554.92,559.42,559.42,559.42,561.42,562.9
2,518.02,519.42,520.22,520.42,521.82,523.02,524.52,528.22,528.12,528.32,529.22,531.52,531.92,533.12,533.52,535.72,539.32,553.82,556.82,559.42,563.82,565.02,565.42,565.42,568.72,569.3
2,518.02,519.42,520.22,520.42,521.82,523.02,524.52,528.22,528.12,528.32,529.22,531.52,531.92,533.12,533.52,535.72,539.42,553.82,556.82,559.42,563.82,565.02,565.42,565.42,568.72,569.3
2,518.72,520.42,521.12,521.72,523.72,524.52,525.72,529.52,529.32,529.82,531.12,533.02,533.82,533.82,537.62,540.92,541.22,555.52,558.32,560.32,564.52,566.12,566.72,566.72,570.42,570.8
2,518.72,520.42,521.12,521.72,523.72,524.52,525.72,529.52,529.32,529.72,531.12,533.02,533.82,533.82,537.62,540.92,541.22,555.52,558.32,560.32,564.52,566.12,566.72,566.72,570.42,570.8
17.0217.0317.04
11.4612.1712.57
2,563.42,566.02,567.5
32,571.032,572.832,574.9
32,569.732,571.432,572.8
32,572.632,573.232,574.9
32,570.832,572.332,573.6
181920212223
12.9814.5115.9517.1418.0218.78
2,569.02,576.52,580.02,586.62,589.02,594.4
32,576.632,584.632,591.032,594.632,597.532,601.0
32,574.132,582.732,590.532,594.032,598.632,600.3
32,577.032,585.832,592.032,595.932,598.832,602.1
32,574.832,584.232,591.232,596.032,599.532,600.9
24252627
20.9022.2623.3325.06
2,600.32,607.52,614.52,619.2
2,608.32,613.92,620.52,627.9
2,608.32,613.02,620.62,627.9
2,609.02,614.92,621.82,628.6
2,609.02,614.12,621.82,628.6
28293031323334
26.2327.1227.8428.1628.6029.5030.91
2,622.22,627.12,630.82,629.02,632.22,634.82,641.5
32,630.732,633.832,637.732,638.532,641.232,644.332,650.5
32,630.432,633.032,638.232,639.232,640.532,644.632,650.7
32,632.132,635.432,639.132,639.632,643.732,645.932,652.0
32,631.732,634.432,639.732,641.132,641.632,645.732,651.8
3536373839
32.2233.5934.4536.0437.18
2,648.62,651.72,658.02,664.02,666.4
2,655.62,661.42,664.52,671.02,673.5
2,655.82,661.42,664.52,671.02,673.3
2,657.42,663.02,665.22,671.82,674.2
2,657.32,662.12,665.72,671.72,674.1
Table 4. Streambed- and calculated water-surface elevation data for the main channel of the Jocko River, western Montana.
[Table 4 continued on sheets 2 and 3. Distance upstream from cross section 1 measured along channel centerline shown on U.S. Geological Survey 7.5-minutequadrangle maps. Vertical coordinate information referenced to North American Vertical Datum of 1988 (NAVD 88). Abbreviations: DS, downstream side ofbridge; US, upstream side of bridge]
1Cross
section
Distanceupstream
from crosssection 1,
inthousands
of feet
Minimumstreambedelevation,
in feet
100-year flood discharge 500-year flood discharge
Existingcondition
(with levee)
Hypotheticalcondition(no levee)
Existingcondition
(with levee)
Hypotheticalcondition(no levee)
Calculated water-surface elevation, in feet, forindicated flood discharge and condition
Reach 1 (Map segment 1)
—Reach 2 Split-Flow 1 (Map segment 1)
—Reach 3 Split-Flow 2 (Map segment 1)
—Reach 5 Split-Flow 3 (Map segment 1)
Reach 4 (Map segment 1)
Reach 6 (Map segment 1)
1 Data for some interpolated cross sections are omitted from the table.2 Cross section not shown on map segment.3 Calculated water-surface elevations in split-flow reaches are for main channel only.
Table 5. Streambed- and calculated water-surface elevation data for split-flow reaches for the 100-yearflood discharge on the Jocko River, western Montana.
[Table 5 continued on sheet 3. Distance upstream from cross section 1 measured along channel centerline shown on U.S. GeologicalSurvey 7.5-minute quadrangle maps. Vertical coordinate information referenced to North American Vertical Datum of 1988 (NAVD 88)]
2,571.02,572.82,574.9
2,576.62,584.62,591.02,594.62,597.52,601.0
2,630.72,633.82,637.72,638.52,641.22,644.32,650.5
2,569.72,571.42,572.8
2,574.12,582.72,590.52,594.02,598.62,600.3
2,630.42,633.02,638.22,639.22,640.52,644.62,650.7
2,571.02,571.32,571.7
2,573.32,583.82,591.32,595.72,598.72,599.3
2,630.82,634.12,640.02,640.12,640.22,644.42,652.5
2,569.72,571.42,572.8
2,574.12,582.72,590.52,594.02,598.62,600.3
2,630.42,633.02,638.22,639.22,640.52,644.62,650.7
17.0217.0317.04
181920212223
28293031323334
Crosssection
11.4612.1712.57
12.9814.5115.9517.1418.0218.78
26.2327.1227.8428.1628.6029.5030.91
Distanceupstream
from crosssection 1,
inthousands
of feet
2,563.42,566.02,567.5
2,569.02,576.52,580.02,586.62,589.02,594.4
2,622.22,627.12,630.82,629.02,632.22,634.82,641.5
Minimumstreambedelevation,
in feet
Main channel Left-flood plain
Existingcondition
(with levee)
Hypotheticalcondition(no levee)
Existingcondition
(with levee)
Hypotheticalcondition(no levee)
Calculated water-surface elevation, in feet, for 100-yearflood discharge on indicated portion of cross section and for
indicated condition
—Reach 2 Split-Flow 1 (Map segment 1)
—Reach 3 Split-Flow 2 (Map segment 1)
—Reach 5 Split-Flow 3 (Map segment 1)
Table 6. Floodway data for the Jocko River, western Montana.
[Table 6 continued on sheets 2 and 3. Floodway determined for Reaches 1, 4, 5, 6, and 8 for existing (1999-2001) conditions. Floodway widthwas expanded to equal the width of the 100-year flood area at all cross sections in split-flow Reaches 2, 3, and 7. Vertical coordinate informationreferenced to North American Vertical Datum of 1988 (NAVD 88). Abbreviations: DS, downstream side of bridge; US, upstream side of bridge]
Reach 1 (Map Segment 1)
12
2 DS2
2 US2
3456
6 DS2
6 US2
789
9 DS2
9 US2
10111213141516
16 DS 2
16 US 2
1717.01
1,460150
140
140550
1,000250130
110
110
190350
70
70
70
350280170750820
1,090130
13 0
130
820900
1,460150
140
140400410150130
110
110
180200
70
70
70
1508090
490400740130
130
130
540750
2,518.02,519.4
2,520.2
2,520.42,521.82,523.02,524.52,528.2
2,528.1
2,528.3
2,529.22,531.52,531.9
2,533.1
2,533.5
2,535.72,539.32,553.82,556.82,559.42,563.82,565.0
2,565.4
2,565.4
2,568.72,569.3
2,518.02,519.4
2,520.2
2,520.42,521.82,523.22,524.72,528.2
2,528.1
2,528.3
2,529.22,531.52,531.9
2,533.1
2,533.5
2,535.72,539.32,554.12,557.22,559.42,563.92,565.0
2,565.4
2,565.4
2,568.72,569.3
0.0.0
.0
.0
.0
.2
.2
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.3
.4
.0
.1
.0
.0
.0
.0
.0
Reach 4 (Map segment 1)
24 430 430 2,608.3 2,608.3 .025 850 580 2,613.9 2,613.9 .026 1,000 400 2,620.5 2,620.5 .027 950 540 2,627.9 2,628.0 .1
28 100 100 2,630.7 2,630.9 .229 150 150 2,633.8 2,634.1 .330 110 110 2,637.7 2,638.0 .331 60 60 2,638.5 2,638.7 .232 125 125 2,641.2 2,641.7 .533 100 100 2,644.3 2,644.6 .334 330 330 2,650.5 2,650.8 .3
35 140 140 2,655.6 2,655.6 .036 650 310 2,661.4 2,661.5 .137 700 180 2,664.5 2,664.5 .038 1,060 870 2,671.0 2,671.4 .439 960 610 2,673.5 2,673.5 .0
1 May include small areas that are not inundated by the 100-year flood discharge and may include some areas where water is ponded inmeander channels (no effective conveyance of floodwater).2
3
Crosssection
(Mapsegments
1-3)
Width ofthe 100-yearflood area,
in feet
Calculatedwater-surface
elevation for the100-year flood
discharge,in feet
Calculatedwater-surfaceelevation withencroachment,
in feet
Increase inwater-surface
elevation,in feet
Floodwaywidth,in feet 1
1
Reach 5 (Map Segment 1)3
Reach 6 (Map segment 1)
Cross sections not shown on map segments.
Calculated water-surface elevations in Reach 5 are for main channel only.
Year
1907
1907
1908
1909
1910
1916
1948
1997
5 Data for Confederated Salish and Kootenai Tribes gaging station below the intake to K Canal(Seth V. Makepeace, Confederated Salish and Kootenai Tribes of the Flathead Nation, writtencommun., 2003).
4 Data for streamflow-gaging station 12388200 from U.S. Geological Survey (2004).
Table 1. Largest estimated and recorded annual-peak discharges for theJocko River, western Montana.
[Abbreviation: e, estimated. Symbol: --, no data]
1 Data for streamflow-gaging station 12388000 from U.S. Geological Survey (1955).2 Data for streamflow-gaging station 12384500 from U.S. Geological Survey (1955).3 Data for streamflow-gaging station 12388000 from Rantz and Riggs (1949).
Discharge of Jocko River, in cubic feet per second
At Dixon11,860
11,540
17,500e
11,540
11,590
--33,700e
42,710
Near Arlee
--
--26,200e
21,480
--21,630
22,660e
52,300
Stream reach(fig. 1)
Drainagearea,
in squaremiles
Crosssectionnumbers
100 500 May 1997
Mouth of Jocko River to Valley Creek(includes reaches 1-5 and part of reach 6)
1380
21-71 4,790 7,510 2,710
Valley Creek to Finley Creek (includes partof reach 6, reach 7, and part of reach 8) 236 72-101 4,400 6,760 2,500
Finley Creek to upstream study areaboundary (includes part of reach 8)
5157 102-112 4,090 6,170 2,300
5 Drainage area and peak discharges are the same as those determined for streamflow-gaging station Jocko River below Big Knife Creek, near Jocko(station 12384500).
1Drainage area and peak discharges are the same as those determined for streamflow-gaging station Jocko River at Dixon (station 12388200).
2 Valley Creek channel joins the Jocko River channel between cross sections 68 and 69. Flood discharges from Valley Creek mix with flood dischargeson Jocko River upstream to about cross section 71.
Table 2. Drainage-area, flood-frequency, and May 1997 peak flow data for selected stream reaches on theJocko River, western Montana.
Peak discharge, in cubic feet persecond, for indicated recurrenceinterval in years or specified date
3 3
3 3 5
4
1
44
Estimated by applying the log Pearson Type III probability distribution to the recorded and estimated peak discharges at U.S. Geological Survey andConfederated Salish and Kootenai Tribes of the Flathead Nation gaging stations.
3
Estimated based on linear interpolation between logarithms of flood discharges at Jocko River at Dixon and logarithms of flood discharges at JockoRiver at Arlee using logarithms of drainage area as the basis for interpolation (Parrett and Johnson, 2004).
4
Table 3. Estimated distribution of flood discharge between the main channel and the left part of the flood plain for split-flow reaches on theJocko River, western Montana.
Peak discharge, in cubic feet per second
100-year recurrenceinterval
500-yearrecurrence interval May 1997Stream
reach
Crosssectionnumbers Main
channelLeft-flood
plainMain
channelLeft-flood
plainMain
channelLeft-flood
plain
Reach 2—split-flow 1
Subreach 17.02
Subreach 17.03
Subreach 17.04
Reach 3—split-flow 2
Reach 5—split-flow 3
Reach 7—split-flow 4
17.02-17.04
17.02
17.03
17.04
18-23
28-34
92-93
Varies 1
2,910
3,710
3,890
3,890
4,390
2,940
Varies1
1,880
1,080
900
900
400
1,460
Varies1
2,910
3,710
4,410
5,070
6,590
3,300
Varies1
4,600
3,800
3,100
2,440
920
3,460
2,710
2,710
2,710
2,710
2,710
2,710
2,380
0
0
0
0
0
0
120
1Distribution of discharge between the main channel and left-flood plain changes throughout the reach.
Maparea
M O N T A N A
Hydrologic AnalysisThe annual-peak discharge that has a 1-percent chance of
exceedance in any year was selected by the CSKT for analysis to beconsistent with other flood studies in Montana. This peak discharge isexceeded, on average, once in any 100-year period and commonly isreferred to as the 100-year flood discharge. However, larger floods canoccur at shorter intervals. To indicate the flood limits associated with
an even rarer flood discharge than the 100-year flood, the CSKT alsoselected the 500-year flood discharge (0.2-percent chance ofexceedance) for analysis.
The 100- and 500-year flood discharges for the Jocko River atDixon were estimated by applying the log Pearson Type III probabilitydistribution to the recorded and estimated annual-peak discharges forthe period of record from 1907-1910, 1948, and 1986 to 2001 usingmethods described by the Interagency Advisory Committee on WaterData (1982). The peak discharges for 1908 (7,500 ft3/s) and 1948(3,700 ft3/s) are considered to be the largest during the entire periodfrom 1907 to 2001; thus, these values were treated as historic peakdischarges (largest peak discharges in 94 years) as described by theInteragency Advisory Committee on Water Data (1982).
The 100- and 500-year flood discharges for Jocko River nearArlee were estimated by applying the log Pearson Type III probabilitydistribution to the recorded and estimated peak discharges at the USGSand CSKT gaging stations for the period of record from 1908, 1916,1948, and 1990 to 2001. The peak discharges for 1908 (6,200 ft3/s) and1948 (2,660 ft3/s) also were treated as historic peak discharges (largestpeak discharges in 94 years).
Flood discharges on the Jocko River change between Dixonand Arlee as Valley Creek and Finley Creek join the river. From themouth of the Jocko River to Valley Creek (fig. 1), the 100- and 500-year flood discharges were assumed to be the same as those determinedfrom the gaged data at Jocko River at Dixon. Upstream from ValleyCreek to Finley Creek, the 100-year and 500-year flood dischargeswere estimated based on a linear interpolation between logarithms offlood discharges at Jocko River at Dixon and logarithms of flooddischarges at Jocko River near Arlee using logarithms of drainage areaas the basis for interpolation (Parrett and Johnson, 2004). The 100- and500-year flood discharges upstream from Finley Creek to the upstreamlimits of the study area were assumed to be the same as thosedetermined from the gaged data near Arlee. Drainage area and selectedflood-frequency data for the Jocko River are summarized in table 2(sheet 1). Recorded and estimated peak-flow discharges for May 18,1997, also are included in table 2.
Hydraulic Analysis
MethodsHEC-RAS, version 3.1.1, a one dimensional, hydraulic-flow
model developed by the U.S. Army Corps of Engineers (2001a,b,c),was used to calculate water-surface elevations at cross sections for the100-year and 500-year flood discharges and for the 1997 peakdischarge. Channel and flood-plain geometry for 113 cross sectionswere surveyed between 1999 and 2001 for use in the model. Bridgegeometries at 9 of the 113 sections located just downstream frombridge crossings were surveyed. Data from surveyed cross sectionsdownstream from the bridges were used to estimate channel and flood-plain geometry at the upstream and downstream edges of the bridgesfor use in the HEC-RAS model. For clarity, only the surveyed crosssection immediately downstream from each bridge is shown on the mapsegments. Data from interpolated cross sections were used at 15 morelocations where the hydraulic-model results indicated that additionalchannel and flood-plain geometry were required. For clarity, only 5 ofthe 15 interpolated sections are shown on the map segments. Overall, atotal of 146 cross sections (113 surveyed cross sections, 15 interpolatedcross sections, and 18 cross sections at the upstream and downstreambridge edges) were used for hydraulic analyses. Data for the crosssections are available in files at the USGS Montana Water ScienceCenter office in Helena, Mont.
Cross sections are numbered in increasing order upstream.The interpolated cross sections are designated on the maps and tableswith a whole number and two digits following the decimal point. Thewhole number corresponds to that of the closest downstream surveyedcross section, and the digits after the decimal point are numbered inupstream order. For example, four cross sections were interpolatedbetween cross sections 17 and 18 (Map segment 1). The cross sectionfarthest downstream was designated as 17.01, while the cross sectionfarthest upstream was designated as 17.04. To describe the hydraulicanalysis, the Jocko River was divided into eight reaches (fig. 1). Allstudy reaches, except the first, begin halfway between the first crosssection in the reach and the next downstream cross section outside the
reach. Likewise, all study reaches, except the last, end halfwaybetween the last cross section in the reach and the next upstream crosssection outside the reach. The first study reach begins at the first(farthest downstream) cross section in the study area, and the last reachends at the last cross section (farthest upstream) in the study area.Study reaches are described in upstream order in subsequent sections ofthe report.
Manning’s roughness coefficients (Henderson, 1966) werenoted in the field at the time of the survey. The roughness coefficientswere subsequently checked and adjusted for consistency during thehydraulic analysis. The elevations of high-water marks, such as debrisdeposits, from the 1997 and earlier floods were surveyed where markscould be found. These high-water mark elevations were compared withwater-surface elevations initially calculated by the model. Thecomparisons were used to verify the roughness coefficients, whichranged from 0.035 to 0.130 for the flood plain and from 0.035 to 0.050for the main channel. Both ranges are similar to values for Manning’sroughness coefficients that were calculated from measured dischargeand channel-geometry and flood-plain data for similar streams (Barnes,1967; Arcement and Schneider, 1989; and Coon, 1998).
Field surveys and elevations were referenced to NationalGeodetic Survey bench marks along the Jocko River valley (Mapsegments 1-3). Information about the bench marks can be obtainedfrom the National Geodetic Survey at http://www.ngs.noaa.gov(accessed July 2005). Additional elevation-control points wereobtained from the CSKT (Map segments 1-3).
Split-Flow EstimationAt many locations along the Jocko River, the 100- and 500-
year flood discharges can overtop channel banks. At most of theselocations, water on the flood plain flows in the same direction as that inthe channel, and the water-surface elevation is the same on both sides ofthe flood plain, as measured on a line perpendicular to the flowdirection. However, along Reaches 2, 3, 5, and 7 (fig. 1), floodwater onthe left-flood plain (left and right defined by looking downstream; forexample, the left bank is on the south or western side of the JockoRiver) can move independently from floodwater in the main channel fora substantial distance along the channel, creating a split-flow condition.Consequently, the water-surface elevations can differ substantially fromone side of the flood plain to the other. In these split-flow reaches,discharge in the main channel and discharge on the left-flood plainwere analyzed separately.
The methods used to estimate the distribution of flooddischarges in the split-flow reaches (table 3, sheet 1) are described inthe following sections. In Reaches 2, 3, and 5, the split-flow conditionswere, at least partly, the result of berms and levees on the left bank(Map segment 1). Although berms and levees are present in all reachesof the Jocko River, only in Reaches 2, 3, and 5 are they long enoughand high enough to create split-flow conditions. The berms and levees
section in the reach and the next downstream cross section outside thereach. Likewise, all study reaches, except the last, end halfway
occur at shorter intervals. To indicate the flood limits associated withan even rarer flood discharge than the 100-year flood, the CSKT also
Water-Surface Elevations andBoundaries of the 100-Year and500-Year Floods and Floodwayfor the Jocko River, WesternMontanaBy Katherine J. Chase and Charles Parrett
IntroductionThe Jocko River flows across the Flathead Indian Reservation
in western Montana and provides irrigation water for several thousandacres of hay and grain that are produced throughout the Jocko Rivervalley. The Jocko River also is an important trout fishery and atributary to the Flathead River. Much of the agricultural land along theJocko River is being considered for residential and commercialdevelopment. This development has the potential to affect the JockoRiver and the fishery. In addition, proposed levee modifications andstream-restoration work could further affect the Jocko River channeland its aquatic habitat.
The Confederated Salish and Kootenai Tribes of the FlatheadNation (CSKT) needed information for assessing the effects ofproposed development and stream-restoration work on the Jocko Riverchannel and fishery. The potential for increased development andstream-restoration work highlighted the need for a better understandingof the boundaries of the 100-year and 500-year floods. To that end, theU.S. Geological Survey (USGS), in cooperation with CSKT, conductedhydrologic and hydraulic analyses of the Jocko River to identify areasalong the river that are subject to flooding.
This report presents water-surface elevation data for flooddischarges having recurrence intervals of 100 and 500 years and depictsmapped flood and floodway boundaries. The flood boundaries showthe extent of flooding under existing (1999-2001) conditions from flooddischarges having recurrence intervals of 100 and 500 years. Themapped floodway boundaries show the area within the boundaries ofthe 100-year flood discharge that is reserved for the passage of floodflows under Montana Administrative Rule 36.15.502 (1995). Thehydrologic analysis is based on recorded annual-peak discharge data forvarious years from 1907 to 1998 at four streamflow-gaging stationswithin or near the study area that extends from the fish hatchery nearArlee to the mouth of the Jocko River near Dixon. The hydraulicanalysis is based on channel- and bridge-geometry data collectedbetween 1999 and 2001.
Description of Study AreaThe Jocko River and its major tributaries, Finley Creek, Spring
Creek, and Valley Creek, are perennial streams. The Jocko Riveroriginates in the Mission Mountains east and north of Arlee and flowsthrough a relatively narrow valley corridor in Lake and SandersCounties before joining the Flathead River near Dixon (fig. 1, sheet 1).The Jocko River valley is bounded by the Mission Range on the eastand mountains along the Reservation Divide on the south and west.Elevations in the basin range from more than 7,700 ft (NGVD 29) inthe mountains to 2,800 ft (NGVD 29) along the valley floor (Kendy andTresch, 1996).
The river channel is incised in some reaches and braidedacross the valley floor in other reaches within the study area. Severalabandoned meander channels form depressions along the river in thebraided reaches. In addition, irrigation-canal berms and flood-protection levees have been constructed on one or both sides of the riveralong much of the study area. The study area was divided into eightreaches, based on hydraulic conditions and the influence of the bermsand levees (fig. 1).
The climate in the Jocko River basin is characterized by warmsummers, cold winters, and moderate precipitation. Based on climate
data from 1896 to 2003 at St. Ignatius, about 5 mi northeast of Ravalli,mean monthly temperatures range from a minimum of 17.1°F inJanuary to a maximum of 84.3°F in July (Western Regional ClimateCenter, 2003a). The average annual precipitation at St. Ignatius is 15.915.9 in. However, the average annual precipitation increasesdramatically with altitude in the basin and probably reaches a value ofabout 80 in. in the Mission Mountains (Western Regional ClimateCenter, 2003b).
Rangeland and irrigated hay and grain production are thepredominant land uses throughout the study area. The Flathead AgencyIrrigation Division manages an elaborate system of canals that divertand distribute water from the Jocko River and its tributaries (Kendy andTresch, 1996). Only the largest canal (the K Canal) is shown on figure1. The only incorporated towns in the valley are Arlee (population 602)and Dixon (population 216) (U.S. Census Bureau, 2005).
Flood Conditions and HistoryMost flood runoff in the Jocko River basin results from spring
snowmelt, spring snowmelt mixed with rain, or spring rains. Recordedflood-discharge data for the Jocko River are available for sites nearArlee, Ravalli, and Dixon.
Flood-discharge data are available for two streamflow-gagingstations in the study area and for two located upstream from the studyarea. A streamflow-gaging station on the Jocko River at Dixon (fig. 1,station 12388200) has been operated by the USGS since 1990. TheCSKT operated a peak-discharge gaging station (crest-stage gage) atthe same location from 1986 to 1989. The USGS also operated astreamflow-gaging station at Jocko River at Ravalli (station 12388000)from 1906-1914. Annual-peak discharge data for this site are available
from 1907-1910 and 1948 and are considered to be equivalent to peak-discharge data at the current site at Dixon. Annual-peak discharge datafor stations 12388200, 12388000, and the CSKT crest-stage gage werecombined for analysis and the combined data will hereafter be referredto as annual-peak discharges for the Jocko River at Dixon.
Upstream from the study area, the CSKT have operated astreamflow-gaging station on the Jocko River below the intake to KCanal near Arlee since 1990 (fig. 1). Also upstream from the studyarea, the USGS operated a streamflow-gaging station, Jocko Riverbelow Big Knife Creek, near Jocko (station 12384500), from 1909-1916. Annual-peak discharge data for 1909, 1916, and 1948 areavailable for this station. Annual-peak discharge data for station12384500 are considered to be equivalent to those at the CSKT sitenear Arlee. Annual-peak discharge data for station 12384500 and theCSKT site below the intake to K Canal also were combined for analysisand the combined data will hereinafter be referred to as annual-peakdischarges for the Jocko River near Arlee.
Large floods on the Jocko River are known to have occurred in1908, 1948, 1964, and 1997. The largest known peak discharge on theJocko River was estimated to be 7,500 ft3/s in 1908 at Dixon (table 1,sheet 1). Peak discharge upstream near Arlee on the same date wasestimated to be 6,200 ft3/s. During the 1948 flood, peak discharges atthe same sites at Dixon and near Arlee were estimated to be 3,700 ft3/sand 2,660 ft3/s, respectively. Although peak-discharge data are notavailable for the Jocko River for 1964, the flood of June 7-9, 1964, isremembered by long-time residents in the area as one of the largest(Seth V. Makepeace, Confederated Salish and Kootenai Tribes of theFlathead Nation, written commun., 2005). The most recent flood onthe Jocko River produced a peak discharge of 2,710 ft3/s (the station atDixon, station 12388200) on May 18, 1997. The peak discharge on thisdate upstream near Arlee was 2,300 ft3/s.
summers, cold winters, and moderate precipitation. Based on climatedata from 1896 to 2003 at St. Ignatius, about 5 mi northeast of Ravalli,
from 1907-1910 and 1948 and are considered to be equivalent to peak-
U.S. GEOLOGICAL SURVEY STREAMFLOW-GAGING STATION AND NUMBER12388200