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FLUVIAL LANDFORMS

Floodplains

• fairly flat & continuous surface occupying much of a valley

bottom

• normally underlain by unconsolidated sediments

• subject to periodic flooding (usually once every year or so)

• surface & sediments somehow relate to activity of present

channel

• exerts influence on basin hydrology through lag

• serves as sediment storage area

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Floodplain deposits consist of

channel fill: poorly sorted silt, sand & gravel

channel lag: coarse materials, fines winnowed

splay: breaks in natural levees; coarser than overbank sediments

colluvium: near valley sides, slope wash & mass movements

lateral accretion: sands & gravels in point bars

vertical accretion: silts & clays

Meander scrolls form on floodplain of meandering rivers through

lateral migration of bends – leave ridges & swales

Cutoffs leave oxbow lakes that become clay plugs

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Floodplains are built primarily by

lateral accretion: building point bars up to the floodplain level &

then shifting

overbank flow & vertical accretion: braided river floodplain is

less thick & regular

Relative importance of vertical vs lateral accretion varies –

vertical is more important where there is frequent flooding &

abundant fines

In an aggrading river, floodplain sedimentation may exceed the

depth to which the river can scour; the sediment is then no

longer part of the active floodplain

In a degrading channel, the floodplain becomes a terrace when

incision prevents the river from flooding the surface every

couple of years

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Floodplain during dry season, northern Australia

Rio Amazonas in flood

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Fluvial terraces

abandoned floodplain

consist of tread and riser (scarp)

can be classified as

erosional/depositional

strath/alluvial (fill)

paired/unpaired

tectonic/climatic

Difficulties of terrace interpretation (eg. from Cody, Wyoming)

Rio Mira, Ecuador

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How do terraces form?

• period of stability and lateral incision

Big Creek, CA southern Israel

• filling and incision

glacial outwash

climate change increases Qs and/or decreases Qw

rise in baselevel due to sealevel increase

tectonic uplift at source & increase in coarse sediments

vegetation clearing

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northern California

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Green River,Utah

Narrows Picnic Area, PoudreRiver, CO

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Alluvial Fans and Pediments

depositional & erosional features at the base of mountains

mountain

piedmont (alluvial fans, bajadas,

pediments, talus slopes)

basin (playa, floodplain)

Piedmont consists mainly of fans & pediments (eroded bedrock

plains)

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Alluvial fans

fan-shaped in plan view & convex in cross-profile

caused by deposition when rivers leave confined channels

location of deposition shifts across fan surface with time, both

laterally & outward from mountain

Death Valley, CA

Adjacent fans coalesce to form bajadas/alluvial aprons

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Fans gradually flatten toward the toe – the steepest areas occur

in the upper fan where coarse sediments, low discharge, &

sediment transport by mass movements (debris flows) occur:

fan profiles tend to be segmented, rather than smooth

Bajada (alluvial apron), w US

The area of the fan is related to the source area

c = f(climate, lithology, tectonism)

Af = c Ad n n = slope of regression line in log plot

Sieve deposits: poorly sorted, lobate form

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Annapurna region, Nepal

Sinai, Egypt

Coastal Peru

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Annapurna region, Nepal

Pleasant Valley, NV

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Quito, Ecuador

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Pediments

erosional bedrock surfaces, often with veneer of sedimentsgenerally fan-shaped in plan view

1 km2 to hundreds of km2 in size

convex or concave across the pediment

longitudinal profile is slightly concave

approximately 2.5° slope

dissected by incised channels & dotted with inselbergs

(residual bedrock knobs)

surface & subsurface weathering, and sheetflow & lateral

cutting by channels are all important

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If pediments erode headward, it could be by

a) lateral planation: arid region rivers with coarse loads migrate

& erode laterally

Pediment,

Mohave Desert, CA

b) parallel retreat: mountain front achieves equilibrium slope,

weathering & erosion maintain slope, and surface

retreats parallel to itself

c) drainage basin hypothesis: lateral planation dominant along

main drainage line, parallel retreat along interfluves

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Henry Mountains, Utah

(pediment first described by GK Gilbert)

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Deltas

Depositional features where river enters a local (lake) or

ultimate (ocean) base level

At the apex of the delta, the river divides into distributaries –

radiating branches that deliver sediment to the extremities

Deposition occurs because of a velocity decrease as the river

enters a body of standing water

Delta form & properties represent adjustment between

fluvial system (Qw , Qs , S, v, w, d)

climate

tectonics

shoreline dynamics

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Basic delta types are

high constructive: fluvial action dominates, high sediment input

relative to marine dynamics; elongate (more mud) &

lobate (more sand)

high destructive: ocean or lake energy high, & fluvial sediments

are reworked

wave-dominated: sediments accumulate as arcuate

sand bars

tide-dominated: sediments linear

The whole delta generally shifts with time

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Estuary, n California

Colorado River deltaCook Inlet, coastal Alaska

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The interaction of river water with standing water depends on the

relative densities of the two:

• hyperpycnal flow: inflowing water is denser due to colder

temperature or higher sediment concentration – 2d plane jet

flow occurs as turbidity current moves along basin floor

• homopycnal flow: density of inflowing & standing water are

equal – 3d axial jet flow, complete mixing close to river

mouth, common in freshwater lake deltas

• hypopycnal flow: ocean water is more dense, mixing is slow,

river water spreads laterally in plane jet flow

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Types of deltas (Ritter et al., 1988)

High-constructive

deltas

High-destructive

deltas

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Lena River delta, Siberia

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Mississippi River deltas (Schumm, 1977)