A floodplain is the flat land immediately

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A floodplain is the flat land immediately
surrounding a stream channel and innundated at
times of high flow.
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Aggradation occurs when deposition is greater
than erosion.
Aggradation of the Rivière des Ha! Ha! in Quebec
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Incision occurs when erosion is greater than
deposition.
Broadstreet Hollow Stream, NY
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• Floodplain sediments derive from deposition of
  overbank suspended sediment or by deposition of
  bedload as the channel migrates across its
  valley

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Levees

• The boundary between channel and floodplain may
  be the site of a natural levee (a broad, low
  ridge of alluvium built along the side of a
  channel by debris-laden floodwater)

Levees form when debris-laden floodwater
overflows the channel and slows as it moves onto
the floodplain.
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Flooding Sedimentation
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Levee Deposits
Coarser sediment
Flood stage
Finer sediment
Finer sediment
The area adjacent to and outside of the channel
serves as an overflow area for excess water and
sediment
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meander review!

• higher velocities on outside of bends lead to
  cutbank erosion
• lower velocities on insides of bends lead to
  point bar deposition

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Point Bar Deposits
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meandering
oxbows (sloughs)

• meander belts

songhua r., china
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Meandering stream flowing from top of screen to
bottom
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Maximum deposition
Maximum erosion
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Meander scars (scroll bars)
Oxbow Lake
cutoff
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The river cuts downward to form a V-shaped
valley.
The river starts to meander
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River cuts from side to side eating into the
valley walls
Floodplain starts to form
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Floodplain continues forming, increased sinuosity
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Floodplain
Alluvial deposits
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Floodplain landforms

• oxbow
• splay
• scrolls
• leveee
• backswamp

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Oxbow lake near the Chippewa River, Eau Claire,
Wisconsin
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Splay

• A deposit of coarse material resulting from a
  levee breach during a flood.

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Meander scroll

• A meander scroll consists of long, curving,
  parallel ridges (scroll bars) that are deposited
  during point bar growth

songhua r., china
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Side looking radar (SLAR) image of floodplain of
an Amazon River tributary in 1971/2 flow is
toward lower right.
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floodplain landforms

• levee coarse sediment deposited near the channel
  during overbank flow
• backswamp low-lying area separated from channel
  by natural levee

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Alluvial Fans

• when streams encounter a sudden change in
  gradient and confinement, e.g. leaving mountains,
  they often deposit alluvial fans
• why? transport capacity suddenly decreases

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• Alluvial fans tend to be coarse-grained at their
  head. At their edges, however, they can be
  relatively fine-grained.

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alluvial fans

• bajada
• coalescing alluvial fans
• death valley

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river terraces
Many stream valleys contain one or more
relatively flat alluvial terraces that lie above
the floodplain.
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River Terraces

• streams may create depositional landforms
  (especially floodplains) and then start to incise
• terraces are abandoned floodplains

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River Terraces
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River Terraces

• Changes in whether a stream is eroding or
  depositing in a given location can be caused by
  several different phenomena eg
• tectonic uplift
• change in base level downstream
• change in climate modifying systems discharge
• which brings us to river response

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River Response
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river response case studies

• skokomish river, WA
• mount pinatubo, philipines
• river restoration engineered log jams (ELJ)

Skokomish River, Washington Response to
intensive upland forestry in steep
landslide-prone terrain
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skokomish river - response

• increased sediment input from landsliding in
  headwater channels
• aggradation of channel bed in main stem
• increased flooding due to decreased amount of
  water channel could hold
• NOT by in creasing peak or mean flows

climate
catchment vegetation
riparian veg.
sediment wood inputs
fast
hydrology
flow obstructions
channel morphology
rate of change
geology
gradient
valley form
slow
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river response case 2
Mount Pinatubo, Philipines Response to massive
volcanic disturbance
6 km3 of pyroclastic materials erupted and
subsequently deposited on flanks of
volcano Abundant fine-grained sediment available
for transport.
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Porac River before eruption
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Pasig-Potrero River after eruption
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Antidunes
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High-Flow Roll Waves Pasig-Potrero River
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High-Flow Roll Waves ODonnell River
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erosion of lahar depoisits
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eruption consequences

• large amount of new sediment available for
  transport
• ashfall deposits
• lahar terraces
• the river bed
• radical geomorphic changes
• decreased grainsize in rivers (cobble/boulder ?
  sand/pebble)
• switch to braided channels (single channel ?
  multi-thread)

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river response

• exponential decay of post-eruption sediment yield

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river response

• Pinatubo eruption effects were generally natural
  phenomena
• BUT, Skokomish changes were not
• what can be done about it?
• river restoration
• the science of properly restoring (as much as
  possible) rivers to their natural state
• the hardest part
• what was the natural state?