A floodplain is the flat land immediately surrounding a stream channel and innundated at 5mes of high flow.

A floodplain is the flat land immediately surrounding a stream channel and innundated at 5mes of high flow.

Aggrada5on occurs when deposi5on is greater than erosion. Aggrada&on of the Rivière des Ha! Ha! in Quebec

Incision occurs when erosion is greater than deposi5on. Broadstreet Hollow Stream, NY

Floodplain sediments form by either: (1) deposi&on of overbank suspended sediment, or (2) deposi&on of bedload as the channel migrates across its valley

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.

Flooding & Sedimenta&on

Levee Deposits Flood stage Coarser sediment Finer sediment Finer sediment The area adjacent to and outside of the channel serves as an overflow area for excess water and sediment

meander review! higher veloci&es on outside of bends lead to cutbank erosion lower veloci&es on insides of bends lead to point bar deposi&on

Point Bar Deposits

meandering oxbows (sloughs) meander belts songhua r., china

Meandering stream flowing from top of screen to borom

Maximum deposition Maximum erosion

Meander scars (scroll bars) Oxbow Lake cutoff

The river cuts downward to form a V shaped valley. The river starts to meander

River cuts from side to side eating into the valley walls Floodplain starts to form

Floodplain continues forming, increased sinuosity

Floodplain Alluvial deposits

Floodplain landforms oxbow splay scrolls leveee backswamp

Oxbow lake near the Chippewa River, Eau Claire, Wisconsin

Splay A deposit of coarse material resul&ng from a levee breach during a flood.

Meander scroll A meander scroll consists of long, curving, parallel ridges (scroll bars) that are deposited during point bar growth songhua r., china

Side looking radar (SLAR) image of floodplain of an Amazon River tributary in 1971/2; flow is toward lower right.

floodplain landforms levee: coarse sediment deposited near the channel during overbank flow backswamp: low lying area separated from channel by natural levee

Alluvial Fans when streams encounter a sudden change in gradient and confinement, e.g. leaving mountains, they o^en deposit alluvial fans why? transport capacity suddenly decreases

Alluvial fans tend to be coarse grained at their head. At their edges, however, they can be rela5vely fine grained.

alluvial fans bajada: coalescing alluvial fans death valley

river terraces Many stream valleys contain one or more rela&vely flat alluvial terraces that lie above the floodplain.

River Terraces streams may create deposi&onal landforms (especially floodplains) and then start to incise terraces are abandoned floodplains

River Terraces

River Terraces Changes in whether a stream is eroding or deposi&ng in a given loca&on can be caused by several different phenomena: tectonic upli^ change in base level downstream change in climate modifying system s discharge which brings us to river response

River Response

climate hydrology geology catchment vegeta&on sedimen t& wood inputs channel morphology gradient flow obstruc&ons valley form riparian veg. fast slow rate of change

River Response: Case Studies (1) Skokomish river, WA (2) Mount pinatubo, philipines Skokomish River, Washington Response to intensive upland forestry in steep landslide prone terrain

Skokomish River Response 1. increased sediment input from landsliding in headwater channels 2. aggrada&on of channel bed in main stem 3. increased flooding due to decreased amount of water channel could hold NOT by in creasing peak or mean flows climate hydrology geology catchment vegeta&on sediment& wood inputs channel morphology gradient flow obstruc&ons valley form riparian veg. fast slow rate of change

River Response: Case 2 Mount Pinatubo, Philipines Response to massive volcanic disturbance ~6 km 3 of pyroclastic materials erupted and subsequently deposited on flanks of volcano Abundant fine-grained sediment available for transport.

Porac River before eruption

Pasig-Potrero River after eruption

Antidunes

High-Flow Roll Waves Pasig-Potrero River

High-Flow Roll Waves O Donnell River

erosion of lahar depoisits

Erup&on Consequences large amount of new sediment available for transport ashfall deposits lahar terraces the river bed radical geomorphic changes: decreased grain size in rivers (cobble/boulder sand/pebble) switch to braided channels (single channel mul& thread)

River Response exponen&al decay of post erup&on sediment yield

River Response Pinatubo erup&on effects were generally natural phenomena BUT, Skokomish changes were not: what can be done about it? river restora&on: the science of properly restoring (as much as possible) rivers to their natural state the hardest part: what was the natural state?

climate hydrology geology catchment vegeta&on sedimen t& wood inputs channel morphology gradient flow obstruc&ons valley form riparian veg. fast slow rate of change