Physical Geography by Alan Arbogast Chapter 16 Fluvial Systems and Landforms Lawrence McGlinn Department of Geography State University of New York - New Paltz

Stream Gradation Streams evolve to carry just the amount of sediment produced by their drainage basin 3 ways streams carry sediment: –Dissolved load – mineral ions in solution –Suspended load – sediment held up in stream by turbulence of water –Bed load – larger particles that roll, slide or bounce along channel bed – Saltation – most common in mountain streams

Types of Sediment Load

Meandering Stream Stream carrying mostly suspended load usually has deep channel curving side-to- side

Braided Stream Streams with mostly bed load will have braided appearance – wide, shallow channel

Graded Stream A stream with just enough discharge to carry its average sediment load – at equilibrium Aggradation – deposition on the bed of the channel due to added load or decreased discharge Degradation – erosion of channel bed due to increased discharge or decreased load The Graded Stream

Evolution of a Graded Stream Base Level – lowest level at which a stream can erode its channel bed – can rise & fall with changes in sea or land level

Niagara Falls Upstream (Lake Erie) 571 ft elev Downstream (Lake Ontario) 243 ft elev Has retreated almost 7 miles in past 12,000 yrs

Development of Graded Stream with Wide Floodplain

Stream Meandering Graded streams may erode horizontally in process called meandering Cutbanks form on outside of meanders (fast), while point bars form on the inside (slow) Stream Meandering

Oxbow Lake Abandoned stream channel cut off from rest of stream by meandering process Oxbow Lakes & Former meanders on Mississippi R.

Floodplain Features Natural Levee – small ridge 1-2 m high – formed by deposit of coarse sediments during flood Backswamp – low lying - poorly drained – small particles

Drainage Basin Changes Steep drop in base level – Meandering stream downcuts, creating Entrenched Meanders San Juan River, Utah Small drop in base level, or climate change – old floodplain becomes alluvial terrace

Alluvial Fans Created by alluvial aggradation in areas of high relief where bedload-dominated streams flow out of mts onto plain – stream sweeps side to side over time, making fan shape – most common in desert climates

Deltas River water slows as it enters ocean, depositing large amts of sediment in triangular pattern called delta – smaller particles farther out in delta Nile River Delta, Egypt Mississippi River Delta, Louisiana

Mississippi River

Human Interaction with Streams Urbanization –More impervious surfaces –After heavy rain, shorter lag time, higher peak flow – more serious flooding

Dams and Reservoirs Dam – engineered obstruction across a river to control its flow – usu. holds back lake or reservoir Why dams? –1. Hydroelectric energy –2. Flood control –3. Enhance river navigation –4. Surface water supply Hoover Dam

Negatives of Dams Expense Refugees from areas flooded by dam Loss of ecosystems & scenery No floods/No soil replenishment in floodplain Stop spawning fish from going upstream