Landform Geography Landforms of the Fluvial System

Stream Gradation Streams evolve to carry just the amount of sediment produced by their drainage basin

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

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

Stream Meandering

Oxbow Lake

Floodplain Features

Drainage Basin Changes

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

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

Chattahoochee and Buford 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

Overland Flow Perennial Streams – water runs all year Ephemeral Streams – water runs only part of year Sources of stream water: Groundwater Melting Ice Surface Runoff

Drainage Basins Drainage Basin – area contributing groundwater and runoff to a stream Drainage basins are divided from each other by topographic barriers called watersheds Drainage Divide – elevated terrain forming rim around a drainage basin Drainage basins vary tremendously in size Basins are nested, smaller within larger

Small, Nested Drainage Basins

Common Drainage Pattern Types

Major US Watersheds

Drainage Density Total length of all streams Drainage Density = Area of drainage basin

Stream Ordering Represents stream size Smallest streams in basin order 1 – order rises when 2 streams of same order come together at confluence

Hydraulic Geometry Geometric attributes of river channels Variables: w = channel width d = channel depth v = velocity of water s = slope (steepness) also called gradient Q = discharge (amt of water flowing) Q = w x d x v (units m 3 /s or ft 3 /s)

Hydraulic Variables

Stream Hydrograph Graph showing fluctuation in stream discharge over time Lag between storm event and highest discharge Base flow – flow rate sustained by groundwater influx

Flooding Flood Stage – stream discharge increases so that water spills out of channel onto adjoining ground Return Period – time between events of a given magnitude, e.g. annual flood, 50-year flood, 100-year flood Larger floods occur less frequently

Mississippi River Floods At least 100-year flood, perhaps a 500-year flood Heavy winter rains saturated ground Stationary high pressure in Southeast in summer, blocking mid-latitude jet stream over Midwest Cool, dry air collided with warm, moist air along jet stream, creating constant precipitation Precipitation ran off into stream channels & rivers

Mississippi River Floods

2009 Carrollton Floods

Fluvial Processes and Landforms Running water is most important geomorphic (landform shaping) process on Earth’s surface All landforms due to either erosion or deposition Erosional landforms occur when sediment, soil, or rock is stripped away from land Depositional landforms occur where sediment accumulates after being dropped

Depositional vs. Erosional Landforms

Hillslopes Most active zones of fluvial erosion due to high relief creating fast-moving, powerful water Rills Gully

Ravine Canyon