Study Guide For EESA05, Chapter 7

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Environmental Science
Ingrid L.Stefanovic

Chapter 7: River Flooding permeability of clay is low; rain and floodwater do not seep into ground as they would in other settings instead water accumulates on the land surface isostatic depression= centre of loading 7.1 An introduction to Rivers water evaporates from Earths surface (from oceans); it exists as a gas in the atms; it precipitates on oceans and lands. Some of the water that falls on land as rain or snow infiltrates into the ground or returns to the atms thru evaporation and transpiration by plants. Much of the water that falls on land, returns to the ocean via surface flow along paths determined by local topography Surface flow, referred to as runoff, finds it way to small streams, which are tributaries of larger streams or rivers Stream: any body of water that flows in a channel The region drained by a single stream is called drainage basin, watershed, river basin or catchment One characteristic of a river is the slope of the surface over which it flows, or its gradient. The gradient of the river is determined by calculating the drop in elevation of the channel over some horizontal distance and is commonly expressed in m/km or as dimensionless number (elevation drop in m divided by horizontal distance in m) Gradient of a river is greatest in its headwaters, decreases downstream, and is lowest at the river mouth, which is its base level Base level of a river is the lowest elevation to which it may erode. Generally, this elevation is at or near sea level, although a river may have a temporary base level, such as a lake A graph showing downstream changes in a rivers elevation is called a longitudinal profile The valley of a river is steeper-sided and narrower in its headwaters than near the river mouth, where a wide floodplain may be present At higher elevations, the steeper gradient of the river facilitates erosion and downstream transfer of sediment Many valleys in glaciated mountains have a U-shaped cross-sectional profilethe eqm form that results from lengthy erosion by valley glaciers Valleys in non-glaciated mt landscapes have V shaped cross-section produced by fluvial erosion and mass wasting Material Transported by Rivers material that it transport is called total load (TL), and consists of bed load, suspended load, and dissolved load bed load comprises particles of sand and gravel that slide, roll and bounce along the channel bottom in rapidly moving waterconstitutes less than 10% of TL suspended load comprises mainly silt and clay particles carried in suspension above the riverbed90% of TL; gives muddy appearance dissolved load comprises electrically charged atoms or molecules, called ions, that are carried in solution in the watermost derived from chemical weathering of rock and sediment in the drainage basin o ions in discharging underground springs, sewage and chemical effluence can be a significant part of the dissolved load of some rivers River Velocity, Discharge, Erosion and Sediment Deposition velocity of water changes along the length of a river, affecting channel characteristics and both erosion and sediment deposition hydrologists combine measurements of flow velocity and cross sectional area of the flow (A) to determine discharge (Q), a more useful indicator of stream flow than velocity alone discharge is the volume of water moving through a cross-section of a river per unit of time and is reported in units of cubic meters/secit is calculated by multiplying the cross-sectional area of the water in the channel by the flow velocity flow velocity and cross-sectional area are related o if cross sectional area decreases, the velocity of water must increase for discharge to remain constant the gradient of a river decreases where it flows from mts onto a plain or into an ocean or lake; in these places, river builds a fan-shaped body of sediment on land referred to as an alluvial fan, or a triangular or irregular shaped deposit in water called a delta flood hazard on alluvial fans and deltas is different from that on the floodplain of a river at the head, or apex of an alluvial fan or delta, the river commonly enters a system of distributary channels that carry parts of the discharge to different parts of the fan or delta. These channels may change position rapidly during a single flood or from one flood to the next, creating hazard that is difficult to predict Channel Patterns and Floodplain formation most features of rivers result from the interaction of flowing water and moving sediment three common channel patterns are: (1) braided: with a large number of intersecting active channels; these have unvegetated sand and gravel bars that divide and reunite the main channel, especially during low flow; these tend to be wide and shallow compared with meandering channels. A river is likely to have a braided pattern if it has an abundant coarse bed load and large diurnal variations in discharge. These conditions are found in areas where the land is rising because of tectonic processes and where rivers receive abundant water and sediment from glaciers (2) anastomosing: with two or more channels and intervening stable islands or bars (3) meandering: with a single channel shaped like a snake; meanders migrate back and forth across the floodplain over years or centuries; water moves faster along the outsi
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