GEO 155 1st Edition Lecture 20 Last Lecture Hydrology and Hydrographs Outline of Current Lecture I Steams as Geomorphic Agents a Work That Streams Do i Sediment Transport ii Degradation iii Aggradation b Equilibrium and Change i Small Scale Channel Form ii Large Scale Causes of Degradation and Aggradation c Slides Current Lecture Fluvial Geomorphology These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute How rives shape land forms Sediment Transport 1 Bed load a Sediment is too large to be picked up by the stream so it moves along the bottom of the river 2 Suspended load a Particles of rock are carried by the stream but they are small enough to remain suspended i The movement of various sizes of the particles is dependent on size and energy of the water body 3 Dissolved load a Rock has been dissolved and carried as solution Degradation Valleys formed when streams wear away a landscape downward Gradual Aggradation River deposits alluvium in a valley which raises the landscape Valley have flattened bottoms Alluvium is lose and easy to erode Stream Terraces Form when flat shells along a valley are revealed o Often times there are layers of degradation and aggradation o Shows the history of degradation and aggradation Equilibrium Stream has exactly enough energy to transport the sediment being supplied to it without eroding or depositing any of it o Sediment brought from Upstream The slopes on either side of the river 1 When extra sediment is added to a river a There is increased work load i Sediment is deposited ii River slope gets steeper b Increased velocity i More energy ii Bigger floods c Deposition of alluvium stops i Works toward equilibrium ii Excess alluvium leveled of Channel Forms Suspended Load Narrow and deep river Particles stay suspended toward the center and toward the surface o Unafected by friction Flows faster More energy Bed Load At the same river channel large sediment accumulates at the bottom o Fast moving water from suspended load does not compensate for the additional sediment from bed load Creates a wide shallow channel o Now capable of moving the bed load Causes of Degradation Energy is greater than workload o Land uplift gives water energy to flow downhill Increases energy Bigger floods o Workload is decreased for example by dams Dams backup sediment decreasing the energy of the channel Once the water passes the dam it is filtered so the energy is increased without any sediment to slow it down This creates erosion downstream from the dam Causes of Aggradation Workload is greater than the energy o Increased sediment caused by more erosion on the nearby slopes o Decrease energy Smaller floods o Example at the mouth intersection of steep canyons water passes through once the stream exits the canyon it spreads out slows down deposits sediment never in the same location After repeated floods a ramp of alluvium is built outside of the canyon known as an alluvium fan
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