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Earth Science - Lecture 30 Notes.docx

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McMaster University
Earth Sciences
Sergei Basik

November 13, 2013 STEAMS Stream Flow and Sediment Transport  A river is not a fixed structure: o Subject to natural processes o Changes course, floods  River a complex network of interconnected channels  Many small tributaries flow to few large streams that flow to one major river  Valleys are eroded over thousands of years  Response to changes in: o Climate o Amount and variability of flow o Size and amount of sediment particles supplied to channels Stream Flow  Streams collect water and carry it across land to ocean  Streams accumulate surface water from watershed (drainage basin)  Drainage basin: o Upstream area from which surface water will flow toward channel o Largest in the world: The Amazon River Basin (about 7,000,000 square km) o Largest in Canada: The Mackenzie River Basin (1,805,200 square km)  Discharge: o Volume of water flowing per unit of time o Average water velocity multiplied by cross-sectional area of stream o Q = VA o Q: discharge or total flow (m /sec) o V: average velocity (m/sec) 2 o A: cross-sectional area (m ) = width x depth  Annual total discharge: o Amazon: 6923km /year o Ganges: 1386km /year 3 o Congo: 1320km /year o Canada (Mackenzie): 325km /year  Point velocities: measured at equal intervals and depths across the channel  Acoustic Doppler current profiler: measures water velocity at hundreds of locations based on shift in sound frequencies due to moving particles Sediment Transport and Stream Equilibrium  Streams carry sediment downstream, eroding material in one place and depositing it in another  Graded stream: able to maintain a dynamic equilibrium (inflow and outflow of sediment in balance)  Streams adjust gradient (channel slope) in response to: o Water velocity November 13, 2013 o Sediment grain size o Total sediment load  Streams begin high in drainage basin o Harder rocks, steeper slopes o Coarser grain sizes o Steeper gradient or faster water to move sediment  Downstream: o Gradient decreases o Sediment is worn down to smaller sizes o Larger flow transports particles on a gentler slope  Eventually the stream reaches a lake or ocean (base level below which a stream cannot erode)  Stream descends from steeper mountainous gradient onto broad valley bottom: o Local base level o Rapid decrease in gradient causes stream to change from erosional mode to depositional mode  Stream reaches base level of lake or ocean: o Abrupt drop in velocity ca
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