Rivers and StreamsChapter 13Streams• A stream is a body of water that is confinedin a channel and moves downhill under theinfluence of gravity.• This definition includes all sizes of runningwater from a tiny trickle to the Amazon.• The total area drained by a given stream isits drainage basinDrainage Basins and Divides• The line separating one drainage basin fromanother is a drainage divide.• The Continental Divide separates drainagebasins of rivers that flow to the Atlantic andGulf of Mexico from those flowing to thePacific and Gulf of Cortez (Gulf ofCalifornia).Floods• Flash floods can occur in a matter ofhours as a result of a single storm.• Flash floods are most likely in areas of highgradients.• Annual floods result from snowmelt orannual rain patterns (monsoons).Gradient• Gradient is the drop per mile (km).• Boulder Creek drops from 8000ft atNederland to 5600’ in Boulder and has agradient of 160 ft/mi (~30 m/km).• The Colorado River through Grand Canyonhas a gradient of about 10 ft/mi (~2 m/km)• The lower Mississippi has a gradient of lessthan 0.5 ft/mi (~0.1 m/km).• Gradients usually decrease downstream.Stream Velocity• Velocity is controlled by both gradient andchannel size.• Steep gradients cause high velocities• Constricted channels cause high velocities.Discharge• Discharge is the total flow in ft3/s (cfs) or m3/s.•1 m3/s = ~35 ft3/s (cfs).• In a section of stream with no tributaries, thedischarge is constant while the velocity andchannel size may vary along the bed.Discharge• Boulder Creek is typically 20-50 cfs, butmay be 500cfs or greater in May and June.• The Colorado at Grand Junction is typically2000 cfs, but may exceed 20,000 in Mayand June.• The lower Mississippi is typically 250,000cfs but exceeded 3 million cfs in 1993 floodLoad•The load is the amount of material transportedby a stream.– Dissolved load is the material in solution.– Suspended load is the material in suspension.– Bed load is the material on the bed• Saltation load is the material bouncing along thebottom.• Traction load is the material dragged along thebottom.Deposition and Erosion• The amount of bed load and suspended loadvary with velocity.– Where a stream widens and slows, it depositsmaterial.– Where a stream narrows and quickens, it erodesmaterial.Bars• Bars are material deposited by streams.– A point bar is a bar on the side of a stream.– A channel bar is a bar in the middle of a stream– A braided stream has many channels bars.• The presence of bars indicates that thesteam is
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