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14 - Streams

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McMaster University
Earth Sciences
Maureen Padden

STREAMS The hydrologic cycle is the interchange of water among sea, air and land. Water flows through streams on the surface to reach lakes and run off into the sea. STREAM  Body of running water moving downhill b/c gravity  Head water: upper part of a stream (i.e. near mountain source)  Stream channel: long narrow depression eroded by the stream into rock or sediment o Stream bed: bottom of channel o Stream banks: to the side  Long profile – cross section along channel showing all gradients from source to mouth  Base level- Limiting level below which stream can erode. This is the change from an erosive environment to a depositional environment. o Faults can change base level o Dams can change base level to upstream; sediments need to be dredged eventually DRAINAGE BASINS Drainage basin: the particular area that water falls in and drains into the ocean via a stream and its tributaries. Focus on watersheds in drainage basic – the upstream area from which the surface water flows towards the channel  Tributary: small stream flowing into large one Divide: boundary or topographic line seperating one draining basin from another – i.e. North American continental divide (f/ Yukon Territory to Mexico). Drainage pattern: the arrangement in a map of a river and its tributaries  Dendritic: resembles branches of a tree; uniformly erodible rock or regolith; most common  Radial: diverge outward; high conical mountains  Rectangular: frequent 90 degree bends; regularly fractured rock – metamorphic/igneous; fractures meeting at 90 degrees form pathway  Trellis: parallel main stream with short tributaries at right angle; tiled resistant rock STREAM PROFILES FLOW  Laminar and turbulent flow both occur  Laminar flow: along the length of the tube; very linear and docile fashion; rare  Turbulent flow: water particles move in a highly unpredictable, high energy fashion; turbulent flow occurs as velocity increases; it leads to erosion DISCHARGE  Volume of water moving past a given point per unit time  (cross sectional area – product of stream width and depth)*(stream 3 velocity)=discharge in m /s  Increases down flow in humid areas b/c: (1) water flowing out of ground and intro river through stream bed (2) tributaries add water to river VELOCITY  Stream velocity – distance travelled by water in a stream per unit time  Key factor in erosion, transportation, and deposition o High velocity: erosion and transportation  Very fine particles require a high velocity to be eroded due to electrostatic forces.  Increased velocity: increased ability to carry/erode large grains o Low velocity: increased deposition of sediment  Controlled by stream gradient, channel shape, channel roughness Stream Gradient  Stream gradient: downhill slope of the bed (m/km)  Decreases downstream; rapids mark local increases in gradient Channel Shape  Narrower channels result in rapid flow  Width altered by external forces: bridges, land slide, etc. Channel Roughness  Smooth channel result in rapid flow  Boulder-strewn channels are rough and increase friction for velocity EROSION Streams erode rock and sediment in three ways: hydraulic action, solution, abrasion Hydraulic action: It’s the ability of flowing water to pick up and move rock and sediment; can erode loose material or break rocks. Powerful at waterfall base b/c force of falling water Solution: Process of chemical weathering and erosion at surface of rock; dissolves and moves material Abrasion: Gri
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