15. RUNNING WATER

 

-Hydrologic cycle- continual recycling of water from the oceans, through the atmosphere, to the continents, and back to the oceans (fig. 15-6); concerned with runoff here.

Running Water

-Water falling on slope that does not infiltrate moves either by sheet flow (continuous film of water moving down slope) or channel flow (water confined to trough like depression).

-Water from streams comes from: 1) sheet flow 2) rain falling directly into stream 3) groundwater

-stream gradient- slope over which stream flows (fig. 15-8); vertical drop/horiz. distance (ft./mi; m/km); source (headwaters) of stream has steep gradient and it decreases downstream.

 

-stream velocity-  how fast water is moving (ft/sec., m/sec.); varies:

-varies across channel width; slower near stream sides and bottom due to friction; fastest at center near top in straight channels (fig. 15-9); fastest near outside of meandering

curves (fig. 15-17).

    -semicircular shaped channel faster than broad-shallow and narrow-deep channel because less water in contact with channel (fig. 15-10).

    -smooth channel faster than rough (boulder filled) channel.

    -varies along channel width also; velocity increases downstream , because:

-1) velocity continually increases due to acceleration of gravity; 2) upstream areas commonly broad, shallow, boulder filled channels; 3) discharge increases as more

tributary streams contribute more water (discharge=velocity X area; Q=VA).

 

-stream erosion- removal or breakdown of solids by running water; deepens and widens a stream.

    -solution-stream water dissolves soluble rocks (limestone).

    -hydraulic action- picking up and moving rock and sediment by water.

    -abrasion- grinding away of stream channel by friction and impact of sediment ("sandblasting"); may form potholes in bedrock (fig. 15-12).

 

-stream sediment transport (fig. 15-14)

    -dissolved load- stream carries ions (Ca, Na, etc.) dissolved from rock by solution.

-suspended load- sediment too light to settle out quickly (clay, silt); carried indefinitely without touching bottom; kept suspended by turbulent flow.

-bed load- larger sedimentary particles (sand, gravel, boulders) that travel on or near stream bottom; includes traction (particles roll or slide along bottom) and saltation    

(particles bounce along bottom; sand) (fig. 15-13).

 

-stream deposition- occurs when 1) velocity decreases, 2) too much sediment for stream to carry.

    -braided stream- stream with numerous dividing and rejoining channels; result of deposition of sand and gravel bars within channel; occur in dry areas where little vegetation is present to slow erosion (fig. 15-15).

-meandering stream- stream with a single sinuous channel with broadly looping curves (fig. 15-16); channel shape asymmetric around curves (steep of outside of bend,

 gentle on inside); low velocity and deposition on inside of bend=point bar deposits (fig. 15-17); high velocity and erosion on outside of bends=cut bank (figs. 15-17);

channel migrates laterally; if migrates until bends meet, an oxbow lake forms (fig. 15-18).

 

-floodplain- low lying flat area next to a stream covered with water during flooding of stream; 2 types:  

1) point bar deposits-result of lateral migration of meanders across floodplain (fig. 15-19).

2) fine-grained sediments (silt, clay) deposited during flooding; velocity of water decreases when stream overflows banks; deposits sediment adjacent to stream forming

natural levees (fig. 15-20).

 

-Deltas- sedimentary deposit formed when a stream enters a standing body of water (sediment deposited when velocity decreases); shoreline builds outward (figs. 15.21).

    -topset beds- sand and gravel deposited by distributary channels; all sediment types.

    -forset beds- sand and silt deposited offshore slightly inclined.

    -bottomset beds- clay deposited in deep water.

    -an overall coarsening upward sequence.

 

-Alluvial fan- fan or cone-shaped pile of sediment formed where stream emerges from a mountain canyon (fig. 15-23); form in areas with little vegetation to slow erosion.

 

-Flood- stream receives more water than its channel can handle; water occupies floodplain.

-property damage from flooding continues to increase (>100 million/yr) despite more flood control projects. Why?

-floodplains are desirable sites; urbanization increases surface runoff and allows runoff to reach main streams earlier.

-USGS maintains stream gauging stations nationwide which are used to construct hydrographs (fig. 15.25; variation in discharge over time) and flood frequency curves

that allow flood recurrence intervals to be determined (fig. 15.26; table 15.2).

 

-Drainage basin- area occupied by a drainage system that contributes water to a given stream (figs. 15.27); drainage basins separated by divides (elevated area).

 

-Drainage pattern- the pattern a stream and tributaries make in map view; controlled by rock type and structure (fig. 15-28),

    -dendritic- tree-like branching of tributary streams; develops in uniformly resistant rock.

    -rectangular- tributaries have ~90˚ bends; forms in rock regularly fractured at right angles.

    -radial- streams diverge from a central area; forms where rocks domed upwards (volcano).

    -trellis- main streams are parallel to ridges, small tributary streams form at 90˚ angles.

    -deranged- irregular drainage pattern in which streams flow in and out of swamps and lakes; developed recently, no organized drainage developed yet (recently glaciated areas).

 

-Base level- lowest level a stream can erode to (fig. 15-29); ultimate (sea level) or local (lake, dam, waterfall; fig. 15-30); base level can change (sea level rises or falls).

-Development of stream valleys- over time, streams become deeper, wider, and longer.

    -downcutting- stream has more energy than is required to carry sediment load.

    -lateral erosion- undercutting valley walls by stream; widens valley (meandering streams cut floodplains).

    -headward erosion- upstream lengthening of stream valleys.