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.
-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.