15_streams_09_post - 15: Surface Processes 2-Streams Why...

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Unformatted text preview: 15: Surface Processes 2-Streams Why this is important • Streams are major component of the hydrologic cycle powered by the sun and gravity. Plate, p. 552 Braided streams in Alaska. Meadering Alaskan river. Photo: G.M. Ashley Photo: P. Kresan Incised meanders, San Juan River, Utah. Photo: T. Bean Streams: 1 Streams: 2 Why this is important Why this is important • Streams carry most of the water that goes from land to sea, in the process transporting billions of tons of sediment. • Running water is the most effective sculptor of the landscape. Incised meanders, San Juan River, Utah. Photo: T. Bean Streams: 3 Iguaçu River, Brazil. Photo: D. Nausbaum Streams: 4 Definitions Water budget • Stream: channeled flow of water of any size (river, brook, creek are loosely defined by size) • Channel: conduit for carrying water • Floodplain: lowlying area surrounding the channel that is normally dry but may become submerged during high stream flows. Fig. 17.2 Gains: rain, snow, melting snow; swamps and lakes; soil moisture; tributaries; upward-flowing groundwater; storm drains, sewage pipes Losses: evaporation, downward-flowing groundwater, pumping Streams: 5 Stream evolution Streams: 6 Fig. 17.3 Stream evolution (a) Sheetwash: overlapping films or sheets of water that move downslope (think of water flowing down a driveway) (b) Faster-moving sheetwash scours a channel. (c) Channel lengthens upslope by eroding at start of channel (headward erosion); secondary streams (tributaries) join the main stream to form a drainage basin Streams: 7 Fig. 17.3 Headward erosion in Canyonlands National Park, Utah. Photo: R.W. Schlische Streams: 8 Drainage basins Divides Drainage divide: ridge that separates one drainage basin from another Flat-lying strata of uniform lithology Isolated high Fig. 17.4 2 sets of fractures Valley & Ridge Streams: 9 Fig. 17.5 Streams: 10 Divides Continental divide: drainage divide that separates streams that drain into the Atlantic Ocean and the Pacific Ocean Divides Continental divide: drainage divide that separates streams that drain into the Atlantic Ocean and the Pacific Ocean Fig. 17.6 Streams: 11 Fig. 17.5 Streams: 12 Stream flow Stream transport • Laminar flow: channelized flow paths are all parallel to one another • Dissolved load: high in humid climates (higher chemical weathering). • Suspended load: material that does not generally touch the bottom • Bed load: material that slides or saltates (bounces) along the bottom • Turbulent flow: irregular flow paths, produced in irregular channels or at high velocities Fig. 17.11 Fig. 17.10 Streams: 13 Review questions Streams: 14 Review questions 2 15-1. The flat-lying area surrounding a river channel is termed the ____________. A. base level B. floodplain C. stream gradient D. thalweg 15-2. A. True / B. False: All water in streams arrives directly from rainfall runoff. 15-3. Headwater erosion occurs near ___. A. the end of a stream. B. the middle of a stream C. the start of a stream. 15-4. A. True / B. False: The North American continental divide splits the continent into two equal halves. 15-5. A. True / B. False: Laminar flow is more likely to occur in irregular-shaped channels with high flow velocities. Streams: 15 3 4 1 15-6. Match the names of drainage patterns to the geometries shown in the maps above: A. 1 = radial 2 = dendritic 3 = rectangular 4 = trellis B. 1 = trellis 2 = rectangular 3 = radial 4 = dendritic C. 1 = trellis 2 = rectangular 3 = dendritic 4 = radial 15-7. Which drainage pattern in the maps above typically forms in flat-lying strata of uniform lithology? A. 1 B. 2 C. 3 D. 4 15-8. Which of the following likely contains the largest-size particles being transported by a stream? A. bed load B. dissolved load C. suspended load Streams: 16 Discharge Streams: cross section • Discharge increases downstream Less friction along channel, higher V, higher Q • Volume of water passing by a given point in a given period of time Fig. 17.13 Q=AV Q=discharge, Fig. 17.9 A=cross-sectional area, V=average velocity More friction along channel, lower V, lower Q Streams: 17 Streams: 18 Longitudinal cross section Cross-section evolution Mostly hard rock: stream erosion dominates; forms slot canyon with subvertical walls. Mostly soft rock: Vshaped valley forms by stream erosion and landsliding (slumping) • V- to trapezoid-shaped cross section Fig. 17.16 • Gradient: the slope of the stream. • Long profile: plot of elevation vs. distance along the stream. --> Gradient decreases downstream. --> Concave-upward curve. • Base level: elevation below which a stream cannot erode Hard and soft rock: stairstep canyon Fig. 17.13 Streams: 19 Streams: 20 Base level Base level • Regional (ultimate) base level: sea level • Regional base level: sea level • Local base level: lake; waterfall • Local base level: lake; waterfall Fig. 17.14 Fig. 17.14 Streams: 21 Base level • Local base level: lake; waterfall Streams: 22 Longitudinal profile: iClicker Q1. Which river is longest? A. Colorado B. Missouri C. Red D. Rio Grande Q2. Which river has its source region at the highest elevation? A. Colorado B. Rio Grande C. Sacramento Waterfalls on the Iguaçu River mark escarpments produced by Paraná flood basalts, along the Brazil-Argentina border. Photo: J.K. Hamblin Streams: 23 All streams are plotted so that their profiles end at 0 km on the xaxis. Streams: 24 D. Snake Longitudinal profile: iClicker Changes in base level Fig. 17.17 Q3. What is the base-level elevation for all of the rivers? A. 0 meters B. 1000 meters C. 2000 meters D. 3000 meters Q4. Which river has the steepest gradient near its source region? A. Colorado B. Missouri C. Red D. Sacramento 1. Raise base level: fill valley with alluvium 2. Lower base level: erode some alluvium to form terraces All streams are plotted so that their profiles end at 0 km on the xaxis. Streams: 25 Alluvium: sediment deposited by river Streams: 26 Meandering stream Channel patterns • Meandering: highly curved pattern Photo: W.C. Hamblin Fig. 17.9 Alaskan river. Photo: P. Kresan Streams: 27 • Cut bank: occurs on outer bank of meander; experiences higher flow velocity and erosion Streams: 28 • Point bar: occurs on inner bank of meander; experiences lower flow velocity and deposition Meandering stream Meandering stream Typically form on: lowgradient floodplains w/ finegrained sediments Two meander loops may approach each other, forming a meander cutoff (short-cut) and an oxbow lake (abandoned meander loop). Fig. 17.22 Fig. 17.22 Streams: 29 Streams: 30 Channel patterns Braided streams • Form in areas of: low discharge and high sediment supply. • Braided streams: characterized by numerous branching and rejoining channels with sediment bars in between. Photo: T. Bean Streams: 31 Streams: 32 Braided streams in Alaska. Photo: G.M. Ashley Stream deposition Stream deposition • Occurs when a stream loses energy due to: decrease in gradient, velocity, and/or discharge. Fig. 17.13 Streams: 33 Fig. 17.22 Streams: 34 Formation of natural levees Location 1-Deep, narrow channel has high flow velocity; sediment is transported and/or eroded. • Natural levees: deposits formed on the sides of channels during floods that may serve as partial barriers against future floods Stream deposition • Alluvial fan: conical accumulation of sediment deposited at the base of mountain range, where a decrease in slope causes a decrease in stream velocity 2 1 2 Location 2-Wide, shallow floodplain has low flow velocity; sediment is deposited Death Valley, CA. Photo: M. Miller Streams: 35 Streams: 36 Fig. 17.21a Stream deposition Stream deposition Fig. 17.25 • Delta: depositional feature (often triangular shaped) produced where a stream enters a standing body of water (lake, ocean) and deposits the bulk of its suspended load and bedload Photo: G.M. Ashley • Distributary channels: branching channels that bring water and sediment to the front of the delta Streams: 37 Streams: 38 Stream deposition Delta deposits a. Top-set beds: relatively coarse, relatively flat lying. b. Foreset beds: inclined, finer-grained than topset. c. Bottomset beds: subhorizontal fine-grained deposits. Fig. 7.34 Natural levees at Vicksburg, Mississippi. Photo: USGS Streams: 39 Streams: 40 Delta evolution Top-set beds Review Questions Foreset beds 15-9. Which of the following concerning discharge is false? A. Discharge decreases downstream. B. Channel width increases with increasing discharge. C. Channel depth increases with increasing discharge. D. Velocity increases with increasing discharge. 15-10. All else being equal, water will flow faster in a stream that has a _______. A. steep stream gradient B. gentle stream gradient Bottomset beds 15-11. V-shaped stream valleys result from the downcutting of stream erosion and _______ . A. dissolution B. isostatic rebound C. saltation D. mass wasting (landsliding, slumping) on the valley sides 15-12. All else being equal, a deep, narrow stream will flow _______ a shallow, broad stream. A. more slowly than B. more rapidly than C. at the same rate as Streams: 41 Streams: 42 Review Questions Review Questions 15-13. The stream gradient of most rivers _______. A. is greater near the source (start) than near the mouth (end) B. is greater near the mouth (end) than near the source (start) C. does not change significantly as the river flows downstream D. is greater for a meandering stream than for any other type 15-18. A. True / B. False: Meander bends migrate in the direction of the outside of the curve. 15-14. A. True / B. False: The ultimate base level is sea level. 15-20. Which of the following is not produced directly by stream deposition? A. alluvial fans B. cut bank C. deltas D. point bars 15-15. Which of the following is not associated with meandering streams? A. cut bank B. point bar C. bar D. oxbow lake 15-16. A. True / B. False: Cut banks form on the inside parts of meander bends. 15-17. Within a meander, where is sediment most likely to be deposited? A. on the outer banks of the meander B. on the inner banks of the meander C. uniformly to either side of the meander Streams: 43 15-19. A. True / B. False: Braided streams form in areas of high discharge and high bedload. 15-21. Which of the following contains the finest-grained sediments? A. topset beds B. foreset beds C. bottomset beds 15-22. At a delta, ____________. A. stream gradient decreases B. a single channel breaks into multiple, radiating distributary channels C. Both A and B are correct. D. None of the above are correct. Streams: 44 ...
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This note was uploaded on 09/25/2011 for the course GEOLOGY 100 taught by Professor Lepre during the Fall '11 term at Rutgers.

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