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Exploring Geology Chapter 16 Rivers and Streams
Goals for this chapter: Processes associated with rivers, such as erosion and deposition Features of rivers, including meanders, floodplains and deltas Importance of rivers to society, such as flooding Headings in notes: 3D Information about what is on the 3D file accessed via the 3D link in the lower right corner of slide, if present INSTRUCTIONS TO STUDENTS Instructions that the instructor can give to students OBSERVATIONS What students might see during an observation exercise EXPLANATION Additional aspects that can be explained by the instructor EXERCISE Instructions to students about possible in-class exercise NOTES Miscellaneous notes Note on PowerPoint Animations The PowerPoint files are set up so that text and figures appear sequentially on slides. As a cue to the instructor, the figure number goes away when the next click will advance the presentation to the next slide. If the figure number is still showing, another click will reveal something else on that slide (or hide the figure number for slides with no animated text). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1
Yukon Delta Bering Sea Observe features on this satellite image that may be related to rivers MEDIA 1600a1_Yukon_Delta.mov 1600a1_Yukon_Delta.kmz 1600a2_Yukon_Delta_VE3.scene Note that north is to the right in image; colors are more vibrant in the media file than the figure on slide. INSTRUCTIONS TO STUDENTS Observe this satellite image of the Yukon Delta, identifying some obvious features that may be related to rivers. Colors are from the satellite data, not actual colors of the delta. OBSERVATIONS Many lakes and wetlands Distributary system Bulge in coastline More vegetation on the delta Light-colored water (sediment rich) along the coast 16.00.a1 2
Yukon Drainage Area Yukon Delta Yukon River 3 MEDIA
1600a2_Yukon_Drainages_pano.mov Zoom and scroll map; drainages in blue EXPLANATION Location of delta Drainage network Runoff and sediments from this large region head toward the delta 16.00.a2 3
Drainage Basins Observe the red and blue areas and their boundary
Red area drained by one stream, and blue area by another: each is a drainage basin MEDIA 1601b1_Drainages.mov 1601b1_Drainages_VE1.scene INSTRUCTIONS TO STUDENTS Observe the red and blue areas of this terrain and their mutual boundary Boundary between basins is a drainage divide 16.01.b1 4
Volume of Flow Versus Time
Plot of discharge versus time is a hydrograph This hydrograph shows discharge (flow) increasing during a flood, then decreasing as flood ends Runoff from steep drainage basin is fast and most water arrives downstream at once EXPLANATION Scientists measure the amount of water flowing through a specific measuring site in a stream Plot these data on a hydrograph, which portrays how the flow rate (discharge) varies through time Higher parts of the graph signify more flow Steep curve indicates a more rapid change in flow rate Runoff from basin with gentle slopes is spread out over time; less peak flow 16.01.b 5
Discharge Versus Basin Shape
Simple drainage basin: runoff reaches main drainage in orderly way Has simple hydrograph More complex basin shape: water shows up at different times EXPLANATION Brown insets are map views of drainage basins Configuration of the drainage network influences when water from a precipitation or snowmelt event reaches areas downstream When water arrives depends on the distance it has to travel and its rate of flow (size of stream, etc.) NOTES The top figure is not in the textbook but included here for comparison (book compares bottom figure here with top hydrograph from previous slide) More complex hydrograph reflects something about basin shape 16.01.b 6
How Do Rivers Vary Over Time?
Rivers vary in discharge during the year due to snowmelt, wet/dry seasons, etc. INSTRUCTIONS TO STUDENTS Observe the pattern on this graph of discharge versus time of year, and suggest some situations that could cause such a pattern EXPLANATION Vary in discharge, such as peak flow during Spring snowmelt, or during a rainy season that begins in the middle of the Summer Smaller fluctuations record individual events, like a storm or especially warm week that results in more snowmelt in the Spring Suggest some factors that might explain this pattern above (many possible answers) 16.03.c1 7
Tributaries and Drainage Networks
Smaller subsidiary channels are tributaries Tributaries spread out discharge over time Types of Drainage Patterns EXPLANATION Dendritic drainages are branching and tree-like, with smaller tributaries feeding into larger ones Radial drainages flow outward in all directions, like off a circular mountain Structurally controlled drainages follow tilted or folded layers, faults, joint patterns Rivers can cut across layers (ridges), like in Appalachian Mountains Structurally controlled Dendritic Radial 16.01.c-d 8
North American Drainage Basins
Observe these drainage basins and find where runoff in your area ends up INSTRUCTIONS TO STUDENTS Observe these drainage basins and try to determine where runoff in your area ends up Can you name the major rivers shown? Answer (from right to left) St. Lawrence, Ohio, Mississippi, Missouri, Arkansas, Rio Grande, Colorado, Snake, Columbia, Saskatchewan EXPLANATION Continental Divide generally used to refer to boundary between drainages that flow to Pacific versus those the flow to the Atlantic and Gulf of Mexico, but other regional divides are important Interior drainage means area does not drain into the sea (drainage patterns disrupted by Basin and Range faulting in Nevada and Utah) 16.01.d4 9
How is Material Transported and Deposited?
Fine particles can be carried in suspension (floating) in water Soluble ions are dissolved in and carried by moving water Sand grains can roll and bounce along EXPLANATION Can introduce terms saltation, traction, solution Cobbles and boulders mostly roll and slide during high flows Material moving on river bed is bed load 16.02.a1 10
What Processes Erode Material?
Clasts can be picked up by turbulence and low pressure caused by moving water Moving clasts collide with other clasts and obstacles, chipping away or launching pieces EXPLANATION Soluble material could include salt, gypsum, and calcite in limestone and travertine Turbulence loosens and lifts pieces of streambed Soluble material is dissolved and removed 16.02.b1 11
Turbulence Viscosity (resistance to flow) and surface tension act to keep water smooth, as in slow-moving water Upward- flowing eddies can pick up loose material Moving water has inertia (tries to keep moving with same speed and direction) EXPLANATION Inertia means that a moving object tends to keep moving in the same direction and with the same speed unless affected by a force, such as from friction or an obstacle At higher velocities or near obstacles, flow becomes more chaotic (turbulent), forming a swirl called an eddy 16.02.c1 12
Observe how river gradient changes downstream
Gradient = change in elevation for a horizontal distance (small blue triangles) Expressed as m/km or ft/mile, degrees, or percentage EXPLANATION Steepest gradient on this figure is 15m/km Note the very large vertical exaggeration that is about 50 times – gradients for most rivers are very low (Lower Mississippi is less than 0.1m/km) EXERCISE Provide students with pairs of elevation drop and distance, and have them calculate gradient Practice converting units, such as feet/mile to meters/kilometer Steeper gradient: river drops more for a given distance 16.03.a1 13
How Does a River Change Down Stream?
T1-T3 = tributaries H = headwaters D = delta M = mouth INSTRUCTIONS TO STUDENTS (With only the map showing) Predict how discharge, flow velocity, and sediment load will vary from the headwaters, past a tributary, across the delta, and to the mouth of the river EXERCISE (More difficult for students) Sketch a graph for each factor where vertical axis represents the amount of that quantity (like water velocity) and the horizontal axis represents distance downstream; sketch one example, such as total sediment load, that would be more difficult for them on their own NOTES The increase in discharge follows from the increase in channel size (cross section) and flow velocity. It is not intuitive that water velocity increases downstream. It looks higher in steep mountain streams, but turbulence greatly slows the average rate. Although purposely not shown and probably no need to mention here (deltas are covered later), water velocity generally slows dramatically near the mouth of the river, as the river encounters a lake or sea; this results in sediment deposition and decrease in maximum grain size. 16.03.a 14
Sediment Size Versus Current Velocity
At high velocity, sand and smaller particles carried in suspension At moderate velocity, silt and clay remain suspended but sand moves as bed load (rolls, etc.) NOTES The main point of this graph is that whether and how sediment is carried depends on stream velocity, and that different sediment sizes can be carried by different mechanisms, depending on the velocity At lowest velocity, sand dropped but silt and clay remain in suspension Observe and interpret this graph of stream velocity versus mode of transport for different sizes of sediment 16.03.b1 15
How Do Rivers Change Downstream?
Observe this profile of a river. How would you describe this pattern to someone? INSTRUCTIONS TO STUDENTS Observe this profile of a river that goes from the Appalachians eastward to the Atlantic Ocean It crosses several provinces, including the Piedmont and Coastal Plain OBSERVATIONS Steeper in the mountains Gentle downstream Very gentle in Coastal Plain Bump in middle EXPLANATION Concave-up shape Drop from Piedmont to Coastal Plain is at the Fall Line Rivers tend to be steep near their origin, such as in hills and mountains Most rivers become less steep (more gentle) down stream (profile is concave upward) 16.04.a2 16
Base Level The lowest level to which a river can erode: base level
High above base level: much erosion Closer to base level: less erosion EXPLANATION Base level can be the sea, lake, or closed basin High above base level, streams and rivers can have steeper gradients and erode sharply into terrain Closer to base level, streams have lower gradient so less erosion Roughness of landscape reflects decreasing gradient of rivers (curved profile) Landscape reflects decreasing gradient and how high above base level Sea level is ultimate base level 16.04.b1 17
Curves in Rivers and Streams
Observe these satellite images of rivers This river is braided This river has meanders MEDIA 1601a1_Brahmaputra_Brading.mov This movie is of a different area, but shows excellent large-scale braiding. INSTRUCTIONS TO STUDENTS Observe the characteristics of each river, identifying smaller features along each river, and difference between the two rivers 16.05.a 18
Curves in Rivers and Streams
Observe the channels in three rivers INSTRUCTIONS TO STUDENTS Observe the channels in three rivers Braided: network of interweaving channels Low sinuosity: gently curved Meandering: very curved; high sinuosity 16.05.a 19
What Processes Operate on Meanders?
Small graphs show profiles across the river channel; observe the channel profiles for different parts of the river Straight channel: symmetric, fastest in center Outside bend: deeper and faster; erosion of cutbank MEDIA 1605b1_Meander_Profiles.mov Movie rotates the block, displaying cross sections of the river in different segments INSTRUCTIONS TO STUDENTS Observe the small profiles across the channel (blue and white graphs) relative to their location on curves or straight segments of the river EXPLANATION Straight channel: fairly symmetric with highest velocity in center Inside of bend: channel shallower and velocity lower, so deposition forms a point bar Outside of bend: channel deeper and velocity faster, so erosion forms a cutbank Erosion of cutbank and deposition of point bar may be balanced, so river maintains width Inside bend: shallow and slower; deposition of point bar Erosion and deposition may be balanced 16.05.b1 20
How Do Meanders Form and Move?
Faster side erodes and deepens Deep side faster and more water, so erosion Inside slower and less water, so deposition Erode outside bend, increasing curvature; migration outward and downstream MEDIA 1605c1_Meander_Formation.mov Movie plays to show how water flows and causes meanders to form INSTRUCTIONS TO STUDENTS Do not take detailed notes here as the information is in the book EXPLANATION Curve begins when the velocity is faster on one side; gets eroded and deepens Deeper side carries more and faster water so get more erosion, forming a cutbank Inside has less and slower water so has deposition, forming a point bar Meander erodes its outside bend, increasing curvature; meander migrates outward and downstream Continued erosion or overflow during a flood can cut off meander and form an oxbow lake Erosion or overflow erosion: cutoff meander and oxbow lake 16.05.c 21
Landforms in the Headwaters of Streams
Waterfall MEDIA 1606a3_Alaska_Headwaters.mov 1606a3_Alaska_Headwaters_VE1.scene Rotate the scene about 180 degrees to duplicate this above view. EXPLANATION Waterfall: where resistant rocks or tributary valley higher than main valley Rapid: turbulent river due to an obstruction, like debris from tributary, landslide, or bedrock in channel Mountain lake: water impounded by some obstruction or where bedrock scoured deeply NOTES Could show photographs of waterfall, rapid, and steep headwaters Rapid: turbulent segment Mountain lake 16.06.a3 22
Landforms of Mountain Streams and Rivers
Incise into bedrock; narrow canyon if incision faster than widening Dump debris when reaching more gentle terrain MEDIA 1606b3_Mountain_River_Landforms.mov 1606b3_Mountain_River_Landforms_VE1.scene Covers a larger area than the figure EXPLANATION Rivers erode into (incise) bedrock; narrow canyon if incision is faster than widening of canyon walls Steep drainages may dump their debris when reaching more gentle terrain Incised channels can become braided as they reach less steep areas NOTES Could show photographs of braided streams and alluvial fans 16.06.b1 Braided channels in less steep areas 23
Observe this view of a braided river
River has steep gradient onto plain 16.07.a1 Glaciated mountains: abundant sediment supply MEDIA 1607a1_South_Island.mov 1607a1_South_Island_NZ_VE2.scene INSTRUCTIONS TO STUDENTS Observe this terrain, which has rivers that start in the mountains and reach the sea EXPLANATION River drains steep mountain range with glaciers and an abundant sediment supply River exits mountains with a steep gradient and flows onto a sloping plain Sediment-rich river is braided all the way to the ocean NOTES The topography and satellite image are from Canterbury, New Zealand, but were flipped for layout purposes in the textbook figure, but are not flipped in the media files Sediment- rich river braided to ocean 24
Close-Up Views of Braided River
Braided channels: steep gradients, abundant supply of sediment, and variable flows MEDIA 1607a2_Mountain_Braided_River.mov 1607a2_Mountain_Braided_River_VE1.scene 1607a4_Closeup_Braided_Stream.mov 1607a4_Closeup_Braided_Stream_VE6.scene Close-ups of different segments of same river shown on previous slide; for textbook layout reasons, first figure is rotated 90 degrees compared to media files and second figure is flipped compared to media files. EXPLANATION Rivers have braided channels where there are steep gradients, abundant supply of sediment, and variable flows Braided channels may be part of river with fairly straight path NOTES These views are of same river as last slide Part of fairly straight river 16.07.a 25
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