Running Water Streams Online

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Running Water Streams Online

  1. 1. STREAMS/(rivers) Running Water
  2. 2. Soil/Bedrock attack <ul><li>When a gully is cut into the ground below the water table it becomes a permanent stream (once formed, it lengthens by eroding backwards) </li></ul><ul><ul><li>when tributary gullies cut below water table a river system is born </li></ul></ul><ul><li>Stream erosion- dominant agent erosion </li></ul><ul><ul><li>Mechanical weathering- abrasion: breakdown of bedrock by grinding action of sand, pebbles and boulders moved by stream </li></ul></ul><ul><ul><ul><li>tools are also worn down and rounded </li></ul></ul></ul>
  3. 3. Running water and chemical weathering <ul><li>Rapidly running water has lifting effect that splits off and moves rock fragments </li></ul><ul><li>Dissolving soluble rocks, minerals and cement- </li></ul><ul><ul><li>forms pits and holes in riverbed </li></ul></ul><ul><ul><li>widens existing cracks and holes </li></ul></ul>
  4. 4. Sediment transport
  5. 5. Removal of weathered rock <ul><li>Load - the amount of sediment carried by a river- depends on velocity and volume </li></ul><ul><ul><li>solution (25%) : dissolved material transported in rivers- mostly form groundwater seepage (mostly Ca and Mg) </li></ul></ul><ul><ul><li>suspension (50%) : sediment transported in the water column- turbulence keeps it afloat though it is denser than water (clay, silt, rarely fine sand)- give muddy look </li></ul></ul><ul><ul><li>bed load (25%) : sediment moved along the bottom of a river (saltation, rolling, and traction) sand, pebbles and boulders </li></ul></ul><ul><ul><ul><li>undisturbed areas (25% suspension, 75% solution) </li></ul></ul></ul>
  6. 6. Types of sediment load
  7. 7. Carrying Power and Load <ul><li>Carrying power - indicated by total amount of sediment transported and by the size of the particles- </li></ul><ul><ul><li>depends on nature of stream, climate, kind of bedrock and season ALSO ON </li></ul></ul><ul><ul><ul><li>Discharge and speed (velocity) </li></ul></ul></ul><ul><ul><ul><ul><li>the volume of water flowing past a given point in the stream at a given time (m³/s) </li></ul></ul></ul></ul><ul><ul><ul><ul><li>speed depends mostly on gradient (  elevation/distance) </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>flood stage flow is fastest and most erosive- scours bed </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Water flow is quickest in top center of streams (no friction from the river banks or bottom </li></ul></ul></ul></ul></ul>
  8. 8. Sediment Deposition click4videoclip <ul><li>Denser/large/rounded/heavier particles often with a smaller surface area to volume ratio fall out first and more rapidly as streams flow from their origin (source) </li></ul><ul><ul><li>Occurs due to drop in speed (energy) (less energy = less ability to transport denser/larger particles) </li></ul></ul><ul><ul><ul><li>From slope decrease, bed widening, obstructions </li></ul></ul></ul><ul><ul><ul><ul><li>Most dramatically when emptying into a lake or an ocean, or into a flat valley at the base of a mountain </li></ul></ul></ul></ul><ul><ul><ul><li>Occurs due to a decrease in discharge </li></ul></ul></ul><ul><ul><ul><ul><li>Discharge decreases during passage through arid region (evaporation and seepage) or when diverted for irrigation or city water supplies </li></ul></ul></ul></ul>
  9. 9. River Valleys <ul><li>V-shaped : mountain streams and high plateaus </li></ul><ul><ul><li>in areas with enough rainfall to wash sides of valley into stream </li></ul></ul><ul><ul><li>might form canyons in dry areas- little or no erosion of valley walls </li></ul></ul><ul><ul><ul><li>canyon formation: depends on bedrock, sediment load, discharge, and climate </li></ul></ul></ul><ul><ul><ul><ul><li>(image- str v sha ) </li></ul></ul></ul></ul>
  10. 10. Stream valley V-shape
  11. 11. Valley Downcutting <ul><li>A stream cannot cut any lower than its base level- (image on next slide) </li></ul><ul><ul><li>the level of the stream or body of water into which it flows: (it may build up or erode down with fall or rise in water level of lower water body) </li></ul></ul><ul><ul><ul><li>near base level the stream slope and speed decrease </li></ul></ul></ul><ul><ul><ul><ul><li>Down-cutting lessens </li></ul></ul></ul></ul>
  12. 12. Base Level
  13. 13. Widening the Valley <ul><li>Occurs throughout river history particularly after down-cutting slows </li></ul><ul><ul><li>weathering and erosion of valley walls by stream and its tributaries </li></ul></ul><ul><ul><ul><li>creates wider valley with broad floor and gently sloping walls </li></ul></ul></ul>
  14. 14. Lengthening the Valley <ul><li>Headward erosion - wearing away of the land at the headwater of a gully or stream valley </li></ul><ul><ul><li>begins to occur when a hillside is stripped of its protective vegetation cover </li></ul></ul><ul><ul><ul><li>Also occurs with waterfalls as they eat backwards at point of the falls </li></ul></ul></ul>
  15. 15. River Stages : Stage Not Age YOUTHFUL STAGE <ul><li>All of the features in each of the three stream stages can be found in the drainage basin: the area drained by a river and all of its tributaries </li></ul><ul><li>Characterized by: </li></ul><ul><li>broad uplands with v-shaped valleys </li></ul><ul><li>steep gradient (high energy) </li></ul><ul><li>coarse bed load </li></ul><ul><li>rapid down-cutting </li></ul><ul><li>narrow, straight channels </li></ul><ul><li>lack of flood plain </li></ul><ul><li>water falls and rapids </li></ul><ul><ul><li>in areas of differential uplift </li></ul></ul><ul><ul><li>obstruction by large boulders (early weathering/erosion stages) </li></ul></ul><ul><li>few tributaries </li></ul>
  16. 16. MATURE STAGE <ul><li>Characterized by: </li></ul><ul><li>less gradient (less energy) </li></ul><ul><ul><li>moderate current but less down-cutting </li></ul></ul><ul><ul><li>medium to small sized particle load </li></ul></ul><ul><li>more lateral erosion (cut banks and point bars) </li></ul><ul><ul><li>meanders start forming </li></ul></ul><ul><ul><ul><li>water flows around curves at different rates </li></ul></ul></ul><ul><ul><ul><ul><li>faster (outer banks) = erosion </li></ul></ul></ul></ul><ul><ul><ul><ul><li>slower (inner banks ) = deposition </li></ul></ul></ul></ul><ul><li>broader valleys </li></ul><ul><ul><li>a lot of tributaries and distributaries </li></ul></ul>
  17. 17. OLD STAGE <ul><li>Characterized by: </li></ul><ul><li>gentle gradient (near base level) </li></ul><ul><ul><li>nearly flat slope = lower energy </li></ul></ul><ul><ul><ul><li>deposits sediments in its channel and on its banks </li></ul></ul></ul><ul><ul><ul><li>fine-grained sediment load </li></ul></ul></ul><ul><li>wide, meandering stream valleys </li></ul><ul><li>large flood plain </li></ul><ul><li>mostly lowland (swamps and oxbow lakes) </li></ul><ul><li>very few tributaries </li></ul>
  18. 18. Stream Development and Deposition
  19. 19. Rejuvenated Rivers <ul><li>Rivers are rejuvenated when an existing river’s gradient is increased due to movement in Earth’s crust </li></ul><ul><ul><li>Increased slope allows river to cut deeper into valley floor (stream and valley are already developed past the youthful stage) </li></ul></ul><ul><ul><li>Step-like terraces might be formed on each side of stream </li></ul></ul><ul><ul><ul><li>Provides evidence of uplift and downcutting in new valley floor </li></ul></ul></ul>
  20. 20. Drainage Patterns and Stream Paths <ul><li>Braided image next slide </li></ul><ul><li>Dendritic image next slide </li></ul><ul><li>Meandering </li></ul><ul><li>Stream avulsion - abandon existing course and/or delta to form a new one </li></ul><ul><li>Stream piracy (stream capture)- lengthening of a river by headward erosion </li></ul><ul><ul><li>headward erosion of one river wears through a divide and captures the headwater of another river (nick point- where capture occurs) </li></ul></ul><ul><li>Stream order - 1 st order: no inflowing streams up to a given point; 2 nd order: one inflowing stream past a given point; 3 rd order: 2 inflowing streams past a given point; 4 th order: etc. </li></ul>
  21. 21. Braided and Dendritic Stream Images
  22. 22. Stream Piracy (arrow = headward erosion of divide)
  23. 23. Stream Order
  24. 24. Divides and Drainage Basins <ul><li>Stream divides - high parts of land that separate rivers and river systems- separate rivers that flow east vs. west, north vs. south etc. </li></ul><ul><ul><li>Great Continental Divide, Appalachian Mts., and a low divide in MN and WI separates Mississippi River system from land sloping toward Great Lakes and Arctic Ocean </li></ul></ul><ul><li>Watershed ( drainage basin ) - the area drained by a river directly and all its tributaries (trunk stream and tributaries) </li></ul><ul><ul><li>largest in the U.S. is Mississippi River system </li></ul></ul>
  25. 25. Drainage Divides and Drainage Basin Images
  26. 26. Water and Wind Gap Formation <ul><li>River cuts into valley and meets a very resistant rock formation that wears away more slowly than surrounding rock </li></ul><ul><ul><li>When down-cutting is at same rate as land is being slowly uplifted </li></ul></ul><ul><ul><ul><li>a narrow, deep notch is cut in the ridge through which the river runs- a water gap </li></ul></ul></ul><ul><li>an abandoned water gap- wind gap- from stream capture </li></ul>
  27. 27. Potholes and Plunge Pools <ul><li>Potholes: form when streams run over an irregular bed and develop whirlpools that swirl sand, pebbles and small boulders and grind deep circular holes </li></ul><ul><li>Plunge Pools: very large potholes at base of waterfalls </li></ul>
  28. 28. Waterfalls <ul><li>Steep slopes and cliffs of rapids and waterfalls form when: </li></ul><ul><ul><li>River flows over hard igneous rocks onto sedimentary rocks in areas of uplift </li></ul></ul><ul><ul><li>Differential glacial erosion between valleys </li></ul></ul><ul><li>Temporary features: </li></ul><ul><ul><li>Waterfalls are areas where stream erosion is greatest </li></ul></ul><ul><ul><li>Undermining: water falling into plunge pool at base of falls erodes rock there leaving overhanging cliff </li></ul></ul><ul><ul><ul><li>Breakage at cliff causes recession of falls </li></ul></ul></ul><ul><ul><ul><ul><li>Especially when rock is fractured or poorly cemented </li></ul></ul></ul></ul>
  29. 29. Meanders <ul><li>Meanders form as riverbed gradually shifts toward the outside bend in the river </li></ul><ul><ul><li>Water is fastest and most erosive on outer banks </li></ul></ul><ul><ul><ul><li>Form cut banks </li></ul></ul></ul><ul><ul><li>Entrenched meanders- form in deep canyons after plateaus are uplifted </li></ul></ul>
  30. 30. Sediment Deposits in Stream <ul><li>Deposits on inner banks </li></ul><ul><ul><li>Point bars (on inside of cut banks) </li></ul></ul><ul><li>Deposits in stream channel </li></ul><ul><ul><li>Sand (gravel) bars (in braided streams and shallow spots in streams) </li></ul></ul>
  31. 31. Oxbow Lakes <ul><li>Oxbows form when river curves become too large and the river breaks through a bend- called a cutoff </li></ul><ul><ul><li>Sediment deposition at ends of abandoned meander- when it fills in an oxbow lake forms </li></ul></ul>
  32. 32. Floodplain <ul><li>Flood deposition on valley floor is instant because friction of land causes river to lose ability to transport particles </li></ul><ul><ul><li>Flood deposits are thicker and coarser near river banks and thinner and finer in floodplain </li></ul></ul>
  33. 33. Floodplain Features <ul><li>Levees- thick deposits built up along stream banks </li></ul><ul><li>Back swamp- murky water in lowest areas away from levees- fine-grained sediment </li></ul><ul><li>Floodplain deposits are very fertile </li></ul>
  34. 34. Deltas and Alluvial Fans <ul><li>Fan-shaped deposits at mouth of river </li></ul><ul><ul><li>deposition in quiet water bodies) (flat profile) </li></ul></ul><ul><ul><li>Deposition due to drop in energy (clay and silt) </li></ul></ul><ul><ul><li>Distributaries may form as delta grows </li></ul></ul><ul><ul><li>Image next slide </li></ul></ul><ul><li>Sloping fan-shaped deposits (on land) </li></ul><ul><ul><li>Sediments are washed down from slopes above to a flat area in a dry region </li></ul></ul><ul><ul><li>Coarse sand and gravels </li></ul></ul><ul><ul><li>Image next slide </li></ul></ul>
  35. 35. Delta and Alluvial Fans
  36. 36. Flood Types <ul><li>Ordinary flood : from heavy or long-lasting rains or winter snow melts over much of drainage basin </li></ul><ul><li>Flash flood: cloudbursts - especially if over a narrow valley </li></ul><ul><ul><li>Runoff </li></ul></ul><ul><li>Flood recurrence interval </li></ul><ul><ul><li>How long between flood small vs catastrophic </li></ul></ul><ul><li>Dam breaks including ice jams or volcanic ash, cinder and lava dams </li></ul>
  37. 37. Flood Prevention <ul><li>Stop removing natural vegetation such as trees, shrubs, and grass- especially in headwaters </li></ul><ul><li>Store runoff in reservoirs/build dams </li></ul><ul><li>Dig drainage channels/spillways (near mouth) </li></ul><ul><li>Build up levees </li></ul>

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