Sec 3 Rivers


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Sec 3 Rivers

  1. 1. Drainage Basin
  2. 2. Stream Ordering
  4. 4. DRAINAGE BASINS   <ul><li>A drainage basin (or catchment area) is an area of land which is drained by a river and its tributaries. </li></ul><ul><li>A watershed is the boundary separating one drainage basin from another. It usually follows the ridge or crest of a hill or mountain. River starts at the source, flows along its course and ends or drains into a lake or sea. </li></ul>
  5. 5. http:// /~geol116/week9/drain1%20copy.swf
  6. 6. DRAINAGE PATTERNS <ul><li>Dendritic Pattern  Can be found where the underlying rock is of the same type. The tributaries join the main river at acute angles and resembles the branches of a tree. </li></ul>
  7. 7. DRAINAGE PATTERNS <ul><li>Trellis Pattern -Develop in an area where the rocks are made up of alternate bands of resistant and less resistant rocks, dipping in the same direction and lying at right angles to the main river valley.  Tributaries flow along the less resistant rocks and join the river at right angles. </li></ul><ul><li>Radial Pattern -Develop on volcanic cones or hills with rivers flowing out from a central high point in all directions. </li></ul>
  8. 8. FACTORS affecting river process River velocity River volume RIVER ENERGY
  9. 9. SPEED OF RIVER (velocity ) <ul><li>Gradient of river the steeper the gradient, the faster is the flow </li></ul><ul><li>Roughness of the channel - the rougher the channel, the slower is the flow, because the water has to overcome the friction of the river bed and banks </li></ul><ul><li>Shape of channel-Larger wetted perimeter (banks and bed in contact with water), the river has to overcome more friction and is slowed down. </li></ul>
  10. 10. VOLUME OF RIVER <ul><li>The volume of river depends on : </li></ul><ul><li>Size of drainage basin  -   the larger the drainage basin, the more tributaries there are and the greater is the volume of water </li></ul><ul><li>Presence of Vegetation  -  Vegetation intercepts the rain and enables the rain water to infiltrate into the ground. This reduces surface run-off and so less water flows immediately into the river. </li></ul>
  11. 11. VOLUME OF RIVER <ul><li>Permeability of rocks  -   If the underlying rocks in the drainage basin are permeable, rain water infiltrate into the ground quickly and there will be little surface run-off flowing into the river. </li></ul><ul><li>Climate If there is low rainfall and high precipitation, the rivermay have a low  volume of flow or even dry up. (Vice-versa </li></ul>
  12. 16. <ul><li>The first rainstorm peaked at 30mm of precipitation. True/False 1.2 The second rainstorm peaked at 12 mm of precipitation. True/False 1.3 The discharge of the river peaked at 74mm. TrueFalse 1.4 The lag times were two hours and then four hours. True/False </li></ul>
  13. 17. <ul><li>1.5 The rising limb was steep in storm 1 because run-off over the hard, dry surface was quick. True/False 1.6 The falling limb was more gentle because water reached the river gradually through the ground, after the rain had stopped. True/False 1.7 The second discharge peak was higher because the second storm had more precipitation than the first. True/False 1.8 After the rain has stopped, vegetation, soil and ground water discharge the excess water quickly through ground stores and underground streams and rivers. True/False </li></ul>
  14. 18. River Basin <ul><li> </li></ul>Journey over a river: Worksheet
  15. 19. http:// River Processes
  16. 20. Erosional Process <ul><li>Corrasion (abrasion) : - The river uses its load to grind against the bed and sides The action would dislodge the materials and carry them away This process operates in 2 ways (verticle--> depth; lateral--> width) Potholes are the product of corrasion </li></ul>
  17. 21. Erosional Processes <ul><li>Attrition: The loosened materials that are being carried away collide against the river sides and bed and against one another Over time, they would become smaller and eventually reduced to fine particles called silt </li></ul><ul><li>Solution (corrosion ): - The solvent action of water dissolves soluble materials and carry them away in solution </li></ul><ul><li>Hydraulic action : - The breaking down of rocks Removing and dragging rocks from the bed and banks of the river by the force of the running water (like a water jet) Usually there are lines of weakness like joints and cracks in the river The work of hydraulic action forms plunge pools (small lakes) </li></ul>
  18. 22. Erosional Features- upper course <ul><li>Potholes : Circular depressions on the river bed Formed by corrosion (abrasion) Most effective in flood conditions Pebbles which are trapped in hollows on the river bed are swirled about in turbulent/ fast flowing water </li></ul>
  19. 23. Erosional features <ul><li>Rapids: A series of very short and fast falls </li></ul><ul><li>Condition: an area of alternating bands of resistant and less resistant rocks </li></ul>
  20. 24. Erosional Features- upper course <ul><li>Waterfalls: - Vertical flows of fast moving water flowing from great heights Formed by 2 ways: -    Due to unequal resistance of rocks  or faulting  Unequal resistance- less resistant rocks are eroded more rapidly than resistant rocks As a result, there is a change in gradient By faulting- displacement of rocks results in a difference in height between 2 rocks, water plunges downwards </li></ul>
  21. 25. Erosional Features- upper course <ul><li>Plunge pools: A large depression at the foot of a waterfall This depression is deepened by hydraulic action of the plunging water Condition: likely to form when the water plunges into less resistant rocks It may be further enlarged by rock debris swirling about by turbulent water at the base </li></ul>
  22. 26. Formation of waterfalls <ul><li>http:// </li></ul>
  23. 27. Erosional Features-Gorge-upper course Valleys which are steep sided, deep and narrow
  24. 28. Water falls
  25. 29. Waterfall/Plunge Pools
  26. 30. Erosion of river bank
  27. 31. Upper course- potholes
  28. 32. Endau Rompin
  29. 33. Rocks after erosion
  30. 34. Transportation <ul><li>Suspension - movement of fine eroded materials(silt and clay)floating in water. </li></ul><ul><li>Solution - movement of minerals dissolved in water. </li></ul><ul><li>Saltation - process by which rock fragments like gravel and pebbles are lifted briefly and then dropped on the river bed. They are thus transported in a series of hops and jumps along the river bed. </li></ul><ul><li>Traction - movement of large rocks such as boulders by being rolled along the river bed. </li></ul>
  31. 35. RIVER DEPOSITION <ul><li>A river deposits its load of eroded materials when it is unable to transport it. This occurs when : </li></ul><ul><li>there is not enough water to transport the load during a day season </li></ul><ul><li>a river flows across a desert where there is a high rate of evaporation. </li></ul><ul><li>a river flows across permeable rocks which allow water to infiltrate into the underlying rocks. </li></ul><ul><li>a river carries a larger load than it can transport. </li></ul><ul><li>there is a sudden change in gradient (e.g. river leaves the mountain and flows onto a plain) </li></ul><ul><li>a river flows into a lake or sea. </li></ul><ul><li>one side of the river is shallower than the other (e.g. the convex bank of a meander) </li></ul><ul><li>there are aquatic plants or rocks obstructing the flow </li></ul>
  32. 36. Floodplains and Levees <ul><li>-->   A floodplain is a low-lying plain on both sides of a river that has repeatedly overflowed its banks and flooded the surrounding areas. When the floods subside, alluvium is deposited on the floodplain. The larger materials, being heavier, are deposited at the river banks while the finer materials are carried and deposited further away from the river. The larger materials at the river banks build up into embankment called levees. </li></ul>
  33. 37. Meanders <ul><li>Meanders are loop-like bends in a river. The water flows round the meander in a spiral manner. This causes erosion to take place on the outer bank and deposition on the inner bank. </li></ul><ul><li>Gradually, a steep river cliff is formed on the outer bank, making it concave in shape. On the inner bank, deposition of alluvial materials produces a gentle slip-off slope and the bank takes on a convex shape. </li></ul>
  34. 38. OXBOW LAKES <ul><li>An oxbow lake is a crescent -shaped lake formed on a river when a meander has been cut through and abandoned. </li></ul><ul><li>When a river meanders in very big loops, the outer bank is so rapidly eroded that the river cuts through the narrow neck of the meander. </li></ul><ul><li>The river then flows straight through the channel. When deposition seals off the cut-off from the river channel, an oxbow lake is formed. It may silt up and eventually dry up </li></ul>
  35. 39. River Meanders
  36. 40. Braided Channel
  37. 41. DELTAS <ul><li>--> A delta is a flat piece of land built-up from layers of sediments deposited by a river where it enters a lake or calm sea. The river may have to branch into smaller distributaries to carry the water to the sea. </li></ul><ul><li>Types of deltas Arcuate delta ---> triangular in shape e.g. Nile Delta Bird's foot delta ---> e.g. Mississippi Delta </li></ul><ul><li>Conditions that favour the formation of deltas A large load of sediment Shallow sea at the river mouth Sheltered coast with weak tides and currents Absence of large lakes along the course of the </li></ul>
  38. 42. Ganges Delta
  39. 43. Nile Delta Case Study Benefits and Problems of Human Intervention