Physical GEOG: Chapter 12 - Rivers

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Physical GEOG: Chapter 12 - Rivers

  1. 1. Factors affecting energy of river Factors affecting former factor Description The steeper the channel, the greater the amount of GPE Gradient of channel conversion and the faster the water moves down the channel. The smoother the channel, the lesser the amount of friction Roughness of channel opposing motion of the water and Speed of river the faster the water moves down the channel. The lesser the wetted perimeter, the lesser the amount of friction Wetted perimeter opposing motion of the water and the faster the water moves down the channel. The larger the drainage basin, the greater the area for collection Size of drainage basin of rainwater and the fuller the river would be. The less vegetation, more surface runoff occurs and less infiltrat- Presence of vegetation ion takes place and the more water flows into the river making it fuller. Volume of river The less permeable the rocks, less infiltration occurs and more Permeability of rocks surface runoff takes place and more water flows into the river to make it fuller. The more the rainfall the lesser Climate amount of water reaches the ground and make it fuller.
  2. 2. Process Subprocess Description Agent: Running water Force of fast moving water Hydraulic action loosens rocks and soil along river banks and bed, are soon dis- lodged. Agent: River bed and sides When rock fragments are Abrasion dragged along the river bed or against the sides of the river, grinding action causes widening and deepening of channel. Erosion Agent: Other eroded particles Eroded materials collide against Attrition one another and this breaks them down into smaller pieces, larger rocks are slowly broken down, rough edges are smoothened. Agent: Carbonic acid CO2 reacts with H2O to form Solution H2CO3. When river flows over limestone, carbonation occurs and the acid dissolves the lime- stone. Heaviest materials tpted this way. Traction Dragged/rolled along river. 2nd heaviest tpt this way. Saltation Series of bouncing motions.Transportation Lightest materials tpted this way. Suspension “Float” along in water. Soluble subs tpted this way. Solution Transporting of dissolved materials.
  3. 3. Factors that can cause deposition: 1. During low precipitation 2. River enters lake or sea, decrease in speed Deposition ------- 3. Increased friction with river sides, decreasing the speed 4. Sudden increase in load, esp during landslides 5.Obstructions (eg. aquatic life) are present DEPOSITION: Small decrease in speed of river enables large particles to be deposited while a large decrease in speed enables smaller particles to be deposited. TRANSPORTATION: Large particles require high river speeds to transport them while smaller particles require small river speeds to transport them. EROSION: From 0.5mm onwards, more energy is required to erode the particle. Below 0.5mm, more energy is required to erode them as these fine particles are highly cohesive and they stick toeach other hence a large amount of energy is needed to dislodge them away from each other.Formation of waterfall 1. When a river flows over rocks of different resistance, the less resistant rock is eroded far more than the more resistant rock, causing a change in gradient of the river course. Over time, the river plunges from a great height to hit the river below with tremendous force. A waterfall forms as a results. Repeated poundings of the river bed may leave a depression at the base of the waterfall. This depression is deepened as rocks and boulders swirl around forming a plunge pool.
  4. 4. 2. Due to the displacement of rocks, there is a difference in height between the two rocks. Water cascades down the scarp to hit the lower rock. Repeated poundings of the river bed may leave a depression at the base of the waterfall. This depression is deepened as rocks and boulders swirl around forming a plunge pool.Formation of gorges When a river flows through an area with resistant rocks by its sides, it can only erode its channel vertically. Over time, a deep, narrow and steep-sided valley known as a gorge is formed.Formation of valleys Upper course valley: Gradient is steep hence it has sufficient energy to erode the rocks in its path and cut deep into the channel. A V-shaped, narrow and steep-sided valleys are formed. Middle course valley: Gradient is not that steep hence it flows with less energy and cuts less deeply into its channel. Lateral erosion is more dominant than vertical erosion hence it is wider. Lower course valley: Exceptionally wide because gradient is gentle. Lateral erosion is dominant giving rise to a broad, flat-floored valley.Formation of floodplains and natural levees When there is heavy rain, a river may overflow its banks. This causes land on either sides of the river to be flooded. As floodwater spread over large areas, it loses speed. When the floodwaters recedes, the river deposits the material it is carrying on the land, leaving behind alluvium on the river banks. Coarser sediments end up nearer the banks as they weigh more hence the accumulation of these sediments form natural levees while the wide flat plains are the floodplains.
  5. 5. Formation of meanders and ox-bow lakes In the outer bend of a river, the river flows faster than in the inner bend as there is less friction against the water and the river sides. This causes erosion to occur in the outer bend due to high speeds and deposition to occur in the inner bend due to the slows speeds. When more sediments are eroded on the outer bend and more sediments are deposited on the inner bend, the river become more curved, forming meanders. With continual erosion and deposition, the meanders become more pronounced and curved towards each other and a narrow neck of land forms in between the curve. With even more deposition and erosion, the meanders will meet and soon damn up the water in the cut-off forming an ox-bow lake. The water in the lake will soon evaporate away.Formation of deltas When a river approaches a sea, its speed decreases. Thus it deposits its sediments at the river mouth. If the rate of deposition is greater than the removal of the sediments, they will accumulate and build up at the river mouth. A delta is formed. When more alluvium is deposited, the delta extends seawards and smaller channels known as distributaries are formed.
  6. 6. Method Description + - Example Straightening of river Reduces chances Flooding down- channel by removing of flood as there is stream may meanders and curves in no chance for occur as these the river. Reduces length deposition to eroded sedi- River re- of river. reduce depth of ments would be ------- alignment river. deposited at the lower course where speed is slow. Widening and deepening Reduces chances Expensive to The Singapore of the river channel to of flood as more carry out and it is River has been increase ability to water is required not possible for re-sectioned contain water. Wetted to cause the flood. LDCs to afford. during the post-Re-sectioning perimeter is smoothened Difficult to carry independence with cement and granite. out in urban years to increase areas as there is the volume of the a high demand river. for land. Walls of sand, stone and Reduces chances Easily over- China has built concrete built along river of flood as more topped by a dykes along the banks to increase water is required larger flood, Yellow River but capacity of river. to cause the flood. expensive to has failed due to construct and constant Dykes maintain. deposition of sediments in the river bed that decreases the depth. Metal cages filled with Divert the flow of Metal cages rust rocks placed at the river water to the cen- easily and when sides. tre of the channel they are spoilt, to prevent erosion they are useless. Gabions of the river sides, Short term pro- -------- no deposition of tection. sediments to cause floods. Walls made up of large Divert the flow of Undermining of stones that line the banks water to the cen- base of revet- of a river. tre of the channel ments cause it to to prevent erosion collapseRevetments -------- of the river sides, eventually. no deposition of sediments to cause floods.
  7. 7. Roots of trees hold the Channel does not Trees might Royal Forest soil together firmly. Veg become shallower block the Department slows down rate of hence chance of sunlight from (RFD) in surface runoff, preventing flood is reduced. other aquatic Thailand came Planting large amounts of water plants in the up withvegetations from entering the river. river, affecting watershed Decreases amount of the river management eroded sediments into the ecosystem. programme to river. prevent floods.

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