RIVERS

1. RIVERS

2. The Hydrological Cycle

3. The Hydrological Cycle • The water cycle is the process by which water travels from the Earth's surface to the atmosphere and then back to the ground again. It is a constant process with the same water going through the cycle over and over again. • There are 1,500 km3 of water on or near the earth's surface. This water circulates between the oceans, the atmosphere, and the land surface of the earth. This pattern of circulation is known as the hydrological cycle (or the water cycle).

4. Water is stored in the oceans, in the ice caps, as groundwater, and in lakes. At anyone time: • 97.3 % of the world's water is salt water in the oceans and seas. • 2.1% is in the Greenland and Antarctic ice caps. • 0.01 % is in rivers and lakes. • Only 0.001 % is held in the atmosphere as clouds and water vapour.

5. Water is transferred: • To the atmosphere by evaporation and transpiration. • From the atmosphere by precipitation, as rain or snow. • To the oceans by rivers, streams, and groundwater seepage.

6. How Rivers are Formed? • When rain falls, some soaks into the soil. • When the soil is saturated, and can hold no more water, the excess rainwater starts to flow across the surface. It flows in small rills or as a continuous sheet of water. This is overland flow. • Surface flow runs into rivers and streams, which are usually permanent features of the landscape. • Some soil water seeps slowly into rivers and streams over a long period. This is called through-flow. • Rivers and streams carry excess water out of the drainage basin.

7. The source of a river may be a spring, a lake or a marsh, but it is generally in an upland region, where precipitation is heaviest and where there is a slope down which the run-off can flow. The uplands therefore form the catchment areas (drainage basin) of rivers. The crest of the mountains is the divide or watershed from which streams flow down the slopes on both sides to begin their journey to the oceans.

8. Drainage Basin/Watershead/Catchment Area

9. The amount of water carried in a river is called its discharge. The discharge usually increases as the river flows downstream and is joined by several tributaries and as more ground water and surface flow enter the it. • Discussion • Explain how river discharge may decrease downstream.

10. Drainage Patterns • Groups of streams merge to form a pattern. The patterns that may be formed by streams as they drain a landmass include: • radial • trellis • dendritic

11. Radial Pattern • A radial drainage pattern develops on a volcanic dome. Streams flow outwards downhill from the dome in a radial pattern just like the spokes of a bicycle wheel.

12. Trellis Pattern • The tributary stream join the main stream at right angle. This pattern is usually guided by the rock structure

13. Dendritic Pattern • A dendritic pattern has a tree-like appearance with its consequent or mainstream joined by many tributaries at an acute angle. This pattern develops on the slope of uniform non-pervious (non-permeable) rocks.

14. River Erosion • Rivers can erode through the following processes • Hydraulic action –This is the loosening and the sweeping away of material by the force of the river water itself. • Solution (Corrosion) – This takes place when water dissolve the minerals in rocks. For example calcium carbonate in limestone is easily dissolved and removed in solution.

15. Abrasion (Corrasion) – this is the process by which stone, sand grains, and other particles carried by the river are thrown against the bed and banks and help to erode them. • Attrition – This is the wear and tear of the transported material themselves when they roll and collide with one another. The coarser boulders are broken down into smaller stones and the angular edges are smoothed and rounded to form pebbles.

16. River Transport • When the river flows it carries with it eroded materials. These comprise the rivers load and may be transported through the following actions. • Traction – The coarser materials such as stones and boulders are rolled along the river bed. • Suspension – Fine clay particles float along in the river water. • Solution – Minerals are dissolved in the water are transported through this action. • Saltation – In this type of transport, particles are bounced along the river bed

17. River transportation

18. River transportation - saltation

19. River Deposition • Deposition is the opposite of erosion. Deposition is where a river lays down or drops the load or material that it is carrying. • If the force of a river drops, the river cannot carry as much load. This is when the river deposits its load.

20. How material is deposited. • Large boulders will be deposited first. They can only be moved when the gradient of the bed is very steep, and the river is full of storm water after a flood. • Gravel and small stones are the next to be left behind. Many rivers deposit an alluvial fan of coarse material where they leave a mountain area, and the gradient is suddenly reduced. • Silt can be carried along even when the river is flowing quite slowly, but most rivers deposit silt along their valleys. There will usually be a silt deposit left on the valley floor after a flood.

21. Fine clay particles settle very slowly. There may be clay deposits on the bed of a lake, or clay mixed in with silt on the valley floor. • The salts which are carried in solution are not usually deposited at all. They are carried into the sea, where they remain. Salt will only be deposited when a river dries up completely in a desert area, or flows into an enclosed 'salt lake' like the Dead Sea where the water evaporates.