1. Teaching Aids Service by KRRC Information Section

2. Geological work of Rivers

3. source - the point at which a river starts tributary - a small river / stream which joins the main river. confluence - the point at which a tributary joins the main river. meander - a bend in the river mouth - the point at which a river enters the sea.

4. Methods of River Erosion 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) 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 (vertical--> depth; lateral--> width) Potholes are the product of this process. Attrition: The term is used for wear and tear of the loosened materials that are being carried away which 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. Solution (Corrosion): - The solvent action of water dissolves soluble materials and carry them away in solution

5. Features of river erosion 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. Rapids: A series of very short and fast falls Condition: an area of alternating bands of resistant and less resistant rocks Cuestas: is a ridge formed by gently tilted sedimentary rock in a homoclinal structure. Cuestas have a steep slope, where the rock layers are exposed on their edges, called an escarpment or, if more steep, a cliff. Usually an erosion-resistant rock layer also has a more gentle slope on the other side of the ridge called a dip slope.

7. Rapids

9. Hogback: is an erosional feature made by streams and carved out from very steeply inclined rocks that have proved resistant to erosion. In most cases, the two strata that compose a hogback are different types of sedimentary rock with differing weathering rates. The softer rock erodes more quickly than overlying hard rock. Over time, the softer rock retreats to a point where the hard and soft rock strata are adjacent. This creates cliffs that steepen as the softer rock continues to erode. Hogbacks are often found as ridges along the "eroded flanks of large, tightly folded anticlines and synclines.

10. Hogback

11. Messa Butte Mesa: Flat-topped, steep-sided upland capped by a resistant rock layer, made up essentially of horizontally layered rocks ; normally found in dry environments. Butte: Small, steep-sided, cap rock-protected hill, usually found in dry environments an erosional remnant of a plateau

12. v-shaped valley This is a valley whose typical cross section is shaped like a 'v'. (a cross section is the shape that you would walk if you walked down one side of a valley and up the opposite side). As the river cuts down into the landscape in an upland area, gravity and mass movements roll material down the valley sides to give the valley a typical 'v' shape.

13. Canyons are similar to valleys, except that they are deep-seated, narrow and surrounded by steep sides. These landforms are created by erosion and also, by activities of rivers, winds, and glaciers

14. waterfall waterfall - A steep drop in the bed of a river causing the water in the river to fall vertically. explanation - Waterfalls occur where a band of harder rock crosses the path of a river. The river is less able to erode this harder rock and leaves it as a step in the river's long profile.

15. Fluvial Terraces Fluvial terraces are remnants of the former floodplain of a stream of river. They are formed by the downcutting of a river or stream channel into and the abandonment and lateral erosion of its former floodplain. The downcutting, abandonment, and lateral erosion of a former floodplain can be the result of either changes in sea level, local or regional tectonic uplift; changes in local or regional climate; changes the amount of sediment being carried by the river or stream; change in discharge of the river; or a complex mixture of these and other factors.

16. alluvial fan - This is a 'fan - shaped' mound of alluvium. explanation - This feature is formed when the speed of flow is suddenly greatly reduced e.g. when a stream rushing down the hillside reaches the gently sloping valley floor. When this happens, the river is unable to carry much of its load and this alluvium (usually sands and gravels) is laid down in a fan shape Fluvial Deposition

17. Braiding description - This is where the river channel is broken up into a number of distributaries. explanation - As a river slowly meanders across its flood plain, it often deposits material in the 'middle' of its channel. Sand and single banks often form small islands in this way.

18. flood plain - This is an area of flat land found on either side of a river. This usually becomes wider as the river nears its mouth. explanation - The flood plain is the area of land which is flooded when a river overflows its banks. In this area layers of alluvium are laid down. The soils are thus thick and fertile and the land is flat.

19. Meanders Description - These are bends (loops) in the river. explanation - Lateral (sideways) erosion of the river channel results in the river forming a winding pattern. (tip ! if a river or stream has an absolutely straight pattern then it is probably artificial i.e. a canal). diagram - As the water flows down a river, its speed is faster on the outside of the meander causing erosion and slower on the inside of the meander resulting in deposition. Young stream Vegetatio n on levee

20. levees Description - These are mounds of alluvium piled up along the river's edge. explanation - These are formed by the river depositing the coarsest part of its load (pebbles, gravel and sand) close to the river channel when the river floods. This happens because usually when the water overflows the river bank its speed (and therefore its ability to transport alluvium) is greatly reduced. Sometimes these are built by people in an attempt to prevent the river flooding

21. ox - bow lake Description - This is a truncated (cut off) section of a meander in the river channel which forms a 'c' shape. explanation - This is formed by the loops in a meander eroding the flood plain until eventually the loop in the meander is left cut off from the main river channel.

22. Delta Description - This is a 'D' - shaped area of sandy / muddy sediment built up into the open water where a river meets the sea or a stagnant water body. explanation - Where a river meets the sea or a lake and its speed is slowed down, then deposition on its bed can lead to braiding. This deposition results in the river bed being built up out into the sea. Deltas form wherever rivers encounter standing bodies of water such as lakes or oceans.

23. River Delta

24. The Ganges and Brahmaputra Rivers combined have formed one of the largest deltas in the world, comprising approximately 105 640 km 2. The Ganges River originates near the Tibet/India border, and then flows southeast across India to combine with the Brahmaputra in the country of Bangladesh. The Brahmaputra River has its source in Tibet along the northern slope of the Himalayas, and flows across Assam into Bangladesh. The drainage basin, approximately 1.6 million km 2 in area, is geologically young, with large volumes of unconsolidated sediment available for transport Bay of Bengal Ganga