RIVERS AND ASSOCIATED LANDFORMS

Prerna Bharti Assistant Professor (Guest) B. N College, Patna University

River

 Running water is the most important agent of erosion on the continents and the stream valleys are the most common landforms.

 Rivers flowing to the oceans drain about 68 % of the Earth's land surface. The remainder of the land either is covered by ice or drains to closed basins.

 River gradually mould the land by eroding away the material in some place and depositing it in other Parts of River

 tributary : a stream flowing into or joining a larger stream  distributary : numerous stream branches into which a river divides where it reaches its delta  downstream : moves toward mouth of river (delta)  Delta : a large, roughly triangular body of sediment deposited at the mouth of a river  Meander : a broad, looping bend in a river  Braided : river is divided into multiple channels by alluvial islands. Braided rivers tend to have steeper gradients

Drainage Pattern

Dendritic  Irregular branching pattern (tree like) in many direction.

 It is common in massive rocks and in flat lying strata

Parallel • Parallel or sub-parallel drainage formed on sloping surface. • Common in terrain with homogeneous rocks. • Development of parallel rills, gullies or narrow channels are commonly seen on gently sloping surface • Streams radiates out from the center of the topograhic high

• common in Volcanic terrain

• Channels marked by right-angle bends

• Commonly due to presence of joints and fractures in the massive rocks or foliation in metamorphic rocks

Consequent Stream: A stream following the slope. Development of slope may be due to sudden tectonic uplift

Subsequent Stream: A stream that develops later on, carving the softer rocks and flow at almost right angle to the original slope of the land Upper course

. Vertical erosion is the dominant process

. River channel narrow and shallow, often lined with large angular boulders

. Quite steep gradient

. Velocity quite low because so much energy used in overcoming friction with the rocky bed and banks of the river V-shaped valleys and interlocking spurs

The transport of large-calibre sediment may cause intensive vertical erosion which produces a steep-sided V- shaped valley profile. Upper course: Rapids and waterfalls Rapids form as a result of the river cutting down rapidly in a localised section of the river. The main characteristics of rapids are distinctly steeper gradients marked by steps in the channel and high turbulence, which is the result of large bed load in the channel or steps. Rapids form: - Over different bands of geology that vary in resistance. Where the river flows from hard to soft rock, the softer rock is eroded relatively rapidly. - Where there is a sudden change in the gradient of the river. - May form downstream of a waterfall.

Rapids Waterfalls Waterfallsare steep steps in the long profile of a river’s course.

Thornton Force (Ingleton,UK) Formation of waterfall and a gorge

Watch a short animation which shows the formation of a waterfall and a gorge Explain the formation of a waterfall. (7)

They may be formed by a band of resistant rock occurring on the river bed. The hard rock forms a cap rock, underlain by the soft rock. Differential erosion results and the softer rock is eroded faster than the hard rock, creating a steep gradient in the bed. The increased velocity gained by the water as it falls over the step further increases the rate of erosion of the soft rock. Eventually the overhanging, more resistant rock collapses due to gravity and the position of the waterfall retreats in an upstream direction. Potholes

Potholes are cylindrical holes ‘drilled’ into the rock by turbulent high-velocity flow. The water contains some of the load being carried by the river and this hits the base and sides of the hole to both widen and especially deepen it, smoothing the edges. This is a particular application of the abrasion process, known as pothole drilling.

Fluvial Landforms – Middle course of river

River Channel: . wider and deeper . Volume of water greater than that in upper course. . Gradient is less than upstream portion of the river course. FLUVIAL LANDFORMS

Alluvial Fan

 Alluvial fans are fan- shaped deposits of water-transported material (alluvium).  They typically form at the base of topographic features where there is a marked break in slope.  Consequently, alluvial fans tend to be coarse-grained, especially at their mouths. At their edges, however, they can be relatively fine-grained. Braided Channel  Braided Rivers exhibit numerous channels that split off and rejoin each other to give a braided appearance. They typically carry coarse- grained sediment down a steep gradient.

Meandering Pattern

 In contrast to braided rivers, meandering rivers typically contain one channel that winds its way across the floodplain. As it flows, it deposits sediment on banks that lie on the insides of curves (point bar deposits), and erode the banks on the outside of curves.

Levee s

 Deltas form wherever rivers encounter standing bodies of water such as lakes or oceans.  The Ganges and Brahmaputra Rivers combined have formed Ganga one of the largest deltas in the world, comprising approximately 105 640 km2.  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 km2 in area, is geologically young, with large volumes of unconsolidated sediment available for transport Bay of Bengal

Bird foot Delta

Young stream Vegetation on levee River system

Tributaries Distributaries Trunk stream

Headwater