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Explain what it is meant by, in a drainage basin, by the terms drainage density and stream order

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Introduction

A) Explain what it is meant by, in a drainage basin, by the terms drainage density and stream order. The drainage basin is an area of land drained by a river and its tributaries. Drainage Density represents an approach to the quantitative analysis of drainage basin. It is the average length of channel per unit area of the drainage basin. It is calculated as shown below. Drainage density is a qualitative analysis of the areal properties of a river basin . Drainage density is useful as a measure of frequency and spacing of streams within the drainage basin. It also helps to determine the texture of dissection of a drainage basin- the extent to which the landscape is cut into by river valleys and in turn influence slope development. Generally, there are 4 categories of drainage density and corresponding texture of dissection. Very low- less than 8km/km sq coarse Medium low- 8-20 medium High density- 20- 200 fine Very high density- more than 200 ultra fines Drainage density and texture of dissection varies in response to these factors: * Permeability of surface- a highly permeable material such as sand tends to give low drainage density because of high infiltration capacity and little water is available as surface runoff to maintain channels. ...read more.

Middle

Have the results been always beneficial? Human activities on rivers can either be deliberate or advertent. The river is an efficient pathway between places and any actions in one place may be transmitted like a chain reaction to have consequences across time and space. Although it is not of man's intentions to bring about disastrous consequences on rivers, their actions are often not beneficial to the environment. Deliberate actions are made to regulate rivers. Dam constructions have been encouraged by growth in water and energy demands and advances in technology. Most dams have achieved their aims of regulating discharge. They are also highly successful in meeting the needs of surrounding communities. However, dams have many environmental consequences that are not beneficial to man. Dams are able to retain high amounts of sediments and this is detrimental to the environment. Since the construction of Aswan High Dam on River Nile, the silt load is lowered throughout the year and the seasonal peak is removed. The Nile now only transports about 8 % of its natural sediments below the dam. ...read more.

Conclusion

For example, the Mississippi basin is reinforced by levees. Yet catastrophic flooding still took place in 1993 that destroyed 7 million hectares of farmland. This shows that flood protection policy may backfire causing worst damage. Advertent changes like urbanization have many negative impacts on rivers. The increase in concrete surfaces and efficient sewage system will promote high and fast discharge. This will in turn lead to an increase in peak flood flows of a river. The below shows the contrast between storm hydrographs of an area before and after urbanizations. A flashier storm hydrograph will make the area more prone to flooding. Chinese experts believe that local deforestation, uncontrolled building and lack of flood control have led to massive floods that threaten loves and homes of millions. Clearing of land for cultivation may cause soil erosion which result in large quantities of sediments being moved into channels and floodplains. Mining can also lead to very major change in channel morphology. As mining often requires the use and deposition of large quantities of water, thus high amount of waste is moved to rivers. Both human activities may lead to the aggradations of the channel bed- the building up of land surface. 1 ...read more.

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