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.
* Rock type
* Vegetation- dense vegetation helps to intercept rainfall, because of infiltration capacity and prevent overland flow. This will inhibit channel development and thus the reduction of drainage density.
* Rainfall amount, intensity and duration
* Human activities
Stream order refers to stream hierarchy- the way the various stream channels in drainage fit together. Identification of the link in a stream network is by the process of stream ordering. Analysis of the stream order in a drainage basin can help in predicting the shape of hydrograph, and hence flood forecasting. This is a quantitative approach to drainage basin morphology by examining the linear properties of the river basin
The Horton’s law of stream number shows that there is an inverse relationship between stream order and stream number. Another equation associated with stream order is the bifurcation ration which is the ratio of one stream order to the next highest. Studies have shown that in a generally uniform region, the bifurcation ration tends to remain constant. This is a useful method in comparing river system and analysis of stream network.
Various methods of stream ordering have been devised, by Strahler remains the most widely used. According to this method, streams at the head of a drainage system are designated as first order streams. It takes as at least two streams of a given order to form a stream of the next highest order. A confluence, where a change of order occurs, is referred as promotion point.
B) With reference to examples, discuss how human activities have influenced rivers. 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. The removal of sediments form the dam leads to a reduction in flood- deposited nutrients on fields; less nutrients for the fish in the south Mediterranean Sea; accelerated erosion of the Nile Delta and riverbed. Channel and floodplain morphology will have to reach a new equilibrium with the changed sediment and discharge condition.
The diagram below shows the upstream and downstream effect of a dam on the cross and long profile.
There has been a significant change in the alluvial morphology of the channel. Downstream degrading flattens the channel slope so much that there is insufficient energy to transport the available sediments. Reduced peak flows lessen the competence of the river. Thus, only the finer particles are transported, leaving behind a coarser bedloads. This eventually forms a protective amour on the river bed, which reduces further erosion.
Humans build dams to improve their lives, yet their intentions may backfire. Dam failure may rare, but they bring about dramatic effects. In 1972, a dam failure in Rapid City, South Dakota resulted in catastrophic flooding that killed hundreds.
Another form of river manipulation is channelization. This involves construction of structures to improve the ability of channels to transmit floods. The river’s capacity is enlarged and this will increase its velocity. Such structures may fulfill their purpose, but not beneficial to man and the environment. The more rapid movement of water along the improved sections of a river channel can aggravate flood peaks further downstream and case excessive erosion.
The lowering of water tables may cause over drainage in the adjacent agricultural land. On the other hand, channels lined with impermeable materials may obstruct soil water movement. This will lead to surface saturation. Thus, we can see that channelization have many negative impacts on the agricultural industry. There have been concerns about the effectiveness of such methods of controlling rivers. 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.