How and why does the load transported by rivers vary? (15)
Sediment in a river comes from many different sources. It could be an exogenetic process meaning the sediment has come from outside of the river by activities such as mass movement; or it could be an endogenetic process where sediment has come from the river channel itself and the material is from the bed and the banks.
There are three types of load-Bedload; the material carried by a river by being bounced or rolled along it’s bed, Suspended load; very small and light material (usually fine clay and silt) transported by the river in suspension, and Dissolved load; products of solution carried by the river even during low discharge. Transportation is the process by which the river carries it’s load; there are four main types of transportation-suspension by which fine clay and sand particles are carried along within the river, even at low discharge, saltation, the process by which sand sized particles are bounced along the river bed by the flow of water, solution, by which dissolved materials are carried along in the river requiring very little energy and finally traction, by which large boulders and pebbles are rolled along the riverbed at times of high discharge.
There are many human factors that cause the load carried by rivers to vary; one factor is that of rapid deforestation. In the Amazon Rainforest, Brazil more than 590,200 sq km of land had been deforested in a period of 10 years, from 1991 to 2001; this was vastly due to human settlement and development of the land. This has meant an increased load in the Amazon River, due to greater soil erosion as the lack of vegetation means less interception and poor protection of the soil from the actions of moving water on its surface, as roots bind and stabilise the soil. This contrasts to the River Congo in Africa which has a smaller load, vastly contrasting of suspended load and dissolved load. This is because the Congo basin is covered by a greater density of vegetation than the Amazon basin which has been subject to significant deforestation.
Another human factor that affects the variations in the load transported by rivers globally is the use of nitrates and phosphates as chemical fertilizers in the agricultural industry; this is a major contributor to the effect on the load of the River Jakara in Kano State, Nigeria, around which a high percentage of the population are subsidence farmers. In 2003 it was observed that there was a peak of 11.46 mg/l on the concentration of nitrate and phosphate in the river, which had entered the river via throughflow and overland flow, due to both chemicals being very mobile in the soil. They affect the load of the river as once carried in solution in the river they react chemically with the surrounding bed rock thus increasing the amount of dissolved and suspended load.
A further predominant human factor affecting the amount and type of load is the use of dams; the construction of dams like that of the Aswan Dam on the River Nile, Egypt, traps sediment upstream causing a significant reduction in the sediment yields downstream. A dam also heavily controls and reduces the river discharge at any point in time which leads to reduced competence and capacity within the river. The load transported in the lower courses of dammed rivers such as the River Nile is comprised of