River Load

River Load

1. Dissolved load consists of soluble materials carried as chemical ions, so there are no measurable particles

2. Large particles only form part of the load during and immediately after extreme events that lead to increases in discharge

   • In these circumstances, the river increases it's competence to allow larger particles to be carried
   • Large boulders and cobbles often form the load in the upper course of a river because it seldom has the energy to be able to transport it downstream

3. In general, the particles decrease in size the further downstream they are (Bradshaws model), this is due to increased erosion with processes like attrition decrease sediment size

4. Total sediment yields tend to increase with distance downstream, mainly due to increases in average discharge and velocity in the lower courses. Here, the river posses a greater capacity and is able to transport more material

5. Spatial variations in load can be seen in rivers across the world due to the following factors

   • Size of drainage basin
     • larger basins with many tributaries have a greater potential for transporting sediment.
   • Rock Type
     • in areas where the underlying geology consists of softer rocks like clay and sandstone, the sediment load consists mainly of sand or clay particles
     • where the rock is limestone, more material will be transported as dissolved load because limestone is soluble
     • Moving water doesn't easily erode igneous rocks (like granite and basalt). Therefore, total sediment yields will be considerably lower
   • Relief
     • in drainage basins with low relief - a small difference in altitude between the source and it's base level - the energy available for erosion and transport is limited
       • such rivers have low loads compared with river that have higher upper reaches in areas of high relief
   • Precipitation
     • low loads are generally found in areas with low precipitation rates. This is because less water is available as runoff compared with drainage basins with high precipitation
     • Seasonal differences in sediment yield occur in areas where the climate has wet and dry seasons - where snow melt in spring adds to normal run off from precipitation
   • Human activity
     • in areas where rapid deforestation is taking place, there have been marked increases in load. mainly caused by increased soil erosion which occurs because the vegetation that protected the soil from the action of moviong water have been removed
       • there is also reduced uptake of water by trees and other plants
         • the result is that soil is washed into the river and adds to the suspended load
     • farmers use of nitrates and phosphates as chemical fertilisers means that these substances enter the river by through flow and overland flow and are then transported in solution
     • the construction of dams will trap sediment, significantly lowering sediment yields downstream