ABSTRACT
All rivers form part of a drainage system, the form of which is influenced by rock type and structure, the nature of the vegetation cover and the climate. For instance, Schumm and Spitz (1996) provided an account of the influence of geology on the lower Mississippi River and its alluvial valley in which they indicated that the river had reacted to uplift, faults, plutonic intrusions, clay plugs and Pleistocene gravels. Indeed, they maintained that the directions of streams are similar to those of the major structural trends of the area, the positions of streams reflecting movements in the rock masses in which the alluvial valley has been formed. An understanding of the processes that underlie river development forms the basis of correct river management. Rivers also form part of the hydrological cycle in that they carry precipitation run-off. This
run-off is the surface water that remains after evapotranspiration and infiltration into the ground have taken place. Some precipitation may be frozen, only to contribute to run-off at some other time, while any precipitation that has infiltrated into the ground may reappear as springs where the water table meets the ground surface. Although, due to heavy rainfall or in areas with few channels, the run-off may occur as a sheet, usually it becomes concentrated into channels that become eroded by the flow of water and so eventually form valleys. In addition, rivers play a part in the coastal environment in that they bring water and sediment
to the coastal zone. They, in turn, are influenced by coastal processes, notably tides. If changes occur within a river system that affect the sediment supply to the coast, in particular, if this is reduced, then erosion may be accelerated in those areas so affected. Conversely, beaches may be expanded where extra sediment is introduced into the coastal zone by changes in river regime. Johnson (1919) distinguished three elements in a shoreline, the coast, the shore and the
offshore. The coast was defined as the land immediately behind the cliffs whilst the shore was regarded as that area between the base of the cliffs and low-water mark; the area that extended seawards from the low-water mark was termed the offshore. The shore was further divided into foreshore and backshore, the former embracing the intertidal zone whilst the latter extended from the foreshore to the cliffs. Those deposits that cover the shore constitute the beach.
