The work that a river does is erosion, transportation and deposition. The amount of work a river can do is dependent on the energy it has and this energy is determined by many factors. The study of water flow in a river channel is known as hydraulics. Hydraulic geometry is the study of the relationships that exist within river channels that ultimately determine how much work the river will be able to carry out. Width, depth, velocity, discharge, channel width, water depth, channel bed roughness and slope angle all change as the river travels from source to mouth and it is these factors either in isolation or in combination with one another that influence how much work the river can do.
A river flows downhill due to gravity and this gives it 2 types of energy - potential and the higher above sea level the river is the more potential energy it will have and kinetic energy, which is generated by the actual movement of water as it travels down slope.
Many factors influence how much energy a river has and therefore how much work it can do. A river uses most of its energy (95% in fact!) overcoming friction with its bed and banks in particular large rocks and boulders. The river has to use energy to flow over these obstacles and therefore does not use it to do work.
Type of flow is also important. In a river channel that is rough and irregular the water very rarely flows in a smooth straight line (known as laminar flow). Instead it is turbulent and flows in many different directions. This water movement is known as eddies. Turbulence creates an upward movement of water that allows a river to pick up material and move it i.e. to do work. However, how turbulent a river is depends on how fast it is flowing. A river with high velocity can overcome friction and still have the energy to erode and transport material whereas a river with slow...