Any energy the river possesses that wasn't lost through friction or used for erosion can be used to transport a river's load. The river's load is obtained either by material that has been washed into or fallen into the river from the valley sides or material that has been eroded by the river from it's beds or banks. There are four main transportation processes.
Traction is when large stones or boulders are slowly rolled along the river bed by the water moving downstream. This only happens when there is high discharge.
Saltation is when smaller stones leap-frog along the river bed and is associated with high energy conditions.
Suspension is when small particles (sand and silt) are carried along suspended in the river current and this is the main factor behind river discolouration.
Solution is when material is dissolved and is transported within the moving mass of river water.