Erosion of Earth's topography entrains sediment in rivers flowing across mountains‚ canyons and other naturally steep landscapes within the catchment. This silt‚ sand and gravel is transported variably by suspension in the water column‚ bouncing along the river bed (saltation)‚ or rolling larger pebbles (traction). With a deceleration in water velocity‚ and therefore carrying capacity‚ as topography flattens‚ this material is deposited in a fan shape at the mouth of the river on the shallower plain of the break in slope‚ becoming progressively finer-grained with distance from the source.