Deposits formed by rivers. An alluvial river is one which flows within its own fluvial sediments, as distinct from one that has incised into the underlying bedrock. A river accumulates deposits because its capacity to carry sediment has been exceeded, and some of the sediment load is deposited. Rivers tend toward a state of dynamic equilibrium, in which they adjust their slope in response to changes in discharge and sediment load. The result is a channel profile that is steep in its source areas but flattens out downstream, and is graded to a slope of zero where the river discharges into a lake or the sea.

Fluvial sedimentary accumulations range from temporary bars deposited on the insides of meander bends as a result of a loss of transport energy within a local eddy, to deposits tens to hundreds of meters thick formed within major valleys or on coastal plains as a result of the response of rivers to a long-term rise in base level or to the uplift of sediment source areas relative to the alluvial plain. Both these processes perturb the graded profile so that it tends to rise, creating space, or accommodation, for sediment. The same processes control the style of rivers and the range of deposits that are formed, so that a study of the deposits may enable the geologist to reconstruct the changes in controlling factors during the accumulation of the deposits.