Deposits formed by rivers. A river accumulates deposits because its capacity to carry sediment has been exceeded, and some of the sediment load is deposited. Such 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. 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. Depositional systems and environments River Stream transport and deposition

Coarse debris generated by mechanical weathering, including boulders, pebbles, and sand, is rolled or bounced along the river bed and is called bedload. The larger particles may be moved only infrequently during major floods. Finer material, of silt and clay grade, is transported as a suspended load, and there may also be a dissolved load generated by chemical weathering. Whereas the volume of sediment tends to increase downstream within a drainage system, as tributaries run together, the grain size generally decreases as a result of abrasion and selective transport. This downstream grain-size decrease may assist in the reconstruction of transport directions in ancient deposits where other evidence of paleogeography has been obscured by erosion or tectonic change. Mass wasting

River deposits of sediment occur as four main types. (1) Channel-floor sediments consist of the coarsest bedload, such as gravel, waterlogged vegetation, or fragments of caved bank material. (2) Bar sediments are accumulations of gravel, sand, or silt which occur along river banks and are deposited within channels, forming bars that may be of temporary duration, or may last for many years, eventually becoming vegetated and semipermanent. (3) Channel-top and bar-top sediments are typically composed of fine-grained sand and silt, and are formed in the shallow-water regions on top of bars, in the shallows at the edges of channels, and in abandoned channels. (4) Floodplain deposits are formed when the water level rises above the confines of the channel and overflows the banks. Much of the coarser floodplain sediment is deposited close to the channel, in the form of levees; silt and mud may be carried considerable distances from the channel, forming blanketlike deposits. Floodplain

The thickest (up to 6 mi or 10 km) and most extensive fluvial deposits occur in convergent plate-tectonic settings, including regions of plate collision, because this is where the highest surface relief and consequently the most energetic rivers and most abundant debris are present. Some of the most important accumulations occur in foreland basins, which are formed where the continental margin is depressed by the mass of thickened crust formed by convergent tectonism. Basin

Thick fluvial deposits also occur in rift basins, where continents are undergoing stretching and separation. The famous hominid-bearing sediments of Olduvai Gorge and Lake Rudolf are fluvial and lacustrine deposits formed in the East Africa Rift System. Fluvial deposits are also common in wrench-fault basins, such as those in California.

Significant volumes of oil and gas are trapped in fluvial sandstones. Placer gold, uranium, and diamond deposits of considerable economic importance occur in the ancient rock record in South Africa and Ontario, Canada, and in Quaternary deposits in California and Yukon Territory. Fluvial deposits are also essential aquifers, especially the postglacial valley-fill complexes of urban Europe and North America.