A delta deposit is a type of sedimentary deposit that forms in the shape of a triangle or fan at the mouth of a river. ALLUVIAL deposits are sedimentary deposits that are formed by the action of water, such as rivers, streams, and floods. These deposits are usually composed of silt, sand, and gravel, and are often found near the shoreline of a river or lake. ALLUVIAL deposits are important sources of minerals and other resources, and are often mined for their valuable contents.