Substrate
Rivers carry large amounts of sedimentand other materials, including pollutants,into most estuaries. Sand and other coarsematerial settle out in the upper reaches of the estuary when the river current slows. The fine, muddy particles, however, arecarried further down the estuary. Theremany of them settle out when the currentslows even more, though the finest parti-cles may be carried far out to sea. There-fore, the
substrate,
or type of bottom, of most estuaries is soft mud