Sediment is earth material. Soil and rock transported from hillslopes and other land surfaces into watercourses are the major sources of sediment in waterbodies. Sediment encompasses particles ranging in size from clay to boulders.It is the natural work of rivers to move these materials across the landscape.oil and rock transported from hillslopes and other land surfaces into watercourses are the major sources of sediment in waterbodies. Sediment encompasses particles ranging in size from clay to boulders. It is the natural work of rivers to move these materials across the landscape.