Sediment refers to the solid particles of rock, soil, minerals, and organic matter that are carried by water. Sediment is laid down in layers of flat sheets at the bottom of the water column when the water's speed slows down. When the water current increases speed, the sediment is carried away again.
If suspended in the water column, the sediment particles act like projectiles against any larger object that resists the flow of the water. When the water carrying the sediment slams onto a resisting object, the suspended sediment acts like an abrasive that scratches and scours the surface of the resisting object.
For example, a rock cliff or concrete wall submerged in water will suffer erosion and abrasion from countless repeated hits by any suspended gravel, sand, or silt particles when a wave slams on to it. The solid particles bludgeon the rock face, each small particle making a small scratch on the cliff or wall. Any soft material is dug out, washed away, or dissolved by the water while the surface of the harder materials are scratched by the particles. After constant wear and tear, even coarse rock will be worn smooth by the waves and sediment.
Water that does not have sediment takes longer to abrade the resistant object but water can still dissolve the salts and minerals of the rock. Eventually the water carves away the softer substrates and leaves only the hardest rock still standing.