Sediment is material that settles to the bottom of a liquid, or layers of sand, soil, and tiny rock particles that pile up over time in rivers, lakes, and oceans. When you shake up a jar of river water, the cloudy bits that drift down and collect at the bottom are sediment.

Rivers carry sediment downstream as they flow, dropping heavier particles when the water slows down. Over millions of years, layers of sediment can build up thousands of feet deep. These layers can eventually harden into sedimentary rock like sandstone or limestone. Scientists study sediment layers like reading the pages of a history book: each layer tells them what happened in that place at that time.

Sediment can cause practical problems too. Harbors and channels need regular dredging to remove built-up sediment so ships can pass through. Reservoirs behind dams slowly fill with sediment, which reduces how much water they can hold. But sediment also creates new land: the rich farmland of river deltas like the Mississippi Delta formed from sediment deposited over thousands of years.