A sandbar is a long ridge or mound of sand that builds up in a river, lake, or ocean, usually just beneath or slightly above the water's surface. Sandbars form when flowing water slows down and deposits the sand it's been carrying, creating an underwater hill or island.

If you've ever waded in shallow water far from shore, you might have been standing on a sandbar. Rivers often have sandbars around their bends, where the current slows and drops its load of sand. Ocean sandbars run parallel to beaches, sometimes creating calm pools of water between the shore and the bar.

Sandbars shift and change constantly. A storm might wash one away completely or build a new one overnight. Boat captains have to watch carefully for sandbars because running aground on one can damage a boat's hull or leave it stuck until the tide rises. The Mississippi River is famous for its shifting sandbars, which caused problems for steamboat pilots like Mark Twain.