A sandbank is the name for an accumulation of sand or gravel in places where river or coastal currents slacken. A sand bar is a sandbank that is an obstruction to shipping, often in the mouth of a river.

Sandbanks build up because the river current is too slow to carry sand along. Sandbanks therefore tend to form toward the end of a flood when the river speed is lessening. They tend to be washed away during a flood.

Sandbanks become noticeable during periods of drought when the water level is low. Otherwise they may remain hidden.

Sandbanks are more common in rivers with mainly sand and silt to carry, rather than with those mostly carrying clay where there might not be enough sand to form a bank. This is why sandbanks are common on some rivers and not others. The Mississippi is famous for its sandbanks. Many rivers draining from glaciers also have sandbanks. One of these is shown in the picture above.