A bar in a river is an elevated region of sediment (such as sand or gravel) that has been deposited by the flow. Types of bars include mid-channel bars (also called braid bars, and common in braided rivers), point bars (common in meandering rivers), and mouth bars (common in river deltas). Bars are typically found in the slowest moving, shallowest parts of rivers and streams, and are often parallel to the shore and occupy the area farthest from the thalweg.

The locations of bars are determined by the geometry of the river and the flow through it. Point bars form on the inside of meander bends in meandering river because the shallow flow and low shear stresses there reduce the amount of material that can be carried there. The excess material falls out of transport and forms the bar.