BARS
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,[1] and are often parallel to the shore and occupy the area farthest from the thalweg.[2]
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.

2nd definition

A bar is a long stretch of beach material (sand or shingle) that joins together two headlands. A lagoon (an area of seawater which is party or completely cut off) usually forms behind the bar. An example of a sand bar is Slaton Ley in Devon.
A bar is a barrier of sand stretching across a sheltered bay. It’s only able to extend across the bay due to the absence of any large river. Bars may form in several ways. One way is when a spit is able to grow right across a bay. A second is when a sand bank develops some distance off the shore, but parallel to it. Waves slowly move the sand bank towards the coast until it joins with the mainland. In both cases a lagoon is usually formed landward side of the bar.

3RD definition

A ridge of sand that blocks off a bay or river mouth. It will create a lagoon behind it is across a non-river bay.