Sandbars are common features of shorelines all over the world. They provide a scenic feature for boaters and swimmers, but a danger as well, as their shape and location constantly change and they are revealed and hidden by tidal action. Some basic knowledge about how sandbars form and endure can help boaters prepare themselves to safely navigate them.
Waves approaching a shoreline begin shoaling and breaking a short distance from the land as the body of water they cross grows rapidly more shallow. As they break at the shoreline, the waves lose their forward momentum and a reverse current is created at the bottom as the water returns.
This current picks up sand, silt and rocks and carries it a short distance out. When the current dissipates into the larger body of water, the material is deposited. The deposit gradually rises in height until an underwater shoal or bar is formed. As the wave action and reverse currents along a straight shore are regularly perpendicular to the shore, the bar grows into a long and narrow shape, and forms parallel to the shore.
The stronger the action of breaking waves at the shoreline, the larger the material that can be deposited on the bar and the higher it may rise. In areas where the bottom is made up of pebbles and shell, bars generally don't form to any height. In areas where waves are high and the bottom is sandy, sandbars can reach an elevation of more than a foot from the surface of the water.
Sandbars eventually can close off a stretch of shoreline and turn the water directly adjoining that shore into a lagoon. Over centuries, sandbars can turn into barrier islands, which serve to protect a coast from storm surge and damaging waves. Reshaping of the land by wave and storm action is a regular feature of coastal ecosystems.
Sandbars are common at the mouths of rivers, where strong currents and eddies in the water vigorously work the deposits of sand or silt at the bottom. The bar formations constantly change their shape and location as the river rises, falls and changes its course. This makes navigating an entrance into the river from the sea a hazardous endeavor, and many boats have been wrecked or grounded on coastal river sandbars.