Shoal

A shoal is a naturally occurring underwater ridge, bank, or bar made of sand or other loose material. It rises from the floor of a body of water, either close to the surface or breaking it, and is a hazard for ships. In fields like oceanography and geomorphology, these features are called sandbanks, sandbars, gravelbars, or simply bars. When two or more shoals are linked by shared troughs or connected through past or ongoing sediment and water-flow processes, they form a shoal complex.

Shoals are typically long and narrow ridges. They form where currents from streams, rivers, or oceans deposit sediment and granular material, making the water shallower—a process known as shoaling. In the sea, shoals can also develop when barrier islands are submerged by rising sea levels or when inactive delta lobes erode and sink. As coastal landforms, they are a type of ocean bank; as fluvial landforms, they appear in rivers, streams, and lakes. A shoal or sandbar may seasonally cut off a smaller body of water from the sea, such as marine lagoons, brackish estuaries, or the mouths of seasonal freshwater streams and rivers. The term "bar" can describe features ranging from a few meters long in a small stream to marine deposits stretching hundreds of kilometers along a coast, often called barrier islands.

The material in a shoal is usually sand, but it can be any loose matter that moving water can shift—soil, silt, gravel, cobble, shingle, or even boulders. The grain size depends on the strength of waves or currents moving the material, as well as what is available to be worked.

Wave shoaling occurs when surface waves move into shallow water, like near a beach. They slow down, grow taller, and get closer together. This behavior is called shoaling, and the waves are said to shoal. Whether they break depends on their original size and the steepness of the slope. Waves also shoal as they pass over submerged sandbanks or reefs, which can be dangerous for boats. Shoaling can refract waves, changing their direction. For instance, if waves cross a sloping bank that is shallower at one end, they slow more at that end, bending the wave fronts like light through a prism. Refraction also happens when waves approach a beach at an angle or if the beach slope varies.

Sandbars, or trough bars, form where waves break. Breaking waves create a shoreward current and a compensating bottom current offshore. Sand carried by the bottom current is deposited at the break point. Other longshore bars may lie farther offshore, marking where larger waves break or where waves break at low tide. A bay-mouth bar forms across a bay’s entrance, while a bay-head bar forms near the shore at the bay’s head. A barrier bar runs parallel to a beach, usually on a low sandy coast, separated from the beach by a shallow lagoon; it forms during high water and becomes exposed at lower levels.

In Russian geomorphology, a peresyp is a sandbar that rises above water (like a spit) and separates a liman or lagoon from the sea. Unlike tombolo bars, a peresyp rarely forms a continuous strip and usually has one or more channels connecting the liman to the sea.

A harbor or river bar is a sediment deposit at a harbor entrance or river mouth, created by freshwater sediment or wave action on the seafloor or nearby beaches. If beaches are mobile enough or river loads are large, deposition can build a sandbar that fully blocks a river mouth, damming it. This can be a seasonal natural process, forming estuaries and wetlands in the river’s lower course, until the bar is eroded by the sea or the dammed river builds enough force to break through. Harbor bars that block boat access can result from coastal construction (like breakwaters or dune destruction), upriver development (dams, reservoirs, riverbank changes, agriculture, water diversions), watershed erosion from habitat changes (deforestation, wildfires, grading), or artificially deepened harbors that need regular dredging.