Shoal

A shoal is a natural submerged ridge, bank, or bar.

It consists of sand or other unconsolidated material. This material rises from the bed of a body of water. The ridge stays close to the surface or rises above it. Because the water becomes shallow, shoals pose a danger to navigation. They are often called sandbanks, gravelbars, or simply bars. In oceanography and geoscience, these features are vital to study. When two or more shoals are connected, they form a shoal complex.

This happens through shared troughs or sedimentary processes.

Shoals form through a process called deposition. This occurs when a stream, river, or ocean current moves sediment. The moving water carries granular material like sand, silt, or gravel. As the water slows, it can no longer hold these particles. The material drops to the bottom, causing localized shallowing. This specific process is known as shoaling. Marine shoals can also form when barrier islands drown. This happens due to episodic sea level rise. Other shoals develop from the erosion of inactive delta lobes.

Waves play a central role in how these features behave. As surface waves move toward shallow water, they undergo wave shoaling. The waves slow down and their height increases. The distance between the waves also decreases. The waves may eventually break against the shallow bank. This can be very treacherous for ships and boats.

Shoaling can also cause refraction. This means the waves change direction. If a bank is shallower at one end, waves slow more at that end. The wave fronts then bend, much like light passing through a prism.

There are several distinct types of these landforms. Sandbars, or trough bars, form where waves are breaking. Breaking waves create a shoreward current. This is balanced by a counter-current along the bottom. Sand is carried by the offshore bottom current. It is then deposited where the current reaches the wave break.

Another type is a tombolo. A tombolo is a bar that forms an isthmus. It connects an island or offshore rock to the mainland. In Russian geomorphology, a peresyp is a specific type of sandbar. A peresyp rises above the water level like a spit. It separates a lagoon or liman from the sea.

Shoals can be found in many different environments. They can be fluvial landforms in rivers, streams, and lakes. They can also be coastal landforms in the sea. A sandbar might only be a few meters long in a stream. However, marine depositions can stretch for hundreds of kilometers. These massive features are often called barrier islands.

Some shoals act as seasonal separators. They can separate a lagoon or estuary from the open sea. This helps create unique brackish or freshwater environments.

Harbor and river bars are specific sedimentary deposits. They form at river mouths or harbor entrances. This happens from freshwater sediment or wave action on the seafloor. Sometimes, a sandbar can grow large enough to block a river mouth. This can dam the river and create wetlands or estuaries.

This process can be interrupted by the sea eroding the bar. Or, the river may develop enough head to break through. Human activities can also affect these bars. Construction of breakwaters or damming rivers can change how they form. Even deforestation or wildfires can cause watershed erosion that affects deposition.

Humans have interacted with shoals since prehistoric times. People have inhabited these areas to exploit marine resources.

In modern times, people choose these sites for views or water access. However, these locations are often prone to storm damage. Some places are even named after these geological features. For example, Muscle Shoals in Alabama is named for its many mussels.

These landforms are deeply connected to coastal erosion and the formation of spits. They can even be precursors to the expansion of beaches and dunes.