A sandbar is a depositional landform built up by the accumulation of loose material like sand, gravel, or coarse sediment. Generally appearing as a long, narrow underwater ridge, a sandbar may be partially or fully submerged beneath the water’s surface. Its physical appearance is always in flux, changing visibly with the tides and the intensity of the water flow around it. It represents a temporary storage location for sediment that is constantly being reshaped by natural forces.
The Essential Anatomy of a Sandbar
The most defining visual characteristic of a sandbar is its profile, which is a subtle ridge rising from the seabed. This ridge is typically elongated and often runs parallel to a coastline in a marine environment. In coastal settings, the sandbar system usually includes a higher point, known as the bar crest, and a deeper area between the bar and the shore, called the trough.
The sand and sediment composing the bar crest are frequently coarser than the finer particles found in the adjacent, deeper trough. Sandbars are most easily identified visually by the way waves interact with them. As incoming waves approach the bar, the shallowing water causes the wave height to increase until it breaks directly over the crest. This visible line of breaking waves serves as a clear indicator of the sandbar’s position, even when the feature is completely underwater.
The visibility of the sandbar changes dramatically with the tidal cycle. At high tide, a sandbar may be entirely hidden, but during low tide, the crest can become exposed, creating a temporary, walkable strip of sand. Coastal areas sometimes feature multiple, parallel ridges in the nearshore zone. The material itself can vary from light-colored beach sand to darker, coarser sediment, depending on the local geology and the energy of the water moving it.
How Water Movement Creates Sandbars
The formation of a sandbar is a direct result of hydraulic forces losing the energy required to transport their sediment load. Waves and currents constantly move sand and other particles, but when the water slows down, this material settles out and begins to accumulate. Coastal sandbars form as wave turbulence excavates a trough and deposits the excavated material onto the ridge’s offshore flank.
In the nearshore zone, a process called longshore drift is particularly effective in building up these features. This occurs when waves approach the shore at an angle, creating a current that moves sediment parallel to the coast. Where this current slows down—often due to changes in the coastline or bathymetry—the sediment is deposited, and the sandbar grows over time.
Sandbars are highly dynamic structures, meaning their size, shape, and position are always shifting. They migrate shoreward during periods of calm wave action and can be pushed seaward or flattened during high-energy events like storms. This constant reshaping means a sandbar observed one season may not possess the same dimensions the following year.
Coastal vs. Riverine Sandbars
The appearance of a sandbar is influenced by its environment, leading to distinct forms in different bodies of water. Coastal sandbars, which include offshore bars and barrier bars, are typically long and straight, lying parallel to the mainland coast. When a coastal sandbar accumulates enough material to rise permanently above the high-tide level, it can become a barrier island, creating a sheltered lagoon or bay behind it.
Riverine sandbars, by contrast, are found within the channels of rivers and streams. These are often called point bars and develop on the inside bend of a meandering river, where the flow velocity is significantly slower. The reduced current on the inner curve causes the river to drop its sediment, resulting in a crescent or elongated shape that conforms to the river bend.
The sediment in riverine sandbars often displays complex internal structures, such as small ripples and dunes, reflecting the changing flow conditions of the river. The morphology of these inland bars is closely tied to the water level, becoming exposed during low-flow periods and fully submerged when the river is high. Differences in shape and composition reflect the primary forces at work: oscillatory wave action on the coast versus unidirectional current flow in a river.