Coastal shorelines are constantly reshaped by the interplay of water, wind, and sediment. This dynamic interaction results in two primary types of coastlines: those dominated by material removal and those defined by accumulation. Understanding these processes helps explain the vast differences between a rugged, rocky cliff face and a broad, sandy beach. This article details the nature of a depositional coastline, the forces that build these features, and the resulting landforms.
Defining the Depositional Coastline
A depositional coastline is a shoreline where the accumulation of sediment (sand, silt, and gravel) is the dominant process. These coastlines actively advance seaward as new landforms are constructed from transported material. They are typically found in geologically mature, tectonically stable areas, such as the Atlantic and Gulf coasts of the United States.
These environments are characterized by low-relief coastal plains and expansive, shallow offshore areas. Wave and current energy is usually low, allowing sediment to settle and build up. This contrasts sharply with erosional coastlines, which are high-energy environments featuring rugged shorelines and a net loss of sediment. Depositional coasts thrive when there is an ample supply of sediment, resulting in a constructive environment where land is added, forming diverse features like extensive beaches and coastal wetlands.
The Processes That Build Coastal Features
The construction of a depositional coastline is driven by mechanisms that move sediment and cause it to settle when water energy decreases. Longshore drift is a highly significant process, acting as a conveyor belt for coastal material. This occurs when waves approach the shore at an oblique angle, causing the swash to push sediment up the beach diagonally, while the backwash pulls it straight back down.
This zigzag movement results in a net transport of sediment parallel to the shoreline, which is essential for creating elongated features like barrier islands and spits. The supply of this sediment is equally important, often originating from large river systems that empty into the ocean, bringing vast quantities of silt and sand.
The role of wave energy is crucial, as deposition only happens when the water’s carrying capacity is reduced. Constructive waves, which have a stronger swash (forward push) than backwash (return flow), deposit material and build up the beach face. Destructive waves erode the beach by having a stronger backwash.
Changes in global sea level also influence the process over long geological time scales. Rising sea levels can cause coastlines to retreat, but the continuous supply and redistribution of sediment ensures that depositional features can often migrate landward rather than simply being drowned.
Major Depositional Landforms
The accumulation of transported sediment results in a variety of dynamic landforms, with beaches being the most common example. Beaches are formed when wave action deposits material along the shoreline, consisting of sand, gravel, and shells. Their profile constantly adjusts seasonally, as constructive summer waves build them up and destructive winter waves tend to erode them.
Spits and bars are elongated ridges of sand or shingle that extend into a body of water from the mainland. A spit forms where the coastline changes direction, causing longshore drift to deposit sediment into the open water. A bar is a similar feature that completely crosses the mouth of a bay, connecting two headlands and often creating a lagoon behind it.
Barrier islands are long, narrow, wave-built islands that run parallel to the mainland coast, separated by a lagoon or salt marsh. These features are common along the U.S. Gulf and Atlantic coasts. They are formed by the continual buildup of offshore sediment shaped by longshore transport, providing natural protection for the mainland from storm surges and wave energy.
Deltas represent a distinct depositional landform created at the mouth of a river where it meets a larger body of water. The large volume of river sediment is rapidly deposited as the current slows, forming a characteristic fan-shaped or triangular landmass. The Mississippi River Delta is a classic example of new land built by fluvial deposition.