Low Sedimentary Coasts

Low sedimentary coasts are built, not destroyed. Where the land meets the sea along a gentle, low-lying margin, the dominant story is one of deposition — rivers dump their load, waves pile up sand, and the coastline slowly grows outward. This is the opposite of a high rocky coast, where waves hammer cliffs and erosion rules. The character of an entire coastline — smooth or jagged, advancing or retreating — depends on which process wins.

Along such low coasts, rivers appear to extend their own length. They do this by building coastal plains and deltas at their mouths, pushing the shoreline further into the sea. The resulting coastline looks smooth, with only occasional incursions of water cutting into the land. These incursions take the form of lagoons and tidal creeks. The land itself slopes very gently into the water, so the transition from land to sea is gradual rather than abrupt. Marshes and swamps may abound along these coasts, thriving in the waterlogged, low-lying ground.

Depositional features dominate here. When waves break over a gently sloping sedimentary coast, the bottom sediments get churned up and move readily. This movement builds a characteristic set of landforms:

• Bars — submerged or partly exposed ridges of sand and gravel built offshore.
• Barrier bars — elongated sand ridges running parallel to the coast, separated from it by water.
• Spits — narrow strips of land extending from the coast into the sea, often curved at the tip.
• Lagoons — shallow bodies of water cut off from the open sea by bars or barrier bars.

These features are not permanent. A lagoon would eventually turn into a swamp, and that swamp would subsequently turn into a coastal plain. The coast thus evolves through a predictable sequence: open water becomes enclosed, enclosed water becomes swampy, and swamp becomes dry land.

But this evolution depends on a steady supply of material. The maintenance of all these depositional features relies entirely on a continuous feed of sediments from rivers, waves, and currents. If the supply stops, the features begin to erode. And even a healthy supply cannot protect the coast from extreme events. Storm and tsunami waves cause drastic changes irrespective of the supply of sediments — they can reshape or destroy in hours what took decades to build.

Large rivers play a special role here. Rivers that bring lots of sediments build deltas along low sedimentary coasts. The delta is the most prominent landform of such a coast, a direct extension of the river's load into the sea.

The contrast between India's two coasts makes this concrete. The west coast of the country is a high rocky retreating coast, where erosional forms dominate. The east coast of India, by contrast, is a low sedimentary coast, where depositional forms dominate. The difference is not just visual — it reflects entirely different sets of processes at work.