How Do River Deltas Form?

Where a river reaches a sea or lake, its flow can slow abruptly. Sand, mud, and gravel carried from mountains and valleys can no longer remain suspended as easily, so they settle. Land built outward by this deposited sediment is a delta. The name comes from the resemblance of some examples to the Greek letter delta, but not every delta is triangular.

A river does not build a delta through only one channel. When sediment collects near the mouth, water can seek lower, easier routes and split into several branches. These distributaries deliver water and sediment to different places. When a new branch becomes active, land may grow in front of it; when supply to an older branch decreases, that part of the delta can sink or erode.

The shape of a delta is not determined by the river alone. Waves and coastal currents can spread delivered sediment sideways and smooth the shoreline. Strong tides can create broad mudflats and tidal channels. If a river delivers a great deal of sediment quickly, its branching channels may extend conspicuously toward the sea. Deltas formed by the same basic process can therefore look very different.

A delta is a living landform. It can expand when sediment supply is sufficient, but erosion and flooding can grow when dams, river engineering, or extraction reduce that supply, or when waves and sea-level rise become stronger. Newly deposited sediment can compact and subside over time as well. Delta land does not simply grow in one direction forever.

Deltas often support wetlands, farms, fisheries, and cities, while also being exposed to floods and storm surges. Understanding the balance between sediment carried by a river and sediment rearranged by the sea helps explain why the intricate channels and islands in aerial images keep changing. A delta is a map continually redrawn by river and sea.