River Delta: How It Forms

By: Earth How · Last Updated: May 17, 2020

River deltas form when streams and rivers carry too much sediment clogging the entrance into the basin. Because increased sediment builds up, this forces the basin entrance to widen forming a delta.

Deltas take different shapes.

For example, if you have too much sediment deposits in excess, a feature like a bird-foot delta forms.

Let’s review the anatomy of how river deltas form.

Deltas are fertile and often overcrowded

When an area floods, sediments spread across the region. When water recedes, soils are rich and fertile for agriculture purposes.

In this diagram, the river enters a lake. The water flow decelerates and loses energy with sediments dropping in. A delta forms depositing a prism of sediments that gradually tapers out toward the lake interior.

Throughout geologic time, deltas deposit layers upon layers of sediment. For example, the diagram below depicts a vertical cross section through geological layers deposited by rivers, deltas and lakes.

Deposits from a series of successive deltas build out increasingly high in elevation as they migrate toward the center of the basin.