Delta landforms are sedimentary wetlands that form at the mouths of rivers where the velocity of the water slows. As the sediment builds, triangular landforms separate the river into two or more channels.
Materials suspended in the river depend on the movement of water to keep them moving downstream. A slight reduction in speed causes the heaviest sediment to fall to the bottom of the river and, as it slows more, finer particles begin to settle to the streambed. The finest of silt travels into the lake or ocean to form rich delta land. As deltas grow larger, they sometimes cut off the flow of the river, causing it to cut new channels and form additional deltas.
Deltas, like other wetlands, tend to be diverse in the plant and animal life forms they support. They absorb floodwaters from the streams along with storms from the larger body of water. They also serve as filters, absorbing pollution from upstream and protecting lakes and oceans.
Some rivers do not have delta landforms. Some move too fast for the sediment to drop out of suspension. Others, including the Columbia River, are subject to powerful waves that keep any sediment from forming. Strong tides, such as those at the mouth of the Amazon, prevent delta formation.