According to Curiosity from Discovery, a delta forms when particles of mud, fine sand and gravel amass at the base of a river where it enters a larger body of water like a gulf or large lake. Deltas build and extend slowly over time, and they tend to organize into three types of land: upper delta plain, lower delta plain and underwater delta. Deltas are fertile ground for vegetation.
According to National Geographic, deltas support diverse ecological systems with freshwater in the upper delta and salt water in the lower two plains. Pollution is absorbed and filtered from excess water runoff caused by upriver storms and floods as it flows towards the delta. Plants such as lilies and hibiscus, as well as wort, a herbal medicinal plant used to treat muscle strains, swelling and depression, lavishly grow in a delta environment.
Shallow, shifting delta waters support a wide array of indigenous creatures, including bass and catfish, shellfish, snow geese and other migratory birds, as well as a variety of bugs, including grasshoppers and robber flies.
According to the Encyclopedia Britannica, river deltas vary "in size, structure, composition and origin" in accordance with influencing factors such as weather conditions, geography and the rhythm of the tides.