Deltas form at the mouths of large rivers, when sediments and silt accumulate rather than being washed away by currents or ocean waves. Over time, a complex set of channels, sand barriers and marshes form at the mouth of the river. As sediments continue to accumulate, the course of the river may even be changed. The name delta comes from the resemblance of the triangular fan-shaped mouth of the famous Nile River Delta to the Greek letter delta (Δ).
Bar-built estuaries and deltas both have large deposits of silt, mud and sand near their mouths, but the processes forming each of these estuaries are quite different. In deltas, sediments are carried downstream and deposited at the mouth of the river. Bar-built estuaries are formed by ocean waves and currents pushing sediments shoreward, building up sandbars and forming barrier islands (Ross, 1995; Sumich, 1996).
The image below is the Mississippi Delta as viewed from space. The city of New Orleans is in the center left-hand side of the image. The massive deposits of sand and silt that make up the delta appear light tan in color, surrounding the peninsula. (Photo: Weeks Bay NEERS site)