Deltas form wherever rivers encounter standing bodies of water such as lakes or oceans. The sudden decrease in energy causes the river to drop its sediment load. Deltaic deposits therefore become finer grained the farther out into the lake or ocean (distal edge). Across the delta, they are coarsest in the distributary channels and finest away from the channels. Many deltaic deposits resemble lake or shallow marine deposits at their distal margins and river deposits at their proximal margins.

Niger Delta, Nigeria
Shatt al Arab Delta

Sediment deposited from Puyallup River, Commencement Bay, Tacoma, WA.  Distributaries near mouth of Mississippi Delta, Louisiana


Delta building into lake.  SE Alaska. Delta and alpine lake.   Banff National Park, Alberta Delta and alpine lake.  Grand Teton National Park, Wyoming

A more detailed explanation of sedimentological features associated with deltaic environments can be found here.