When a channel has built above the surrounding delta plain, the next major flood can overtop and cut through the levees to open a new channel which may build a new system. The old channel is eventually abandoned and filled with bog and meadow deposits. This process creates fan-like deltas as channels shift, divide, and build outward over thousands of years. A delta's structure is like a layer cake of shelf marine sands at the base, then overlying sandy channels with muddy levees and adjacent mud and peat marshes, and a top layer of fresh water meadow soils. V. Coastal Environments - Estuaries and Lagoons A. Estuary Environments 1.
Estuaries are semi-enclosed embayments open to the sea at one end.
The drowned mouths of rivers formed by rising sea level and Scandinavian fjords formed by ancient glaciers extending out to sea at times of lower sea level are classic examples. Estuaries are places where fresh water meets and mixes with ocean salt water. Estuaries are characteristic of low-gradient, broad coastal plains and broad shelves on a passive margin. Estuaries are strongly affected by tides and river discharge. Estuaries are short-lived geologically and soon fill with sediment.