1) As a pond develops seeds are flown in by birds and land animals come to inhabit the pond. These are the pond pioneers.
2) As more creatures arrive the debris on the bottom increases. Pondweed, and other submergent vegetation, appears and soon grow all along the bottom.
3) Emergents then appear on the edges of the pond. Over time, sometimes hundreds of years, as ponds plants grow, die and decompose, layers of debris build up. These layers of decaying matter raise the pond floor over the years.
4) After some time, the pond floor is close enough to the bottom that emergents can grow all the way across the floor. When this happens, the ponds becomes a marsh. Many interesting creatures can reside in the shallow muddy waters of marshes. (Marshes can be created in other ways also.)
Source: Online Content Site 1 (provided by edg.)
