need info on sawgrass and fresh water marsh ecology

[allen woody]

We have a marsh in East Texas ,created by beaver dams, that has gone
from open water areas with wild rices and various plants that feed
migratory waterfowl. The fish population has been decimated, perhaps by
upstream pollutants.   The sawgrass has made inroads and has nearly
taken over and has squeezed out other plants. The winters have been
very light and the sawgrass has not faced a solid freeze in several
years.   We want to know what to do to protect the area from being
totally overrun, thus destroying the habitat for ducks, fish et al. a.
woody