They result in hydrologic changes that significantly alter the chemical integrity of the upstream salt marsh ecosystem. The once strongly saline environment changes to one that is brackish or fresh water. Over time, the freshening of salt marshes causes a transformation in the vegetation. Salt marsh grasses and rushes may die off and be replaced by common reed. What once was a low grassy meadow, is turned into a tall reedy thicket.
Common reed blocks tidal flows even further, reducing estuarine organisms' access to the salt marsh.
The export of organic matter is also reduced or eliminated where tidal exchange is restricted or cut off by a tide gate, flapper valve, or dike. Together changes in vegetation and the reduction of the detrital food base cause a major shift in wildlife use and a decrease in species diversity. A reduction of tidal flow also causes a dramatic change in soil chemistry. Sea water contains large quantities of sulfur. In the normally anoxic marsh soil conditions, it is reduced by soil bacteria into sulfide.