Roads crossing small creeks may have culverts installed to allow tidal water to pass beneath them. In many cases, these culverts are too small to pass enough tidal water to maintain natural salt marsh vegetation upstream. Some culverts have tide gates or flapper valves allowing fresh water to leave but not allowing tidal waters to enter the upstream marsh. At some road crossings, a culvert has not been provided at all, and tidal flow has been eliminated altogether. Bridges may have similar restricting effects if the openings are not wide enough to maintain salt and brackish marshes upstream.
Tidal restrictions have many detrimental affects to healthy salt marsh systems.
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.