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Flooding and a lack of drainage are other ways that salt marshes can be disrupted.
Excessive rain can pick up stray oil, metals, and sediments from roads before it finds
its way to salt marshes. These impurities, as mentioned before, cause a variety of
problems for salt marsh vegetation. Some of the oils and metals can be absorbed by
the plants and cause contamination and or poisoning. The sediments can cover over
fresh plant life and either cause minor physical damage or death by smothering. In
severe cases of rainfall where unusually large amounts of water are dropped onto and
drained into salt marshes, salinity imbalances occur. Such great volumes of fresh water
dilute the marshes so that some native plant species can be taken over by foreign species.
This has happened in both Australia and California salt marshes. Flooding problems are somewhat common in salt marshes because of their natural topography. Most marshes are located in flat areas and are the site of the runoff collection of large areas. Also, the soil has low levels of water conductivity because it is already mostly saturated with water. (Graphic: http://www.ocean.udel.edu/courses/mast200/lecture22/sld015.htm )
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