Home Archived Aug 4, 2016

USGS Geology in the Parks


saltmarsh cordgrassMarshes:

Marshes are flat areas level with or just above the local water table. This relationship might be due to runoff, ponded water resources such as lakes, or proximity to the coast and sea level. Marshes may be salt, fresh or brackish, which is a mix of salt and fresh water. Most coastal marshes are either salt or brackish. The image at left is a of a salt marsh cord grass from the Florida Everglades. Levels in salinity (concentration of salts) show this degree of variation due to runoff. Fresh water flowing out from groundwater, surface flow and discharges from human use all act to dilute the salts in the system. Marshes also act to trap sediments flowing in runoff; as such they also act as filters for heavy metals and other pollutants that collect in flowing water.

Estuaries:blue crabs in a net

Estuaries are interfaces between rivers and the ocean, bodies of moving water that are generally enclosed on three sides and protected from the ocean currents and waves. They are flushed by tides, which mixes the salt water and fresh. Estuaries Some of the larger ones, such as Chesapeake Bay (which produced the blue crabs shown at right), are believed to have been formed previous river channels that have been drowned by rising sea level with the end of the last ice age. Estuaries, due to their gradient of salt to fresh water, are very important to species that have portions of their life history spent in both types of environment. Animals such as crabs, molluscs, shrimp and fish spend either portions or the majority of their lives within estuaries. Predators also inhabit the area to take advantage of the abundance of prey.

Click here to download a fact sheet about the San Francisco Bay Estuary Field Station.


Scientist collecting sample of sedimentMangroves are a woody plant that are able to grow in brackish conditions. An especially important ability of these plants it their ability to survive in anoxic soil conditions, or soil that is depleted of oxygen. Oxygen becomes depleted in these systems by respiration that occurs during the decomposition process by bacteria. This ability to live in anoxic conditions allows mangroves to live in areas that are at or slightly below sea level that have large amounts of biotic (living) activity. Mangroves act to stabilize the sediment and serve as a nursery for many economically important species of fish, crabs and shrimp.


Florida Everglades Marsh and Mangrove Status: A USGS online poster describing research on mass change of marsh and mangroves in a study area of the Everglades

Wetlands Photo Gallery: This gallery covers everything from geology to plant species and gives information on current research

Florida Wetlands: A USGS fact sheet about research in the Florida Coastal Wetlands

South American Nutria: A USGS site about an introduced beaver-like rodent and its effects on the Blackwater National Wildlife Refuge, MD


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