Geologicllay speaking, Chesapeake Bay is very young. It was created by the death of the last Ice Age, some 12-18,000 years ago. As the glaciers retreated and the polar ice caps shrank, the huge volume of melting ice caused sea levels to rise. The rising ocean in turn engulfed the coast and flooded the river valley of the ancient Susquehanna river, creating the Chesapeake Bay. The Bay we know today is nearly 200 miles long, fed by 48 major rivers and 100 small tributaries draining a 64,000 square mile basin. Earth and water continue to compete for this territory, redrawing the shoreline as land is built up in some areas and lost beneath waves in other places. People have also redrawn shorelines, often on a much faster scale than nature. Excessive clearing and poor land management have increased upland erosion, sending tons of sediment downstream. As a result, communities that were once important ports are now landlocked. On the other side, the construction of seawalls and breakwaters has interfered with the natural flow of sand, causing beaches to rapidly erode.
Saltwater mixes into the Bay from the Atlantic Ocean. Freshwater flows from the Bay's tributary rivers, with about 50 percent coming from the Susquehanna. Saltwater is heaver than freshwater, so it tends to "creep" up the Bay along the bottom while the freshwater flows down from the tributaries on the surface. As a result the Chesapeake ranges from totally freshwater areas in the North and upstream in its rivers, to areas near the Bay's mouth that are about as salty as the ocean. Thus the Bay can support both fresh and marine life forms, plus those that can tolerate fluctuating salinity levels. The variety of conditions supports some 2,700 species. All are linked in a complex, interdependent web of producers and consumers. From the eagle's huge nest high in a wetland tree to the worms in the Bay's bottom sediments, from the microscopic free-floating plants to the pine trees along the Shenandoah, all have a part in maintaining this system's balance.
The Bay's physical nature as an estuary is both the source of its richness and the source of its vulnerability. The overall proportion of fresh and salt water in the Bay depends largely on the amount of rainfall that is carried to the Bay from the Chesapeake's major rivers. During a wet year, the entire Bay will be somewhat fresher than normal, and conversely a dry year will result in higher than average salinities. The Bay is also dependent on the quality of freshwater flowing from the tributaries. Pollution flowing to the Chesapeake tends to stay there - either in the water column, the bottom sediments or the Bay's living resources.
For more information call:
Chesapeake Regional Information Service
1-800-662-CRIS
pweeg@shore.intercom.net