Blackwater National Wildlife Refuge is part of the Chesapeake Bay ecosystem and encompasses two rivers that share its name. The Little Blackwater flows into the Blackwater River south of the refuge Observation Site, and the Blackwater River empties into Fishing Bay, an arm of the Chesapeake. The Chesapeake Bay formed some 15,000 years ago when ice caps melted and sea levels rose, flooding the Susquehanna River basin. Today it is the largest estuary in North America, where fresh and salt water meet and mix, supporting abundant populations of microorganisms, fish, crabs, oysters, and clams. Many fish and shellfish use the marshes and tributaries as nursery grounds because these areas provide nutrients carried in by the tide as well as protection from predators. At one time, forests covered most of the watershed and protected the shallow bay, but development led to tree clearing and increased use of pesticides and other chemicals, and erosion and flooding have dramatically affected the bay and its inhabitants. The wetlands of Blackwater National Wildlife Refuge are vital to the bay’s health because their plants trap sediments and filter runoff flowing into the estuary, absorbing excess nutrients, toxins, and heavy metals.