Manassas National Battlefield Park lies in the Potomac River watershed, part of the largest estuary in the United States, the Chesapeake Bay Watershed. A watershed is an area of land in which all water runoff and tributaries drain into a specific body of water. All of the water from the park, including streams, rivers, and storm runoff eventually contributes to the Chesapeake Bay, depositing all of the nutrients and chemicals it acquires along the way. The Chesapeake Bay watershed hosts over 100,000 tributaries that stretch through 64,000 square miles of land to flow into the bay. The water from the watershed mixes with the Atlantic Ocean in a partially enclosed body of water, called the Chesapeake Bay estuary. The mixture of freshwater and salt water forms brackish water that provides critical habitat for an array of plants and animals. Estuaries serve as the spawning and nursery grounds for many aquatic creatures. The underwater grass beds and shallow waters provide protection from predators for juveniles to mature safely. Migratory birds stop in estuaries to feed and rest in the wetlands and marshes during migration season.