The Susquehanna and James are only two of the more than 50 major rivers flowing through this region. Together, these rivers pour 2.5 million cubic feet of freshwater and huge amounts of sediments, minerals, and nutrients into Chesapeake Bay each year. Five of these rivers, the Susquehanna, Potomac, Rappahannock, York, and James, provide 90 percent of the Bay's freshwater volume. The largest of these, the Susquehanna, accounts for fully half of the freshwater discharged into Chesapeake Bay. The huge volume of freshwater that flows into the Bay makes these waters 10 percent less salty than those in the nearby open ocean.

The Bay itself is an estuary - a place where fresh river water mixes with salty ocean currents. It is the largest estuary in the United States and one of the largest in the world. The Bay was formed at the end of the last Ice Age, when melting glaciers caused sea levels to rise worldwide. This generally slender, shallow sliver of bay water stretches 200 miles from its northern border at the mouth of the Elk River in Maryland, to its southern outlet between Cape Henry and Cape Charles. Its deepest portions trace what in ancient times was the path of the Susquehanna River; its shallower parts were formed when land was flooded by rising ocean waters. As much as 40 miles across at its widest point, Chesapeake Bay follows a generally narrow channel with an average width of between five and 10 miles. Bay waters today cover a surface area of 2,500 square miles. Its waters have a mean low water volume of over 65,000 cubic yards and an average depth of 23 feet.