The Gunpowder River is actually formed by the joining of Big Gunpowder Falls and Little Gunpowder Falls. Big Gunpowder begins in the extreme southern part of Pennsylvania's York County. From there, the Big Gunpowder flows in a generally southeast direction through the length of Baltimore County until it reaches Joppa, MD where it is joined by Little Gunpowder Falls.

Little Gunpowder Falls, as its name implies, is much smaller. Throughout its length, it forms the much of the boundary between eastern Baltimore County and western Harford County until it reaches Big Gunpowder. From there, the Gunpowder River is mainly a tidal estuary of the Chesapeake Bay that continues to form the boundary between Baltimore and Harford.

Interesting Facts about the Gunpowder River: Both the Big and Little Gunpowder rivers flow through beautiful hilly agricultural and forest land for much of their length. In fact, there are no less than four separate state parks that form Gunpoweder State Park, the largest state park in Maryland. At two points, the Big Gunpowder is dammed up to form major resevoirs for the Baltimore Metropolitan area; these lakes are known as Prettyboy and Loch Raven. By the time the Gunpowder reaches the Bay, however, it is no longer a rural river, but a suburban river that passes under I-95 and many bedroom communities for Baltimore. Nonetheless, the Gunpowder State Park on the Chesapeake Bay amazingly gives one the impression that this is still a somewhat remote river.