The Gunpowder River is a 6.8-mile-long (10.9 km) tidal inlet on the western side of Chesapeake Bay in Maryland, United States. It is formed by the joining of two freshwater rivers, Gunpowder Falls (often referred to locally as "Big Gunpowder Falls") and Little Gunpowder Falls.
Gunpowder Falls 
The Big Gunpowder, 56.7 miles (91.2 km) long, begins in the extreme southern part of Pennsylvania's York County. From there, the Gunpowder flows in a generally southeast direction through the length of Baltimore County until it reaches Joppatowne where it is joined by the Little Gunpowder and the Bird River, becoming simply the "Gunpowder River" once it passes the Amtrak train bridge that runs from Chase to Joppa. Joppa was an important deep-water seaport and rival to the port of Baltimore in colonial American times, until sedimentation of the Gunpowder River caused by upstream development made its harbor inaccessible to large ships.
Little Gunpowder Falls 
Little Gunpowder Falls, as its name implies, is much smaller than the mainstem Gunpowder Falls. Throughout its 25.2-mile (40.6 km) length, it forms much of the boundary between eastern Baltimore County and western Harford County until it reaches the 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 counties.
Both the Gunpowder and Little Gunpowder Falls flow through hilly agricultural and forest land for much of their length. In fact, there are no less than four separate state parks that form Gunpowder Falls State Park, the largest state park in Maryland. At two points, Gunpowder Falls is dammed to form major reservoirs for the Baltimore metropolitan area: Prettyboy Reservoir and Loch Raven Reservoir. By the time the Gunpowder reaches the Bay, however, it is no longer a rural river, but a suburban river that passes under Interstate 95 and many bedroom communities for Baltimore. Nonetheless, Gunpowder State Park on the Chesapeake Bay may give one the impression that this is still a somewhat remote river.
Both streams flow over the Fall Line separating the Piedmont Plateau from the Atlantic Coastal Plain just downstream of US Route 1. This creates some class III whitewater opportunities for kayakers and canoeists when the water is reasonably high.
- Minebank Run
- Long Green Creek
- Sweathouse Run
- Haystack Branch
- Jennifer Branch
- Bean Run
- Foster Branch
- Taylor Creek
See also 
- U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National Map, accessed April 1, 2011
Baltimore County Watershed Management Program