GMCO Upper Potomac River Map. The best tool you can have for finding fish is a good map. ive heard 90% of the fish live in 10% of the water. The GPS on you phone may be good for finding a good Taco shop, but not for finding fish. This map shows access points, good Wading areas, underwater hazards and routs to navigate. This is the only navigation map for the upper Potomac, a must have for fisherman.