My local outdoor recreation store has a Hydrological map of the U.S, from Muir Way, on display. Every time I go into the store I try to purchase the map off of their wall, and each time (no matter the employee) I get the same response: "If I asked management if the map was for sale, I might loose the pleasure of seeing it everyday". For years I tried to buy this map from them - seriously. But they would never sell it! One day I randomly stumbled across Muir Way's website, saw the hydrological map listing, and instantly made the connection.. that is the map on display at the store! No hesitation, I bought the largest size of this map that they make.
I usually would have never considered the hydrological map, instead vouching for a topography map or something alike; once you see this thing in person, though, you will never forget it.