The term probably derives from sailing—a reach is an extended run on one tack. In inland water—lakes or rivers—it is a comparatively long, straight stretch between two bends. In Life on the Mississippi, Mark Twain informs us that Fort Adam’s Reach “is five miles long. A reach is a piece of straight river, and of course the current drives through such a place in a pretty lively way.” In coastal waters, reach is more or less synonymous with sound. It is an arm of the sea that extends inland, or a passageway between islands. Unlike a fjord, it does not end in a glacier; unlike an estuary, it is not the entry to a river. It is generally narrower than a bay. Long Reach, adjacent to Harpswell Sound, off the Maine coast, is an example.

Franklin Burroughs