The following can be seen only from the river, or will have a different look or sound from waterlevel. MOUNT VERNON -- the classic view. SMOOTS COVE -- Maryland, just below Wilson Bridge. A natural underwater spring, a yacht hangout and a semi-submerged hulk resembling a submarine, once used for training divers. WILSON BRIDGE -- Hank Snyder describes the shriek of rubber tires on metal gratings as "What I imagine the descent into hell sounds like." And wear a hard hat. ALEXANDRIA WATERFRONT -- beautiful, especially on a calm day, in late afternoon, when it's reflected in the river. Also: look for Interarms and for John Dean's former townhouse, the white one on the corner. NATIONAL AIRPORT -- a pleasant geometrical pattern, rows of sun-shined planes neatly aligned against the black convex hangar roofs. THE MONUMENTS -- without crowds, and in their perfect settings. KENNEDY CENTER -- stark white, plain box, clearly an architectural misfit on the intimate Potomac riverfront. But sometimes, in spring or fall, you can catch rehearsals through open doors. GEORGETOWN -- slogans for the racing crews painted on the seawall; the Potomac Boat Club, the oldest on the river; The Washington Canoe Club, green-and-white turrets, a ramshackle landmark. BRIDGE PILINGS -- of the Aqueduct bridge that carried barges and water from the C&O Canal to a canal on the Virginia shore. One is on the river, just upstream of the Key Bridge. The other is on the Georgetown shore hard by the Potomac Boat Club. THE C&O INCLINE PLANE -- for the canal, hidden in a cove on the District side. You find it. FLETCHERS COVE -- named for the Fletcher family, which has been on the river for five generations. Location of Fletcher's Boat House. MATHER GORGE -- magnificant 200-foot-deep gorge below Great Falls. Should be seen by commercial raft unless you are a skilled whitewater canoe or kayaker/paddler.