When the level of a lake or sea rises dramatically, its waters may spill into river valleys and the crevices between hills, creating drowned valleys. Drowned valleys are not as steeply sided or deeply cut as fjords; indeed, they include the gently sloped basins of coastal estuaries such as the upper Chesapeake Bay. The term ria, now considered synonymous with drowned valley, originally referred to inlets where landforms rise from the water at precise right angles. Drowned valleys are common along the North American East Coast, where water covers shorelines exposed ten thousand years ago during the last major ice age. Many of the coves and inlets now plied by oyster boats and recreational boaters were once feeding grounds for the land animals of the Pleistocene. In coming centuries, as sea level rises and pushes inland, drowned valleys will likely become more numerous around the world.