There are said to be only four places in the world where two major rivers--with entirely separate watersheds--converge at their mouths to form a common delta. Three are famous, having loomed large in the histories and economies of their regions: the Sacramento, San Joaquin delta in California; Tigris, Euphrates delta in Iraq; and the Ganges, Brahmaputra delta in Bangladesh. The fourth is Merrymeeting Bay in Maine. It is unfamiliar to most people,...