From there down it is, as we have seen, a different thing, an arm of the sea and a sluggish extension of the river, shading from fresh to salt, called a river still but neither river nor sea in its ways, affected rhythmically and obscurely by both of them and subject to its own complex laws as well. In Indian and Colonial times this estuary was the part of the river that counted most for men, because of the bounty that came from its waters, the fitness of its shores for farming, and its navigability for boats and ships in a region where land travel was laborious and whose colonists depended on commerce with a European homeland. Its shores and those of the big tributary embayments--"drowned rivers," they have been called--are thickly sprinkled with traces and remembrances of three and a half centuries' people and events. Mount Vernon, old Fort Washington, Gunston Hall on Mason Neck where quiet George Mason lived and thought ... Aquia Creek where George Brent took his Piscataway bride to live apart from the Marylanders, Potomac Creek where John Smith found the river's namesakes living and another wily captain later tricked Pocahontas into captivity, Port Tobacco and Nanjemoy with memories of brokenlegged Booth, Chotank that gave its name to a whole forgotten way of life, Nomini of the Carters, the Machodocs and the Wicomico and the Saint Mary's and the historic rest.... Some of the big creeks are silted in now with mud washed down off the land in the old days, but in the flatter country toward the Bay most of the larger ones are still pretty and useful harbors for pleasure boats and for the fleets of varied commercial craft that go out to gather the estuary's crabs, oysters, clams, perch, striped bass, shad, and other edible creatures, including even eels for the European market. From hillsides, mellow mansions look down on the water that used to be their highway to the outside world, some crumbling, others proudly maintained. Aquatic life in the upper freshwater stretches has been somewhat diminished and changed by pollution and silt, by dredging and filling, and by other activity. Runs of spawning shad and herring and perch still arrive there in spring, fortunately a season when heavy river flow keeps oxygen levels high. Along the whole estuary there is an abundance of air-breathing creatures, most noticeably birds, that reflect the wealth in its waters. They are strikingly numerous in the marshes that occur here and there next to the open river but more commonly up the tributaries, perhaps the richest biological areas in the whole river. Herons and egrets, ducks and geese, coots and grebes, hawks and ospreys and even a few bald eagles--a stirring sight so near to Megalopolis--are among the larger birds that congregate to live directly or indirectly off the life in the water, dependent on it.