The full main stem Potomac, carrying the water from the combined North and South Branches and the Shenandoah and the other upper tributaries down through the Blue Ridge water gap and across the rolling Piedmont and the Fall Line, is at its most typical in the 39 miles from Harpers Ferry to Great Falls. Seneca as originally proposed would inundate 35 miles of this stretch, together with islands and bottomlands, forests of big hardwoods, meadows and productive fields, and that much-used segment of the publicly owned C. & O. Canal, with the trail along its wooded towpath. Even reduced in size and designed as strictly a water supply structure, it would have many of the same effects. There is special and tranquil beauty in this piece of the river, which makes a fine float trip and is much fished, as well as a lot of historical significance dating back to the Senecas and the Piscataways and before. Here these things are not forgotten and removed from men's reach but are available to metropolitans who go to the trouble to seek them out, as many do. Nor is there anything else around to take their exact or even approximate place if they were gone. It has been pointed out that if the metropolis grows according to predictions, a major part of that growth is going to be upriver, and the main stem of the Potomac will have the same relationship to the metropolis of the future that Rock Creek has to the Washington of today. Thus the decision that is made about the main stem in our generation is similar to the decision that planners had to make about Rock Creek three-quarters of a century or more ago. Those planners decided magnificently well, bequeathing to the future an urban stream and park unique in this country and perhaps the world, a treasure that the public is presently defending against other, newer, subtler threats than mere damming or encroachment. A reservoir above Seneca clearly could not mean that sort of thing. It would be a useful lake, but devoid of the changeless tone of the Potomac as it flows there now. The reservoir's proper functioning would require fluctuations in its level, with occasional ugliness at the shoreline, and if it would permit a great deal of happy water-skiing and flat-water fishing, the same opportunities are going to be available to Washingtonians in the nearby estuary when it is suitably cleaned up, even though the section immediately adjacent to the metropolis may take a good while to bring up to swimming standards.