Far out, though possibly not very far off in time, is the likelihood that future water planners will be able to count on some degree of control over a given region's rainfall and snow. Through experimentation, this subject is rapidly being excised from the mists of superstition that once surrounded it, and the Department of the Interior has an active program of research and study in the West, with tremendous implications. But, yet again, present planning cannot take it into account except in the sense that, along with some of the other technologies already mentioned and undoubtedly others that have not yet even emerged to view, it adds to the near certainty that future planners are going to have a much wider range of alternative methods at their disposal, to choose from and mix as may seem best. And this, in turn, reemphasizes the wisdom of flexibility in present planning and the need to keep big irreversible decisions to a minimum. The upper Potomac estuary from Little Falls down to the vicinity of Marshall Hall and Mount Vernon or below contains a great deal of fresh water, an accumulation made up of inflows from the river above the Fall Line, local storm runoff and tributary flows, and treated sewage returned to the tidal river. The volume of this water that would be available for use without salinity has been variously estimated. At low tide, there would be 9 billion gallons of fresh water in the upper estuary from Chain Bridge to the mouth of the Anacostia River; In the 10 mile stretch from Chain Bridge to the District of Columbia's Blue Plains treatment plant, 15 billion gallons; and, from Chain Bridge to the saltwater front near Indian Head, Maryland, 100 billion gallons. Most of the time now it is afflicted with heavy pollution, as will be detailed in the next chapter of this report. But it does constitute a large natural reservoir of potentially usable municipal and industrial water, whose attractiveness for these purposes, as well as for all others, will grow steadily as the pollution is brought under better and better control. These facts have led some opponents of any and all major reservoirs in the Basin to conclude that the water in the upper estuary is a presently satisfactory reserve with which to face any foreseeable metropolitan shortage of supply from the upper Potomac.