The assumption has strong appeal, but it appears to be too risky to serve as a basis for adequate present planning to meet looming demands. That even now the water in the estuary's uppermost reaches, above the main metropolitan treatment-plant outfalls, would be usable for short emergencies by the installation of relatively simple pumping equipment below the falls, cannot be doubted. That in the long run the major part of the freshwater tidal river at and below Washington is likely to be a valuable source of metropolitan water, maybe a principal source, is quite possible. Its use is and will be a strong consideration in longterm planning--another good reason, in fact, for flexibility. But the truth is that right now enough doubt and ignorance exist in regard to its exact potentiality that it should not be counted on to provide a safe margin of supply under all conceivable conditions during the next twenty years or so, for which planning provisions need to be more rigid and definite. The doubts and unknown factors have to do mainly with the quality of this water, which comes under discussion later. In abridged summary of relevant facts at this point, it may be observed that unless all sewage and sewage effluents were collected and diverted to points well beyond the limits of the upper estuary, use of its water for periods beyond a few days of emergency would become essentially a form of recirculation of waste waters--with, at this time, the main drawbacks that we noted in regard to that process and certain others besides. For, under the low-flow conditions that would bring about its use, the effluents in the river below the mouth of the Anacostia would penetrate upstream as water was pulled out below the falls and would reach the pumps in fairly short order, probably moving in a tongue up the main channel. With the radical improvement in the functioning of the metropolitan treatment plants that must be achieved, and other measures to relieve pollution in this part of the river, valid objections to such recirculation will of course weaken and ultimately disappear. But no one can reasonably expect that these things are not going to take a certain amount of time--quite conceivably enough time to run the city up against an emergency it could not handle without other, more standard sources of auxiliary water. Besides the matter of consolidating and improving treatment of collectible wastes, there are certain other diffuse and stubborn sources of pollution, as will be seen, for which good counter measures simply do not yet exist--among them are surface runoff during local storms and overflow from combined sewer systems.