The Seneca reservoir as proposed in 1963 provided floodwater storage calculated to reduce metropolitan damages by 46 percent. This is a significant though not startling amount of reduction, and it constitutes the most economical one-shot measure of protection that could be attained. However, if the construction of Seneca is precluded for the time being or for good, that measure is not available. Second-best, by Army calculations, would be a combination of several large multipurpose reservoirs on main tributaries farther upstream. But quite aside from other considerations of desirability, these could only be justified economically if a great part of their stored water were destined to furnish massive flow augmentation to ease pollution in the upper estuary. As will be noted in the following chapter, recent studies have raised doubt that such augmentation would be likely to help the estuary nearly as much as had been thought and it is no longer being considered a primary tool for that purpose. This leaves passive devices and local protection works as the main available instruments for coping with floods at the metropolis. They will probably be most effective if applied in a carefully selected combination of means, with levees and other protective works installed where feasible and desirable, and backed up in other areas by zoning, flood warning systems, and good design including flood-proofing, elevated structures, and similar devices. Some of these principles of design are already being incorporated in new buildings and renewal projects, but the task of planning and locating such things as levees usefully on a flood plain containing a good part of "monumental Washington," the beauty of which is a national concern, is not going to be simple. A good program must be instituted soon, and the extent of the Federal interest in the lands involved should considerably ease the job of coordination. Interestingly, certain floods to which Washington is susceptible can be partially guarded against only by such approaches as the ones mentioned above, and not at all by upstream dams. One of them occurred in August of 1933, when a hurricane pushed the water in the estuary upstream and raised it to flood stage at the capital. [Illustration] [Illustration] [Illustration] III THE CLEANSING OF THE WATERS Streams have always had to carry and digest wastes that enter them through drainage from the land. It is one of their functions in the scheme of things, and so well have they performed it through the millennia that human beings have been able to take it for granted. Within limits that might be considered normal, the ability of running water to handle loads of waste is phenomenal, and in earlier times those normal limits were seldom exceeded, for even in populated areas the general lack of sanitary sewer systems kept the loads from being concentrated.