In terms of dissolved oxygen, dilution of such a body of water for quality improvement appears to decrease in unit effectiveness as the volume of dilution is stepped up, which means that past a certain minimal point of improvement it gets expensive and requires unreasonable amounts of storage. In terms of nutrients, one authority has calculated that about 20,000 cubic feet per second would be required to reduce the nutrient level in the upper estuary to a point where it would be only twice that of a normal and healthily "rich" section of the upper Chesapeake Bay. Some augmentation below the point of diminishing returns will undoubtedly be needed, not only for the estuary but to keep the river alive in its gorge above Washington during periods of low flow. But as a main tool for the metropolitan river, it will not substitute for achievement of the best possible standard treatment followed by advanced treatment and other techniques. Obviously, just as in water supply, an ultimate cure for water quality ills is going to consist of a "mix" of solutions, different techniques being applied to the situations they are best suited to deal with, and combinations of them being worked out where combinations are what is indicated. Already the same kind of sophisticated mathematical models of given bodies of water--including the Potomac--that are being used to study solutions for water supply problems are being put to use on water quality as well, weighing the benefits and drawbacks of various combinations of means. And, just as in water supply, ultimate "hard" technology is undoubtedly going to make better solutions possible, while a strong and meaningful start is possible with the technology that is on hand. * * * * * Silt is a truly Basinwide problem. The individual tiny grains of soil that mass to sully and choke the estuary may have originated anywhere in the thousands of square miles of drainage above. They constitute an economic loss at their points of origin as well as a trouble all along their downstream course of migration. The basic-physical ways of preventing silt are twofold and easily defined: first, the maintenance of proper land cover--vegetation or humus or mulch which blankets and anchors the soil particles and prevents falling or flowing water from dislodging them--and second, structural approaches that control the flow of water and can also serve to trap eroded material. These latter can be anything from good contour plowing practices to a major reservoir with a certain silt capacity built into it. [Illustration]