The reason most such encroachment is bad, with or without a dam upstream, with or without levees, is that it establishes the certainty of further and larger flood damages in the future, with the certainty of further and larger expenditures to combat them. It has been pointed out that no such thing really exists as flood _control_, but only a given degree of flood _protection_. Economics and technology dictate that reservoir capacities devoted to the storage of flood water, for example, be considerably smaller than the maximum runoff conceivably possible. This means that sooner or later there is going to be a great flood against which the reservoir or reservoirs will not suffice. If the reservoirs' presence, as is most often the case, has directly encouraged a lot of flood plain speculation and construction downstream, then the great flood is going to do more damage than was ever done before, and more reservoirs and other protective measures, most often Federally financed, are going to be demanded, at a price that rises sharply as less desirable sites and methods have to be employed, and with frequently catastrophic scenic effects. These considerations apply to small watersheds as well as large ones. This costly cycle, which frequently makes the general public pay both in tax money and the sacrifice of amenities to protect the investment of a relatively few who profit from the wrong kind of flood plain use--in plain words, makes the public subsidize their ventures--has established itself widely. In some places, of course, certain kinds of development can take place only on the flood plain, and planning for its structural protection may be amply warranted, with equitable cost-sharing. But the difference between this sort of flood plain use and the much more common, thoughtless, quick-profit type needs to be more widely recognized and established in policies at all levels of government. The subject has been much studied. In August 1966 the findings of a distinguished Task Force on Federal Flood Control Policy, which made detailed recommendations for injecting some sense into the situation, were submitted to the attention of Congress by President Johnson. At the same time, he issued Executive Order 11296 on the subject, directing all Federal executive agencies with influence in such matters to do everything possible to discourage uneconomic and unwarranted use of the nation's flood plains. This, of course, includes the present Potomac planning effort. [Illustration: FLOOD PLAIN DELINEATION POTOMAC RIVER AT HANCOCK, MD.]