Rivers are supposed to run out of their banks occasionally. Topographically, stream flood plains--the expanses of flat bottomland that have been deposited over long periods of geological time by the streams they border--are similar to what legal terminology calls "attractive nuisances." Men have always known that they were dangerous and yet have always utilized them to some degree, because they contain the best farm land, are convenient to water, and are easier places in which to build houses and factories and roads than are the safer hills and uplands. In times before engineering technology was able to erect such effective control structures as today, populations who had lived along "flashy" watercourses long enough to learn their habits tended to build their more valuable structures back away from the parts of the flood plain that got wet most often, leaving those parts for cropland and timber, or sometimes for shacktown, promenades, and parks. Thus long-settled countries and regions have often developed through trial and error a degree of what is now called "passive flood protection," which simply means recognizing that the flood plain is sometimes a rather perilous place, and treating it accordingly. It was valid in past ages, and it is still valid today. The Potomac Basin has been inhabited by civilized men since long before modern engineering evolved. Possibly early town-builders' wariness of floods contributed to the fact that the problem of flood damages here, though quite real, is somewhat less severe than in certain other sections of the nation. At specific points of concentrated flood plain development--Petersburg, W. Va., on the South Branch; Cumberland, Md., and the areas upstream from it on the North Branch; and metropolitan Washington at the head of the estuary--figures show significant amounts of average annual destruction by rampaging stream waters. In headwater areas or small urban watersheds scattered throughout the Basin, there are a number of other places where some damage takes place, whether agricultural or structural. The total average annual damages for the Basin, as computed in the 1963 Army _Report_, amount to about $8.6 million. Along small streams, whether urban or rural, the same principles apply as along large ones, and the proper protective measures are similar if smaller in scale. Leaving the worst parts of the flood plain in fields or parks is the usual and effective form of passive protection. Where existing development demands structural measures, it has been common practice to cover streams over as sewers or to confine them to straightened concrete channels that sluice rainwater and mud away as fast as they will flow--though often this is not fast enough, as is shown by occasional messy and costly overflows of Four Mile Run between Arlington and Alexandria. And the loss of pleasant brooks and creeks through such practices is a heavy price to pay.