In the suburbs, more modern separate storm and sanitary sewers are the rule, but they too have some problems of a kind we noted in relation to the upper river. Investigations on Rock Creek revealed steady dribbles of raw sewage entering the creek or its tributaries from a large number of storm-sewer outfalls and other places. Partly these flow from malfunctioning individual septic systems in outlying areas, surreptitious connections of house sanitary sewers to the storm system, breaks and leaks in sanitary sewers, and such things. Partly too they seem to come from the fact that some sanitary sewers are having to carry more sewage than they were designed to handle, so that their overflow valves leak more or less constantly into the storm sewer system. The capacity of sewage collection systems is related to planning. If a pipe is laid down to a fringe area where county zoning maps indicate only limited development is going to be permitted, its size is gauged to that kind of development. But if the zoning is changed later and three times as many houses are hooked up to the line as were originally envisioned, trouble results. Rock Creek is heavily affected by such sewage, and the chances are that the situation is much worse on many other urban drainways, for their longstanding degradation or sheer disappearance from view has lost them the alert defenders who watch over Rock Creek in its pleasant valley. Out of the storm sewers whether combined or separate, off of the roads and streambanks and hillsides, down the urban tributaries or directly overland into the estuary, comes still another big jolt of organic and bacterial pollution every time there is a heavy rain. This is surface runoff, the washings of the street and parks and sidewalks and rooftops. Besides debris, it contains vast hordes of bacteria and many kinds of organic oxygen-demanding substances, of which animal droppings are only one easily definable example. Around a city the size of the Washington metropolis, this runoff would constitute a worrisome pollution problem even if the matter of sanitary wastes were thoroughly in control. [Illustration] Ships and large boats in the estuary, in accordance with an unfortunately persistent nautical tradition, generally discharge toilet wastes and garbage directly into the water on which they float. Some of these are coastal or transoceanic vessels, both commercial and naval. Many more belong to the fleet of pleasure boats which have been increasing at Washington despite the water's unpleasant state to which they add their bit, degrading the element that is supposed to provide the enjoyment for which the boats were built. It is not a problem limited to the Potomac estuary, but widespread these days and the focus of much concern among public health and pollution control authorities, conservationists, and the boat and marina industries themselves.