No easily applied method of testing can effectively establish the guaranteed absence of human disease germs. The traditional "Coliform Count" plays safe, as it must. It measures the concentration of certain easily spotted "indicator" organisms that do not themselves make people sick but are always voluminously present in the fecal discharges that can carry harmful germs, and it gauges the danger by the concentration of these indicators. However, concentrations of coliform bacteria, originating in animal manure or elsewhere, may invade a stream through runoff from rural lands without having any meaningful relationship to human disease germs. Counting them under such circumstances is a little like measuring the depth of the proverbial well by the length of the pump handle. Furthermore, no one really knows how easy or how hard it may be to catch given diseases by swimming. In this country, outbreaks of leptospirosis, an illness common to man and certain animals, have been traced to swimming holes, and other links are obvious. On the other hand, some careful British investigations turned up a good many quite healthy people who habitually splashed about in sea water teeming with pathogenic organisms of one sort or another. Sea water and fresh water have vastly different qualities, but the subject is presently full of confusion, and it needs much research. Land runoff in general furnishes a large amount of pollution of all classes, and in all parts of the upper Basin except the least-used forest sections. Besides bacteria, heavy loads of organic material may be washed into streams in regions with high densities of livestock or poultry, and some pollution of this sort is found practically everywhere. The wild and domestic animal population of the Basin above Washington has been estimated to produce wastes equivalent to those of about 3.5 million people. Much of this is dealt with by the "living filter" of the soil, but much also reaches the streams, associated with sediment from erosion producing rains. And the sources, particularly in areas such as those along the Shenandoah and the Monocacy and other streams with wide rich valleys, are numerous and diffuse.