Organic materials are pervasive enough in the upper estuary that during periods of even normal flow their decay pulls oxygen levels down. Under usual conditions this B.O.D. grows worse and worse downstream and reaches a peak in the neighborhood of Mount Vernon, though its effects continue to be felt below. Fish kills among the rugged resident species that predominate in these reaches of the river are not uncommon, the shoreline windrows of deceased carp and perch periodically adding their essence to what metropolitans have come to accept as the Potomac's normal summer smell. And along with the organic materials are heavy concentrations of bacteria. The organic and bacterial load enters the estuary from many sources, most of them local, for only a little of this material comes down from the upper river. A significant amount of it issues from the network of small urban watercourses like Rock Creek. Many of these were covered over as storm sewers or troughed in concrete long ago, but they continue to serve their age-old function of draining the lands they traverse, even if through cast-iron gratings. A good bit of the organic load in these tributaries consists of raw human waste, incongruous and particularly obnoxious around a modern city. The bulk of it is released in periodic surges when local rainstorms overload the old-fashioned combined sewer systems of the District of Columbia and Alexandria. In dry weather these systems send both collected sanitary wastes and street drainage down to the cities' respective treatment plants, but during storms when street drainage is heavy the sewers' capacity is exceeded and overflow gates gush mixed stormwater and sewage out into the streams, which carry it to the estuary.