FIGURE 3-2 Mean August dissolved oxygen concentrations in the Mississippi River below St. Paul, Minnesota. Note the ''sag" in the curves: the oxygen level decreases immediately downstream of the city due to oxygen consumption by microorganisms that degrade wastes; it increases again further downstream due to atmospheric reaeration once the wastes are biodegraded. SOURCE: Reprinted, with permission, from Johnson and Aasen (1989). © 1989 by the Journal of the Minnesota Academy of Science.

Phelps did not call himself a limnologist, he identified himself as an expert in "the science of rivers"—with knowledge of the biology, chemistry, and geology of river systems (Phelps, 1944). His work helped to advance the field of stream limnology.

Early research on how sewage affects streams was an important source of the scientific basis for federal legislation to control water pollution. For example, the opening paragraphs of the 1972 Clean Water Act state that its objective is "to restore and maintain the chemical, physical, and biological integrity of the nation's waters." The act requires states to issue permits—known as National Pollutant Discharge Elimination System (NPDES) permits—to municipalities and industries that discharge wastes to waterways. For municipal sewage treatment plants, two primary requirements specified in the permits are reductions in BOD (the basis for the Streeter-Phelps equation) and suspended solids in the waste to levels that are based on the capabilities of standard sewage treatment technologies. Thus, the early work by Streeter, Phelps, and others examining the effects of pollution on streams helped to provide the technical basis for this permit system.

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