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Figure 1-5 Line graphs of DO observations for the Upper Mississippi River from 1953 to 1997 and bar charts of worst-case DO before (1961-1965) and after (1986-1990) the CWA for (a) reach scale, (b) catalog unit scale, and (c) major river basin scale. Source: USEPA STORET. (a) For Rf1 reach 07010206001 (UM 811.6-844.7). (b) For catalog unit 07010206 (UM 811.6-879.8). (c) For major river basin (07).

creasing 3.5 mg/L from before to after the CWA. At the broader scales, an improvement is detected, but it is not as large (a before and after difference of 1.7 mg/L at the catalog unit scale and 1.5 mg/L at the major river basin scale). This is because the larger spatial units contain stations both near and far from point source outfalls. In spite of the unavoidable introduction of data noise, however, the signal linking point source discharge to downstream DO is still detectable at the broader scales using the data mining and statistical methodology developed by the study authors. Readers should note that, in this example, the worst-case DO concentration was below the benchmark threshold of 5.0 mg/L at all three scales before the CWA and above the threshold at all three scales after the CWA.

Section A of Chapter 3 provides background on the relationship between BOD loading and stream water quality and discusses the two key physical conditions (high temperature and low flow) that create "worst-case" conditions for DO. Section B describes the development and application of a set of screening rules to select, aggregate, and spatially assess before- and after-CWA worst-case DO data drawn from USEPA's STORET database. Section C presents the results of the comparison analysis of worst-case DO from before and after the CWA for reach, catalog unit, and major river basin scales.

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