Pacific Northwest Basin

Percentile

California Basin: 18

Percentile

Figure 3-50 Before- and after-CWA frequency distributions of worst-case DO aggregated by major river basin for n = 7 RF1 reaches with paired before and after data sets: California (18). Source: USEPA STORET.

Percentile

Figure 3-50 Before- and after-CWA frequency distributions of worst-case DO aggregated by major river basin for n = 7 RF1 reaches with paired before and after data sets: California (18). Source: USEPA STORET.

Based on the two different statistical tests, and discounting the 7 river basins with limited data, 8 of the 11 remaining river basins can be characterized by a statistically significant improvement in worst-case DO using at least one of the two tests. Mixed results (yes and no) were obtained for two basins with the Kolmogorov-Smirnov test indicating no significant improvement for the Tennessee (6) and the Pacific Northwest (17) basins, whereas the paired t-test indicated significant improvements (yes) in these basins. Overall, there is a statistically significant improvement in worst-case DO trends using both statistical tests at 6 out of 11 river basins with sufficient data. Of the five basins with at least one "nonsignificant" change, three basins (Missouri River, Arkansas-Red-White, and Pacific Northwest) had a mean worst-case pooled DO level greater than 5 mg/L in the before time period and were less likely to be targeted for improved point source pollution control. It is also noteworthy that in the 25-year interval between the before- and after-CWA periods, there were no statistically significant conditions of degradation of worst-case DO for any of the major river basins. It is also noteworthy that when all 311 paired reaches are analyzed together, both tests indicate significant national-scale increases in worst-case DO [see Figure 3-51 and top row (All USA) of Table 3-3].

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