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Source: USEPA STORET.

Source: USEPA STORET.

The "signal" of DO improvement has been clearly identified at both the catalog unit (watershed) and RF1 reach level spatial scales. In contrast to the results obtained by aggregation of the data at the catalog unit scale, the "before" and "after" "signals" identified at the smaller scale of an RF1 river reach appear to provide a better indicator of the long-term improvements in "worst-case" dissolved oxygen.

On the Upper Mississippi River in the vicinity of Minneapolis-St. Paul, Minnesota, for example, the mean tenth percentile dissolved oxygen levels for station records averaged over the catalog unit 07010206 showed an improvement from 3.7 mg/L "before" to 5.4 mg/L "after" the CWA (see Figure 1-5). This trend is clearly an improvement in water quality that reflects the substantial reductions in effluent loading from the St. Paul METRO facility, the "worst case" summer conditions (~ 0 to 2 mg/L) observed in the oxygen sag zone about 10 miles downstream of the Twin Cities, and the much higher DO conditions observed in other reaches of the Upper Mississippi River upstream of the Twin Cities, including tributaries to the Upper Mississippi River.

Figure 3-25 Location map of catalog units with improved and degraded DO and distribution chart of the 10 catalog units with the greatest before- versus after-CWA improvements in worst-case DO. Source: USEPA STORET.

Figure 3-25 Location map of catalog units with improved and degraded DO and distribution chart of the 10 catalog units with the greatest before- versus after-CWA improvements in worst-case DO. Source: USEPA STORET.

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