Jason Williams 

In the United States, the Clean Water Act (CWA) is the primary legislation driving surface water quality management. Its goal is to “restore and maintain the chemical, physical, and biological integrity of the Nation’s waters.” Section 305(b) of the CWA requires states to document CWA progress by reporting whether applicable water quality standards are achieved for all state waters every two years. Developing strategies for increasing the proportion of waters achieving standards requires diagnosing factors driving 305(b) data temporal trends. This analysis demonstrates how systematically analyzing 305(b) data in new ways can help document CWA progress (or lack thereof) and associated drivers. Idaho 305(b) data were used to evaluate the relative contribution of assessment progress and restoration to 2002-2022 Idaho 305(b) temporal trends. Assessment progress was defined as assessing unassessed waters and correcting assessment errors. Restoration was defined as changes from not achieving to achieving all assessed standards because water quality improved. From 2002-2022, the percentage of Idaho stream kilometers achieving all assessed standards increased from 24% to 32%. Systematically evaluating reasons for stream status changes revealed this trend was driven primarily by assessment progress, specifically progress monitoring previously unassessed waters in good condition and correcting prior assessment errors. More stream km changed from impaired to unimpaired because prior assessment errors were corrected than because water quality improved; in each report ≤ 5% of all stream km changing status resulted from water quality improvement. As of 2022, more state stream km were impaired (39%) than unassessed (29%) and restoration success rates will likely become the primary driver of 305(b) temporal trends in the future. Systematically analyzing 305(b) data in new ways may help develop new empirically driven strategies for accelerating CWA progress and merits further investigation.