Monitoring programs can do more than distinguish healthy streams from those in poor biological condition: They can also generate the data sets needed to identify potential causes of poor conditions through causal assessment. We have developed rapid, screening-level causal assessment (RSCA) tools that take advantage of extensive regional monitoring programs to evaluate potential causes of poor conditions in the SLRR watershed in southern California in an automated and standardized fashion. RSCA evaluates habitat degradation, salinization, eutrophication, and altered temperature. This study also provided an opportunity to develop an first-cut effort to evaluate flow modification. We found that habitat degradation was the most common likely cause of poor biology, affecting 60% of sites with poor biology. However, salinization, flow modification, and eutrophication also affected a majority of sites. In contrast, temperature alteration was ruled out as a likely cause at most sites in poor condition. Sites along the lower mainstem of the SLRR tended to have multiple stressors as likely causes. RSCA and similar tools allow watershed managers to take advantage of the wealth of data monitoring programs provide, allowing them to go beyond identifying problems and start working towards solutions to fix them.