Anthropogenic activities can result in the conversion of perennial rivers and streams into long-term, non-perennial systems, ultimately causing devastating effects on freshwater and riparian habitats, and wildlife. The Lower Kern River (LKR) historically functioned as a perennial-like river system; however, the river currently ecologically functions as an ephemeral river due to a series of weirs and canals that often divert 100% of flows away from the river. We present findings and lessons learned on how an unexpected, rapid dewatering event impacted the LKR ecosystem and fish community. We deployed temperature loggers (n=1 logger/site; n=5 sites) and measured stream physiochemical parameters (e.g., salinity, pH, etc.) before and during rapid water loss in the Kern River during summer and fall 2024. Underwater cameras, eDNA, and field surveys were conducted to survey the fish populations pre- and post-dewatering of the river. Preliminary results suggested that 1) water temperatures exceeded recommended thresholds for aquatic wildlife (>30̊C) throughout the river during summer 2024, and 2) rapid water loss resulted in multiple mass fish deaths (>3,000 fish) compared to the upstream reference site in early fall 2024. Important lessons learned included 1) adapting to drastic, unexpected changes when launching a new research project linked with a lawsuit, and 2) recognizing the importance of exploring the regional water politics that impact our local river to better inform research efforts. While data collection is still ongoing, these preliminary results highlight that ecosystem downgrading happens when perennial rivers are routinely dewatered, and thus, ecologically function as long-term ephemeral rivers, which can potentially have long-lasting effects on long-term ecosystem health.