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. To understand the impact that rapid water loss linked with anthropogenic activities has on river ecosystems, we studied the recently rewatered Lower Kern River in Bakersfield, CA and subsequent regression of water levels due to weir repairs and water diversions. Specifically, we surveyed rapid water loss effects on aquatic ecosystem health and wildlife before, during, and post critical water levels on a perennial river that ecologically functions as an ephemeral river. 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. Mosquito fish (Gambusia affinis) and largemouth bass (Micropterus salmoides) were the dominant taxa (> 70%) observed prior to water loss; however, Catfish and bullhead taxa (Ictalurus spp. and Ameiurus spp.) also dominated the fish community post-water loss. Interestingly, we only observed the endangered rainbow trout (Oncorhynchus mykiss) prior to the dewatering event. While data collection is still ongoing, these preliminary results highlight the effects of rapid water loss due to dewatering rivers and massive water diversions. 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.