Excessive water diversions have substantially altered natural river flows throughout riverine systems within California (USA), often leading to drastic shifts in the availability of suitable habitat to support freshwater wildlife. The Lower Kern River (LKR) historically functioned as a perennial-like river system; however, 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 on how an unexpected, rapid dewatering event impacted fish community dynamics in the LKR. A planned visual fish transect survey was conducted late August 2024 along 2.4 km of the LKR to identify fish taxa and their abundances. Rapid dewatering of the LKR began downstream of our site concurrent to the timing of our survey, with the dewatering expanding throughout our site in less than seven days; therefore, we conducted a second survey one week after the first survey to compare changes in the fish community before and after the dewatering event. Photographs and videos taken were analyzed using ImageJ software to confirm fish identification, measure total lengths, and to determine life stages. River water was also sampled for fish eDNA during the first survey to confirm and expand field taxa identifications. The fish community before dewatering was dominated (>80%) by mosquitofish (Gambusia affinis) and largemouth bass (Micropterus salmoides). Rapid dewatering of the LKR resulted in a mass fish death event (>2,000 fish), which demonstrated that the fish community was instead dominated by mosquitofish (Gambusia affinis), yellow bullhead (Ameiurus natalis), and bluegill sunfish (Lepomis macrochirus). Juvenile fish were the most frequent life stage among taxa. Fish eDNA results added the Sacramento sucker (Catostomus occidentalis) to the taxa list. One taxon of interest was the endangered juvenile rainbow trout (Oncorhynchus mykiss), which has been observed in the LKR by scientists and citizens, but was not detected using eDNA. Freshwater ecosystems provide critical habitat for fish communities, which are affected by water availability and quality; therefore, it is important to find alternative pathways that support anthropogenic freshwater needs without sacrificing fish populations.