Water level drawdowns are increasingly common in lakes and reservoirs worldwide as a result of both climate change and water management. Drawdowns can have direct effects on physical properties of a waterbody (e.g., by altering stratification and light dynamics), and can also have emergent effects on the waterbody’s biology and chemistry. However, the emergent effects of drawdown remain poorly characterized in small, thermally-stratified reservoirs, which are common in the landscape. Here, we intensively monitored a small eutrophic reservoir for two years, including before, during, and after a month-long drawdown that reduced total reservoir volume by 36%. Our study aimed to quantify the effects of water level change on reservoir physical, chemical, and biological properties. During drawdown, stratification strength (maximum buoyancy frequency) and surface phosphate concentrations both increased, contributing to a substantial surface phytoplankton bloom. The peak in phytoplankton biomass was followed by cascading changes in surface water chemistry, with sequential peaks in dissolved organic carbon, dissolved carbon dioxide, and ammonium concentrations that reflect biogeochemical processes associated with bloom degradation. Dissolved oxygen concentrations substantially decreased in the surface waters during drawdown (to 41% saturation), which was associated with increased iron and manganese concentrations. Combined, our results illustrate how changes in water level can have emergent effects on coupled physical, chemical, and biological processes. As climate change and water management continue to increase the frequency of drawdowns in lakes worldwide, our results highlight the importance of characterizing how water level variability can alter complex in-lake ecosystem processes, thereby affecting water quality.