Lake Kapowsin in Pierce County, formed approximately 500 years ago when the Electron Mudflow from Mount Rainer dammed a valley creek, was recently designated Washington’s first freshwater aquatic reserve. In light of this new designation, the Washington Department of Natural Resources contracted University of Washington Tacoma staff to collect summertime water quality data to be used in future watershed management decisions. Field sampling was carried out once a month from June to October 2016 at three stations along a lake transect from inlet to outlet. Water samples were collected at surface and near-sediment depths to monitor chlorophyll a, total suspended solids, turbidity, and alkalinity, and in situ vertical profiles of temperature, pH, specific conductivity, and dissolved oxygen were measured. Secchi depth was also recorded and phytoplankton samples were collected by vertical net (20 µm mesh) tows. Elevated chlorophyll concentrations in the water column, coupled with a shallow Secchi depth, give indication of high algal productivity leading to poor light penetration. Consequently, Lake Kapowsin, despite its shallow depth, experiences thermal stratification between June and September due to the inhibited light penetration. This thermal stratification, in concert with the high algal productivity and bacterial aerobic respiration at the bottom of the lake, results in anoxic conditions and reduced pH in the bottom waters. These resultant conditions lead to a release of phosphorus and, to a lesser degree, nitrogen from the sediment. Ultimately, the persistent anoxic bottom waters capped with warmer waters makes the lake more suitable for warm water fish than cold water fish. The elevated productivity and anoxia in Lake Kapowsin is likely due primarily to naturally occurring eutrophication owing to the flooded forest inundated during the creation of the lake.