Streams often exhibit diurnal variations in hydrology and chemistry, yet in urban settings, the extent to which these patterns reflect influences from infrastructure remains poorly understood. Recent literature has highlighted hydrology and water quality may be important to characterize during baseflow when observations are not complicated by stormwater runoff. This study investigates diurnal variations in stream baseflow and water quality in Black Creek, a heavily urbanized watershed in Toronto, Canada. The aim of this research is to determine the degree to which observed diurnal patterns in hydrology and water quality reflect natural processes (i.e., photosynthesis, evapotranspiration), and/or extraneous water contributions to baseflow from dry-weather sewer overflows, and/or leakage losses from sewers. Stream water was sampled for ten 24-hour dry-weather days during summer 2025. Samples were analyzed for total nitrogen, inorganic carbon, organic carbon, and major ions. Key physiochemical parameters were measured in-situ, and high frequency discharge, wastewater flow, and precipitation data supported hydrological characterization. Repeated measures ANOVA was used to quantify parameter differences across 24 hours. Results indicate significant hourly variation for all parameters. Subsequent paired Student’s t-tests reveal that these variations closely resemble diurnal processes associated with respiration and photosynthesis. Preliminary discharge data also suggests a damped and lagged diurnal wastewater signal within streamflow, suggesting wastewater may influence stream water quantity. Results are expected to improve understanding of the mechanisms driving short-term variability in urban water quantity and quality, characterize potential contribution of wastewater derived baseflow, and inform future monitoring associated with planned sewer infrastructure upgrades.