Rain events cause large changes in physicochemical and biological conditions in lakes. Stormwater may modify water conductivity, increase pollutant and sediment input and promote flushing effects, turbulence and mixing of the water column due to water inflow, air temperature decrease and strong winds. In turn, changes on physicochemical conditions have an effect on the dynamics of primary producers and higher trophic levels. In the urban context, catchment response to rain takes place in a few tens of minutes and the monitoring of urban runoff impacts on lakes should take advantage of high frequency monitoring technologies, which provide appropriate time resolution for analysing these phenomena. Improving our understanding of stormwater runoff impacts on lake ecosystems is critical to define strategies for water resources and catchment management and to cope with land use changes due to population growth and increasing urbanization as well as climate change impacts on hydrological processes. Lake Pampulha in Brazil (mean depth: 5.0 m, 197 ha) is an urban water body that has been monthly monitored since September 2011. Water temperature, conductivity and pH were measured every 0.5 m in a middle site in the lake. Flow of Lake Pampulha main tributaries are monitored every 10 minutes since October 2011 using automatic sensors. The General Lake Model (GLM) was used to model water temperature and mixing in the lake. Simulated water temperature showed good agreement with observed water temperature (rmse = 0.70°C). Model results and moni toring data showed that stormwater runoff are responsible for mixing the water column and disrupting algal growth during wet summer.