Billings reservoir is the biggest artificial water body (100 km² - 35m max depth) inside Sao Paulo (Brazil) metropolitan area, a urban agglomeration of 20 million inhabitants. Its waters, used for power generation, water supply, flood control and urbanistic and recreation purposes, are affected by the urban area growth since years 1950 and have been studied since 1970’s. Located at the borders of humid subtropical climate zone (23.7ºS), its particular operation to attend the demand of power generation and flood control combined to meteorological driving forces creates a very complex hydrodynamics, marked by water circulation and short events of stratification and mixing during spring-summer seasons. To understand the effects of this behavior over the dispersion of constituents and the sediment re-suspension a 3D hydrodynamic model (Delft-3D) was set up and calibrated based on conventional field campaigns conducted in the period 2007-2010. Results show that some compartments of the reservoir are more affected by meteorological forces rather than hydraulic operations, pointing to new parameters for monitoring and data acquisition and are the key for its sustainable management.