Constructed wetlands are used throughout the world to filter toxins from treated wastewater and to increase wildlife habitat. We investigated biological and chemical changes in treated sewage effluent as it passed through the Tres Rios Constructed Wetlands Demonstration Project in Phoenix, Arizona. Wetland influent water had very low levels of total coliform bacteria (< 4 CFU mL^-1) and no detectible E. coli, but both total coliform (> 300 CFU mL^-1) and E. coli (> 4 CFU mL^-1) increased at the wetland outflow. Heterotrophic bacteria also increased by more than 3 orders of magnitude from the inlet to the outlet of the constructed wetland. Chemical and physical analyses indicated that coliform proliferation was associated with the wetland flow patterns, with highest total coliform levels found in stagnant areas where residual chlorine levels were lower. In addition, coliform growth was significantly correlated (r = 0.51; p < 0.05) with water pH throughout all sampling sites. E. coli levels tended to be highest directly adjacent to waterfowl nesting areas, and microbial source tracking methods are being utilized to identify pathogen sources (human vs. other mammal vs. avian). By identifying the environmental factors controlling bacterial regrowth in the Tres Rios Wetland, this work will aid in construction of models to predict patterns of pathogen proliferation, ultimately leading to improved control of disease-causing bacteria in managed wetland systems.