To determine factors influencing the hydraulics of constructed wetlands, the following studies have been conducted at the free-water surface Sacramento Constructed Wetlands Demonstration Project; friction surveys, a mass water balance, tracer studies, and vegetation growth and density analysis. The coefficient of friction, Manning’s n, was identified to range from 2.1 s/m1/3 to 7.6 s/m1/3. Vegetation densities ranged from 55 stems per m2 to 200 stems per m2 during the period of study. The values calculated from the Sacramento Project are significantly higher than typical values reported for natural open channels, but are similar to values reported for densely vegetated treatment wetlands. Increased flowrates tended to decrease the Manning’s n value. Overall, an average head loss, measured as the water elevation difference between inlet and outlet, of 1.7 cm was observed. The water balance identified water losses, composed of evapotranspiration and percolation, on the order of 40 percent of influent flow. Results of mesocosm experiments indicate that transpiration may account for up to 25 percent of these losses. Tracer studies indicate half-cell residence times of 5.3 days, or 110 percent of the nominal residence time. Although the wetland cells continue to behave in a plug flow regime, a moderate degree of mixing is observed. This is attributed to the presence of vegetation, small interior microchannels and recirculation eddies, and the presence of the open water areas at middle of the half cell that assists in mixing by redistributing flow along the width of the cell.