Analysis of streamflow measurements and continuous records of streamflow provided insight into interaction of the groundwater system with the Clackamas River in northwestern Oregon. This report assesses gains and losses of the Clackamas River based on streamflow measurements made during previous hydrologic studies, decades of continuous streamflow data, and a detailed suite of streamflow measurements made in September 2006. Gains and losses were considered significant if, after accounting for tributary inflows and withdrawals, the difference in streamflow from a measurement site to the next site downstream exceeded the streamflow measurement uncertainty. Streamflow measurements made in 1987, 1992, and 1998 indicated minor gains and losses. Comparison of continuous records of late summer streamflow of the Clackamas River at Estacada to sites at Clackamas and Oregon City indicated gains in some years, and no losses. Analysis of streamflow measurements of the Clackamas River from Estacada to Oregon City during low-flow conditions in September 2006 enabled an estimation of gains and losses on a reach-by-reach scale; these gains and losses were attributable to the geomorphic setting. During late summer, most groundwater discharge occurs upstream of Estacada, and groundwater contributions to streamflow downstream of Estacada are minor.