Abstract: As a consequence of the dam,the physical transport of dissolved matter is profoundly affected by human activities in reservoirs.A threedimensional numerical model,Environmental Fluid Dynamics Computer Code (EFDC),is used to study the physical mechanisms of mass transport in the Falls Lake Reservoir in North Carolina,USA.The spatial and temporal variations of the mean age and the residence time of a tracer are analyzed.Under steadystate conditions with constant in/outflows,the dissolved matter transport is dominated by river discharges.Under dynamic conditions,the phase of the water storage is also an important factor.The geometric feature and the function of the reservoir and the distribution of the tributary runoffs are the major factors that contribute to the differences of the local residence times.