Although the hydrological services of individual wetlands have been much acknowledged, their cumulative effect at the watershed scale has not been assessed as extensively. In this study, a distributed hydrological modeling platform, the PHYSITEL/HYDROTEL, was used to (i) investigate watershed hydrological processes with and without abundant wetlands, and (ii) quantify the effect of wetlands on streamflow, specifically baseflow and quickflow in the Upper Nenjiang River Basin (Northeast China). The modeling results showed that wetlands can strongly affect quickflow and baseflow, consequently modifying the watershed hydrograph. The study demonstrated the flow regulation services of wetlands at various temporal (i.e., daily, monthly, seasonal and annual) and spatial scales (i.e., sub-watersheds of different sizes). Specifically, wetland presence seemed to have stronger influence on baseflow and quickflow in the summer season than in the winter. The cumulative effect of wetlands on quickflow and baseflow quantity enhanced with the increasing wetland area. Anthropogenic activities such as reclamation and settlements could affect the regulation services of wetlands to some extent. These findings indicate that wetlands exert significant effects on watershed hydrological processes by modifying the temporal variability of quickflow and baseflow in this region. Therefore, it is important to protect and restore wetlands in the upstream portion of a watershed as a means of sustaining flow regulation and water resources in river basins.