Based on the investigation into hydrology and water quality at the five inflow rivers (Dasha River, Baisha River, Shuangshi River, Fushi River and Wojiang River) of Dashahe Reservoir from March 2011 to February 2012, we analyzed the inflow volume, nitrogen concentration and phosphorus concentration at these inflow rivers and calculated the total external loadings in order to reveal the catchment impacts on the reservoir water quality and provide substantial information for land use regulation and water quality protection. The results show that Baisha River is the biggest tributary with an average volume of 1.01 m³·s^-1, which is located at the southwest of Dashahe Reservoir. Fushi River is the smallest one with an average volume of 0.23 m³·s^-1 at the northwest. The annual average concentration of total nitrogen (TN) ranged from 1.62 mg·L^-1 to 4.37 mg·L^-1, and the annual average concentration of total phosphorus (TP) ranged from 0.08 mg·L^-1 to 0.36 mg·L^-1 at all the five tributaries. Fushi River had the highest annual concentrations of both TN and TP. Dasha River had the lowest annual TN concentration and Baisha River had the lowest annual TP concentration in terms of measurements. The annual loadings of TN and TP for Dashahe Reservoir were estimated to be 176.7 tons and 13.7 tons, separately. Among all the five inflow rivers, Wojiang River at the north of reservoir had the most significant contribution to nutrient inputs with 33% of TN loading and 32% of TP loading. Shuangshi River at the west had the smallest TN contribution with 12% of total loading and Dashehe River at the southwest had the smallest TP contribution with 9% of total loading.