Nonpoint source (NPS) pollution from agricultural areas is one of the most critical problems threatening the nation's water resources. Surface runoff is a major carrier of NPS pollutants. Riparian buffers have been proposed as a means of reducing transport of NPS pollutants from agricultural areas to streams. Using rainfall simulation techniques and natural runoff monitoring, plot studies were conducted to evaluate the ability of an established multi-species riparian buffer system to reduce sediment and nutrient in agricultural runoff. The buffer system evaluated contained three types of vegetation: cool-season grasses, warm-season grasses, and woody plants. Results indicated that the effectiveness of the riparian buffer was determined by the characteristics of the pollutants, buffer width, vegetation type, and permeability of the buffer soil. Under the simulated rainfall and runoff condition, the 6 m wide grass buffers removed 77% of the incoming sediment from surface runoff while the 3 m wide buffers removed 66%. Differences between 6 m and 3 m wide buffers were significant (P < 0.05) for sediment and nutrient removal. A warm-season grass buffer composed of switchgrass (Panicum virgatum L. cv. Cave-n-Rock) was more effective in reducing transport of sediment, total-N, NO3-N, total-P and PO4-P in runoff compared to a cool-season grass buffer. Under natural rainfall conditions, the 7.1 m wide warm-season grass (switchgrass) buffer removed 95% of sediment, 80% of total-N, 62% of NO3-N, 78% of total-P, and 58% of PO4-P in surface runoff. The 16.3 m wide warm-season grass/woody buffer removed 97% of sediment, 94% of total-N, 85% of NO3-N, 91% of total-P, and 80% of PO4-P in runoff. Clay and soluble nutrient reduction were related to the degree of runoff infiltration. Even though the warm-season grass buffer was effective at removing sediment and sediment-bound nutrients, the warm-season grass/woody buffer increased the removal efficiency of soluble nutrients by over 20%. The combination of dense, stiff, warm-season grasses, and woody vegetation in a riparian buffer system improves the removal of NPS pollutants from agricultural runoff. These results can be used to improve the design of riparian buffers for improved water quality in agricultural areas.