Riparian buffers prevent sediment and phosphorus (P) from reaching streams, but their accumulation in buffers is seldom measured. This study's objectives were to determine accumulations of sediment and P in a multi-species riparian buffer, and characterize spatial-temporal patterns of P in soil water and ground water. The buffer was planted in 2000, below a steep-sloping field in row-crop production under no-tillage management in Iowa's Loess Hills. Topographic surveys were conducted in 2002, after the buffer was fully established, and again in 2005. Mapped differences in elevation showed sediment accretion was associated with concentrated flow pathways and lateral flow along the buffer-crop margin. About 32% of the buffer's outer switchgrass (Panicum virgatum L.) zone had sediment accumulations exceeding 4 cm (1.6 in), which totaled 14.5 Mg ha-1 contributing area (2 t ac-1yr-1). Among five soil plots, total P in accreted sediment varied from 7 to 55 g m-2, totaling 9.6 kg P ha-1 contributing area (8.5 lb ac-1). Phosphorus concentrations in soil water were greatest beneath the switchgrass, compared to the crop and the buffer's inner vegetation zones (p<0.05). Concentrations in soil water and ground water were also greater where sediment accumulated, presumably due to increased infiltration of runoff. Sediment and P trapping occurred despite no-tillage management on the contributing hillslope, and relatively dry conditions during the study. This emphasizes the importance of installing multiple, complementary conservation practices in sensitive environments. Considering seasonal risks of runoff when selecting buffer species and anticipated runoff patterns when designing buffer areas could reduce subsurface P losses through riparian areas.