Sediment and Nutrient Trap Efficiency of a Small Flood‐Detention Reservoir

Significant amounts of sediment and nutrients are removed from storm runoff by small flood‐detention reservoirs such as Callahan Reservoir in central Missouri, which stores 1 cm of runoff from its 1,460‐ha drainage area. The purpose of this study was to compare the trap efficiencies of sediment and nutrients and determine which factors affect them. During a 3‐year study, this reservoir trapped an average of 85% of the incoming sediment, 77% of the total sediment phosphorus (P), and 37% of the inorganic nitrogen (N). Sediment leaving the reservoir was clay and contained about 23% of the inflowing total sediment P. Sediment and P trap efficiencies (TE) for individual storms were related to concentrations of sediment and P (solution and sediment) in runoff, respectively. Average TE of total solution P (35%) was slightly lower than the TE of NO 3 ‐N (40%). Annual TE of ortho‐P ranged from 8 to 64% and averaged 43%. Dissolved organic and hydrolyzable P yields were greater in outflow than in inflow. These increases, which were <1% of the total P budget, may have been due to biological activity in the reservoir.