The objective of this study was to evaluate effects of conservation practices actually implemented in reducing sediment and nutrient loads at field and watershed scales. We obtained locational information of 10,028 conservation practices implemented in the St. Joseph River watershed from USDA - Natural Resources Conservation Service (NRCS). Considering expected water quality impacts and simulation ability of the Soil and Water Assessment Tool (SWAT) model, 5,583 of them were selected and incorporated into modeling at a hydrologic response unit (HRU) level by adjusting associated parameters. A calibrated SWAT model was used to estimate load reduction effectiveness of the selected practices. Model results indicated that many of the practices will likely reduce pollutant loads between 10% and 50% at field scale. Variability in load reductions among many practices was high. Most conservation practices reduced less than 1% of the loads when calculated for the entire watershed, but amount of the load reduction was still large and thus their cumulative long-term effects were expected to be significant. Conservation crop rotation and no-till, which were the most widely applied conservation practices in the study watershed, provided the greatest sediment load reduction, while conservation crop rotation and cover crop reduced the greatest amount of nutrients. Conservation crop rotation, cover crop, no-till, and mulch-till sometimes increased loads of soluble nutrients, resulting in the overall decrease in their effectiveness. Comparison of the spatial distributions of the selected conservation practices and simulated pollutant loads showed existing conservation practices were not targeted for areas producing relatively greater loads. The findings of this study demonstrated different effectiveness of conservation practice are at the different spatial scales, suggesting application area, field scale effectiveness, and placement of the practices are equally critical in achieving watershed scale water quality improvement.