Technical Abstract: Beasley Lake watershed in the Mississippi Delta is a 915 ha intensively cultivated watershed (49-78% in row crop production) that was monitored for the herbicide metolachlor from 1998-2009. As part of the USDA Conservation Effects Assessment Program (CEAP), the watershed was assessed for the effectiveness of agricultural best management practices (BMPs) implemented during the study period including vegetated buffer strips, conservation tillage, conservation reserve program (CRP), constructed wetlands, and quail habitat buffers. A variety of crops have been grown in the watershed including conventional tillage cotton, corn, milo, and conservation tillage soybeans. The purpose of the present study was to examine the influence of varying BMPs and/or land-use (cropping patterns) on observed metolachlor concentrations in Beasley Lake water. Metolachlor was applied to cotton, corn, milo and soybeans for weed control during spring (March-May) of 1998-2002, 2005, and 2007-2008. During those years, average mass of metolachlor applied was 12.7 kg with peak application during 1998 and 2007-2008. Spring metolachlor concentrations were measured in Beasley Lake water approximately monthly during the study period. Spring metolachlor concentrations in Beasley Lake ranged from below detection (< 0.02 ppb) to 3.1 ppb with an average of 0.16 ppb. A simplified model was produced to predict lake water metolachlor concentrations based upon herbicide applications. The model was in good agreement (R2 = 0.875) when BMPs encompassed <48% of the total watershed but in poor agreement (R2 = 0.197) when BMPs encompassed > 53%. Linear regressions used to determine associations of metolachlor with BMPs and/or cropping patterns showed decreasing ratio of lake metolachlor mass:applied metolachlor mass with increasing conservation tillage (R2 = 0.342), CRP (R2 = 0.309), and all BMPs combined (R2 = 0.458) while ratio of lake metolachlor mass:applied metolachlor mass increased with increasing conventional tillage cotton (R2 = 0.459). As a result, metolachlor levels in Beasley Lake after herbicide application are mitigated >3.7-fold when >50% of the watershed area have implemented BMPs and >3-fold when <9% of the watershed area are planted in conventional tillage cotton. Thus, greater conservation tillage, CRP, and combined BMPs in conjunction with decreased conventional tillage cotton presumably leads to decreases in applied metolachlor reaching the lake.