Abstract <p>Most crop production systems rely on pesticide usage to minimize pest damage to crops and maximize yield. Herbicides are the most common pesticides used in agriculture and their off-site transport to water bodies occurs frequently, and subsequently impacts water quality, which is a worldwide concern. Long-term trends in herbicide concentrations in agricultural streams and rivers and the environmental factors associated with these trends are limited and needed to develop watershed conservation strategies. To address this research gap, in a 10-year study, instream atrazine, metolachlor, and simazine concentration were measured in the Cedar Creek watershed, a tributary of the St. Joseph River, Indiana, USA. The aim was to determine if trends occur in atrazine, metolachlor, and simazine concentrations in Cedar Creek, and if these herbicide concentrations are better predicted by time than other environmental factors. Our decade-long watershed study indicated that no annual or seasonal trends occurred in atrazine and metolachlor concentrations, and their concentrations were better predicted by climatic, hydrologic, and land use variables. Atrazine concentrations during the spring regularly exceeded the drinking water standards, which indicates a need for targeted conservation efforts to reduce atrazine concentrations after application. Annual and spring downward trends for mean simazine concentrations were observed and attributed to declining simazine usage. These findings provide information that will help guide watershed management and monitoring efforts within large agricultural watersheds.</p> Highlights <p>•&#xa0;Long-term assessment of atrazine, simazine, and metolachlor trends in the U.S. Midwest.</p> <p>•&#xa0;Atrazine and metolachlor showed no annual concentration trends.</p> <p>•&#xa0;Simazine concentrations declined, likely to reduced usage and cover crop adoption.</p> <p>•&#xa0;Atrazine often exceeded drinking water standards in spring.</p>