Changes in herbicide use in the Midwestern United States have been substantial over the last 5 years. Most significant is a tripling in the use of glyphosate (N-(phosphonomethyl)glycin). Over this same time period (1997-2001), atrazine use increased by 20 percent and acetochlor use increased by 10 percent, while cyanazine use decreased by 99 percent, alachlor use decreased by 70 percent, and metolachlor use decreased by 55 percent. Previous studies have documented that herbicide flushes occur in midwestern streams during runoff events for several weeks to several months following application, and that herbicide concentrations in mid-western streams during flushing events are related to rates of herbicide use. The objective of this study is to determine the distribution of glyphosate and its primary transformation product aminomethylphosphonic acid (AMPA) in mid-western streams during post-application and harvest-season runoff events. Water samples will be collected in 2002 during two post-herbicide-application runoff events and one harvest-season runoff event from 53 sites on streams in the Midwestern United States. All samples will be analyzed at the U.S. Geological Survey Organic Geochemistry Research Laboratory in Lawrence, Kansas, for glyphosate and 20 other herbicides.