Water-quality samples were collected from three sites in the lower Mississippi River in Louisiana during April 1991 through December 2003 by the U.S. Geological Survey and analyzed for selected herbicides, herbicide degradation products, and nutrients (nitrogen, phosphorus, and silica). When this study first began in April 1991, water samples were analyzed for selected herbicides and herbicide degradation products by gas chromatography/mass spectrometry. As the study progressed, there were changes and additions to the herbicide compounds analyzed, which resulted in lower analytical reporting levels or the quantification of additional herbicides or herbicide degradation products. Alachlor ethanesulfonic acid was analyzed by solid-phase extraction and enzyme-linked immunosorbent assay from 1993 through 1998. In March 1998, analyses for the acetamide degradation products, ethanesulfonic acid and oxanilic acid, by high-performance liquid chromatography with diode-array detection, were added. This method was replaced by liquid chromatography/mass spectrometry in August 1999.