In July and August 1988, the Illinois Environmental Protection Agency conducted an intensive stream quality survey of the lower Big Muddy River main stem. The survey focused on water quality, fish populations, contaminants in fish, instream habitat, and sediment chemistry in a 104-mile segment of the Big Muddy River between Rend Lake and the Mississippi River. The Big Muddy main stem survey was designed to assess stream quality in the vicinity of mine drainage sources and major municipal point source dischargers. A major objective of the project was to identify sources of acid mine drainage contributing to water quality degradation and fish kills in the Big Muddy River upstream from the public water supply intake at Royalton, Illinois. IEPA staff successfully identified two abandoned mines believed to be the major sources of mine-related water quality problems in the Big Muddy River in the vicinity of the Royalton. Water quality samples collected in summer 1988 at 14 main stem sites indicated Big Muddy River water quality was considered between fair/good and was generally not significantly impacted by point source dischargers. Main stem biotic integrity evaluated from fish population sampling at six sites was considered fair.