The Illinois River located in northwestern Arkansas and northeastern Oklahoma is influenced by point and nonpoint sources of nutrient enrichment. This has led to increased algal growth within the stream, reducing water clarity. Also, sediment runoff from fields, pastures, construction sites, and other disturbed areas, in addition to frequent streambank failure, has increased sedimentation within the stream and decreased water clarity. A study was conducted by the U.S. Geological Survey in cooperation with the Arkansas Department of Environmental Quality and the U.S. Environmental Protection Agency to characterize the increased turbidity by determining the organic and inorganic composition and mass of suspended material in the Illinois River from August 2005 through July 2007. Water-quality samples were collected at four sites on the Illinois River (listed in downstream order): near Viney Grove, Arkansas; at Savoy, Arkansas; south of Siloam Springs, Arkansas; and near Tahlequah, Oklahoma. In general, turbidity, total suspended solids, suspended-sediment concentration, organic material concentration (measured as volatile suspended solids and ash-free dry mass), and chlorophyll a concentration were the greatest in samples collected from the Illinois River at Savoy and the least in samples from the most upstream Illinois River site (near Viney Grove) and the most downstream site (near Tahlequah) from August 2005 through July 2007. For example, the suspended-sediment concentration at the Illinois River at Savoy had a median of 15 milligrams per liter, and the total suspended solids had a median of 12 milligrams per liter. The Illinois River near Tahlequah had the least suspended-sediment concentration with a median of 10 milligrams per liter and the least total suspended solids with a median of 6 milligrams per liter. The turbidity, total suspended solids, suspended-sediment concentration, organic material concentration, and chlorophyll a concentration in samples collected during high-flow events were greater than in samples collected during base-flow conditions at the Illinois River at Savoy, south of Siloam Springs, and near Tahlequah. For example, the median turbidity for the Illinois River at Savoy was 3 nephelometric turbidity ratio units during base-flow conditions and 52 nephelometric turbidity ratio units during high-flow conditions. Organic material in the Illinois River generally composed between 13 and 47 percent of the total suspended material in samples collected from August 2005 through July 2007. Therefore, most of the suspended material in samples collected from the sites was inorganic material. Overall, the highest percentage of organic material was found at the Illinois River near Viney Grove and at the Illinois River near Tahlequah. The Illinois River south of Siloam Springs had the lowest percentage of organic material among the four sites. In general, the percentage of organic material was greater in samples collected during base-flow conditions compared to samples collected during high-flow conditions. The mean seasonal concentrations and percentages of organic material were the least in the fall (September through November) in samples collected from August 2005 to July 2007 from the four Illinois River sites, while the greatest concentrations and percentages of organic material occurred at various times of the year depending on the site. The greatest concentrations of organic material occurred in the summer (June through August) in samples from sites on the Illinois River near Viney Grove, at Savoy and south of Siloam Springs, but in the spring (March through May) in samples from the Illinois River near Tahlequah. The greatest percentages of organic material (least percentages of inorganic material) occurred in the summer in samples from the site near Viney Grove, the winter and summer at the site at Savoy, in the spring, fall, and winter (December through February) at the site south of Siloam Springs, an