E. E. Morris Jim C. Petersen M.E. Darling J. E. Terry 1984 A water-quality assessment was made of Illinois River, Muddy Fork, Spring Creek, and Osage Creek in northwest Arkansas. Data were collected to calibrate and verify steady-state digital, stream, water-quality models. The models were then used to simulate changes in instream diel-minimum dissolved-oxygen resulting from changes in nutrient loading. The city of Fayetteville proposes to divert part of its projected wastewater-treatment plant discharge to Illinois River. Muddy Fork, Spring Creek, and Osage Creek currently received effluent from the cities of Prairie Grove, Springdale, and Rogers, respectively. The diel-minimum dissolved-oxygen standard for each of these streams is 4.0 mg/L under projected loadings. Data collected indicate that none of the four streams meet Arkansas state standards for diel-minimum dissolved oxygen, total phosphorus, and fecal coliform bacteria. Computed dissolved-oxygen deficits indicate that benthal demand is the principal reason for dissolved-oxygen not meeting standards. Model simulations indicate that Spring Creek and Osage Creek can meet dissolved oxygen standards with stringent effluent limits imposed at the inspecting waste water-treatment plants; Muddy Fork and Illinois River can not. (USGS) application/pdf 10.3133/wri834092 en U.S. Geological Survey, Water-quality assessment of the Illinois River basin, Arkansas reports