Dissolved gas data were collected downstream from a moderately sized midwestern reservoir over a period of 10 years. Among 281 observations from August 1983 through April 1994, the gas pressure differential (▵P) averaged 119 mm Hg (116.0% of saturation) and discharge averaged 245.0 m3/s. A maximum ▵P of 251 mm Hg (133.6% of saturation) was associated with record release rates during the flood of 1993. Three-fourths of the 281 observations exceeded the U.S. Environmental Protection Agency's criterion of 76 mm Hg as ▵P (110% of saturation). Periodic examinations of live fish and of fish collected from fish kills demonstrated sublethal and lethal gas bubble trauma in fish downstream from the dam. The occurrence of periodic gas supersaturation-induced fish kills was linked to continued high dissolved gas pressures during periods when the discharge from the reservoir was substantially decreased. Lower discharge rates decreased river depth and lowered compensating hydrostatic pressure. The occurrence of gas supersaturation at this reservoir is of interest because of the potential for gas supersaturation at other reservoirs where gas supersaturation might not be predicted.