A study of the Yocum Creek Basin conducted between July 27 and August 3, 1993, described the surface- and ground-water quality of the basin and the streamflow gain and loss. Water samples were collected from 12 sites on the main stem of Yocum Creek and 2 tributaries during periods of low to moderate streamflow (less than 40 cubic feet per second). Water samples were collected from 5 wells and 12 springs located in the basin. In 14 surface- water samples, nitrite plus nitrate concentrations ranged from 1.3 to 3.8 milligrams per liter as nitrogen. Orthophosphorus concentrations ranged from 0.01 to 0.06 milligrams per liter as phosphorous. Fecal coliform bacteria counts ranged from 9 to 220 colonies per 100 milliliters, with a median of 49 colonies per 100 milliliters. Fecal streptococci bacteria counts ranged from 37 to 1,500 colonies per 100 milliliters with a median of 420 colonies per 100 milliliters. Analyses for selected metals collected near the mouth of Yocum Creek indicate that metals are not present in significant concen- trations in surface-water samples. Diel dissolved oxygen concentrations and temperatures were measured at two sites on the mainstem of the stream. At the upstream site, dissolved oxygen concentrations ranged from 6.2 to 9.9 milligrams per liter and temperatures ranged from 18.5 to 23.0 degrees Celsius. Dissolved oxygen concentrations were higher and tempentture values were lower at the upstream site than those at the downstream site. Five wells were sampled in the basin and dissolved ammonia was present in concentrations ranging from 0.01 to 0.07 milligrams per liter as nitrogen. Dissolved nitrite plus nitrate was present in wells, with concen- trations ranging from less than 0.02 to 6.0 milligrams per liter as nitrogen. Volatile organic compound samples were collected at two wells and two springs. Chloroform was the only volatile organic compound found to be above the detection limit. Analysis indicated that 0.2 micrograms per liter of chloroform was present in one spring-water sample. In springs sampled, nitrite plus nitrate concen- trations ranged from 1.4 to 7.0 milligrams per llter as nitrogen. Dissolved ammonia plus organic nitrogen concentrations ranged from less than 0.2 to 0.49 milligrams per liter as nitrogen. Orthophosphorus concentrations ranged from 0.01 to 0.07 milligrams per liter as phosphorus. Fecal colfform bacteria counts ranged from 3 to 200 colonies per 100 milliliters, with a median of 18 colonies per 100 milliliters. Fecal streptococci bacteria counts ranged from 110 to more than 2,000 colonies per 100 milliliters with a median of 350 colonies per 100 milliliters. Large producing springs 1ocated in the mid to upper reaches of the basin contribute most of the flow to Yocum Creek. Streamflow increased an average of 29 percent on the mainstem of the stream. One losing reach was discovered on the mainstem of the stream and two losing reaches on tributaries to the mainstem. Surface flow steadily decreased along these reaches to the point where surface flow was not present, and the streambed became dry. These observations suggest that significant interaction exists between the underlying Springfield aquifer and surface flow in the Yocum Creek Basin.