A water-quality assessment of the Rock Creek (Cass) watershed was made June 12 and 13, 1974. Nutrients were resampled November 7, 1974. Surface waters within the watershed were calcium bicarbonate types with very similar concentrations of major dissolved constituents. Samples collected June 13 contained between 7.4 and 14 milligrams per litre dissolved nitrate (as N), between 0.05 and 1.3 milligrams per litre dissolved phosphate (as P), and between 2.3 to 4.0 milligrams per litre dissolved organic carbon. Samples collected November 7 contained less than 0.22 milligrams per litre dissolved nitrate (as N). The concentrations of fecal coliform bacteria ranged from 450 to 14,000 colonies per 100 millilitres. The concentrations of fecal streptococci bacteria ranged from 600 to 8,500 colonies per 100 millilitres. The bed materials of an upper Rock Creek site contained moderately high concentrations of dieldrin and chlordane (17 and 19 micrograms per kilogram, respectively) and also contained less than 6 micrograms per kilogram aldrin, DDD, heptachlor epoxide, and PCB's. Bed materials in a lower Rock Creek site contained less than 4.2 micrograms per kilogram of all the above compounds except PCB's. Biologic diversity indices of 2.7 and 2.2, which do not typify healthy benthic communities, were found in the upper and lower Rock Creek sampling sites, respectively. The benthic communities were dominated by midge larvae at both sites. Diatoms (Navicula sp) dominated the phytoplankton communities at both sites.
