In October 1977, water-quality monitoring stations were established on five small streams that drain the Bull Mountains and also on the Musselshell River to document present water-quality conditions in a coal area of south-central Montana. Relatively static water-quality conditions exist throughout the annual flow cycle on the small streams but water quality varies with time on the Musselshell River. The near absence of surface runoff in the Bull Mountains during the study and the dominance by the base-flow component account for stability of water quality in the small streams. High-mountain runoff coupled with storms and prairie runoff impact the base flow of the Musselshell River. Bicarbonate and sulfate were the principal anions and are present in nearly equal proportions in all small streams. Except for West Parrot Creek, magnesium was the most dominant cation. West Parrot Creek, which consistently contained the smallest levels of dissolved solids, had sodium rather than magnesium as the principal cation. Fattig Creek was highest in dissolved solids with an approximate concentration range of 900 to 2,100 milligrams per liter. Suspended-sediment discharge in the streams was relatively small; no stream exceeded 0.32 ton per day. The Musselshell River had dissolved solids concentrations that ranged from about 450 milligrams per liter during spring runoff to 1,800 milligrams per liter during periods of base flow. The sodium sulfate-type water, which is common during base flow, is diluted during runoff with water having principal ions of calcium, magnesium, and bicarbonate. Suspended-sediment loads ranged from 0.56 to 37,300 tons per day and correlated directly to stream discharge. (USGS)