Strip mining for coal in southwestern Washington may be affecting the water quality of streams. To investigate these possible effects, five streams were selected for study of water quality in each of the two coal bearing areas: the Centralia-Chehalis coal district, and Kelso-Castle Rock coal area. In the Centralia-Chehalis coal district, three of the streams have drainage basins in which mines are active. Water in streams that drain unmined basins is typical of western Washington streams and is characterized as a mixed water because calcium, magnesium, sodium, and bicarbonate ions predominate. A change in anionic composition from bicarbonate to sulfate in streams draining mined areas was not sufficient to change the general water composition and thus make the streams acidic. The largest downstream changes in water quality in both mined and unmined drainage basins were observed during summer low-flow conditions, when minimal dilution, increased water temperatures, and low dissolved oxygen concentrations occurred. High dissolved solids were found in the mined drainage basins during this period. High concentrations of iron, manganese, and zinc were present in the bottom sediments of the mined basins. Moderate concentrations of chromium, cobalt, copper, and zinc were also found in the bottom sediments of a few unmined basins. Streams with substrates of gravel-cobble or gravel-coarse sand had the most diverse benthic fauna and a higher number of ubiquitous taxa than streams with sand-silt substrates, which had the most dissimilar fauna. Mayflies, stoneflies, and caddisflies were rare at the site most affected by mining. The erosion potential of a basin appears to be related to the average basin slope and the amount of forested areas. Strip mining for coal in steep basins may lead to massive movements of unconsolidated spoils after vegetal cover is removed if the land disturbed is graded to pre-mining slopes. (Lantz-PTT)