The impact of Douglas fir needles and twigs, western hemlock needles, and red alder leaves on dissolved oxygen and thus on the quality of mountain stream water was examined. The mean COD, 90-day BOD, and BOD rate coefficients were, respectively, 454 mg O2/g, 110 mg O2/g, and 0.125 for Douglas fir needles, 947 mg O2/g, 110 mg O2/g, and 0.056 for Douglas fir twigs, 570 mg O2/g, 200 mg O2/g, and 0.049 for western hemlock needles, and 882 mg O2/g, 286 mg O2/g, and 0.047 for red alder leaves. The 90-day values of BOD and K1 for the leaf material could be estimated accurately by tests of 20, 20, and 60 days, respectively, for Douglas fir needles, western hemlock needles, and red alder leaves. The BOD of leaf material exposed to fluctuating temperature exerted a 5-day BOD 4.0, 2.4, and 4.2 times greater than the standard temperature BOD5 for Douglas fir needles, western hemlock needles, and red alder leaves, respectively. Toxicity of a leachate extracted from each species was determined on guppies and steelhead trout fry. The concentration of material needed to produce toxic effects was very high, so high, in fact, that oxygen depletion probably would be responsible for death long before the leachate effect. Data on oxygen depletion obtained in this study will be useful in developing a predictive model for water quality management on forested lands.