Drift of Ephemeroptera and Plecoptera in Two Colorado Rivers

Abstract

Mayfly and stonefly drift were studied during the ice-free season in two Colorado mountain rivers. Eleven and 12 species of mayflies and stoneflies, respectively, were collected in 416 samples, taken at eight daily time periods. Drift rates ranged 1-499/100 m3. Baetis bicaudatus and Baetis tricaudatus were the most prolific mayfly drifters in all months in both rivers. B. tricaudatus exhibited a bigeminus pattern in the Dolores River in August and September, 1975, with high August drift rates of 499/100 m3 and 158/100 m3, respectively, during post-sunset and pre-sunrise hours. Baetis bicaudatus also exhibited a bigeminus pattern in August and September in both rivers. Larger Baetis nymphs drifted more at night than in the three daytime periods in May, June in the Dolores River, and larger B. bicaudatus drifted at night in September. These times correspond with months representing larger nymphal sizes and seem to support the hypothesis that larger nymphs drift predominantly at night, after reaching some mi...