Drift of Ephemeroptera and Plecoptera in Two Colorado Rivers

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 minimal size threshold that might increase their risk as prey for sight-feeding fishes. Ephemerella inermis drifted at densities of 6-9/1003 in May and July, respectively, in the Dolores River, and Rhithrogena sp., Ephemerella inermis and Cinygmula sp. drifted at peak rates of 11, 9 and 8/100 m3, respectively in May, June and July in the Dolores River. All other mayfly species drifted infrequently and/or at low rates of less than 2/100 m3 in both rivers. Stoneflies, except Chloroperlidae, drifted at low densities of less than 2/100 m3 in both rivers. For most species, these low numbers corresponded with low relative benthic abundance, making it difficult to ascertain propensity to drift. The relatively high benthic abundance of Claassenia sabulosa and Hesperoperla pacifica, coupled with their near absence in the drift, would suggest a low behavioral drift tendency, consistent with most drift studies that have included Plecoptera. Chloroperlid nymphs representing the genera Suwallia, Sweltza and Triznaka, drifted in the Gunnison River at the relatively high peak densities of 16-26/100m3, with a marked nighttime periodicity. Their peaks represented large, pre-emergent sized nymphs, and occurred in May, June and August, 1975, corresponding with emergence of various species throughout the summer.