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Biomass of secondary producers was determined by quantitatively sampling the benthic macroinvertebrate community. The river-wide mean invertebrate dry weights (gm/m2) for the Colorado and Gunnison rivers are presented in Figure 14. There were significant differences in the Colorado River by sample period. The highest macroinvertebrate biomass estimates were found in spring 1994; this was followed by a decline, with the lowest levels reached in spring 1995. An increase in fall 1995 brought the levels up to those observed in fall 1994. COLORADO RIVER GUNNISON RIVER Mean Detritus Mean Detritus 100 Igo .... R .._ TT -- .._ ...... o ? a .e' 10 - 10 o ......:: g ........ A 1 ' 1 Spring 1994 Fall 1994 Spring 1995 Fall 1995 Spring 1994 Fall 1984 Spring 1995 Fall 1995 PERIOD PERIOD Figure 13. The mean and 95% confidence interval of the detritus values calculated for the river-wide substrate sample: collected in 1994 and 1995 in the Colorado (left) and Gunnison (right) rivers. 10 COLORADO RIVER Mean Invertebrate Dry Weight 10 GUNNISON RIVER Mean Invertebrate Dry Weight E - H L __. .... ........... .. .. .......... _._- .......... _.................... ; ? C .... ....... .... C ..... __ .. ... ..... m W 0 g Spring 1894 Fall 1994 Spring 1995 Fall 1995 ? Spring 1984 Fall 1994 Sprang 1985 Fall 1995 PERIOD PERIOD Figure 14. The mean and 95% confidence interval of the invertebrate dry weight values calculated for the river-wide substrate samples collected in 1994 and 1995 in the Colorado (left) and Gunnison (right) rivers. The Gunnison River demonstrated the same temporal pattern as the Colorado River with decreasing biomass levels over the first several sample periods followed by a significant increase in the fall of 1995. The only significant difference between the two river systems was found in the fall of 1995 where the Gunnison River with a biomass of 5.2 gm/m2 was significantly greater than in the Colorado River (1.3 gm/m2). The movement and distribution of organic materials in the Colorado and Gunnison rivers were determined by quantifying drift rate, invertebrates and organic matter. These results can be seen in Figure 15 for each time period within the Colorado River. Macroinvertebrate and organic drift densities (gm/m3) were highest in the spring (1994 and 1995) and lowest in the fall. Highest invertebrate drift was in spring 1994 and the lowest in fall 1995. Highest detrital drift was in spring 1995, corresponding to the increased water turbidity. In general, the 16