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Table 5. Eight general categories of habitat types on the Colorado River. CATEGORY HABITAT TYPE CATEGORY HABITAT TYPE Low Velocity Types Pool Backwater Types Backwater Debris Pool Backwater Pool Rootwad Pool Embayment Eddy Edge Pool Shoal Types Sand Shoal Riffle/Eddy Cobble Shoal Run Types Shoal/Run Slackwater Types Slackwater Run Pocket water Scour Run Shore Run Vegetation Overhanging Vegetation Undercut Run Associated Types Inundated Vegetation Run/Riffle Other Types Isolated Pool Riffle Types Riffle Cobble Bar Shore Riffle Rootwad Pile Riffle Chute Sand Bar Shoal/Riffle Tributary Chute Island Rapid Irrigation Return Boulder Biological Parameters-The primary producer trophic level in the Colorado River and Gunnison River was measured as instantaneous biomass (chlorophyll-a) estimates. These data, expressed as river-wide mean values (Figure 24) for each sample period, indicated that riffles had significantly higher periphyton biomass than runs (paired t-test, P=0.05). The only exception was the fall of 1995. Both habitat types Figure 23. The river-wide summary of habitats mapped in the demonstrated the same temporal Colorado River during September 1996. pattern with an initial increase between the spring and fall of 1994 followed by a large decrease over the winter of 1994. The subsequent increase between the spring and fall of 1995 was greater than the previous increase from spring to fall in 1994. Highest concentrations occurred in the fall of 1995. When compared to the Colorado River, the Gunnison River had higher periphyton concentrations in riffles and lower concentrations in runs (although not significant at the P=0.05 level) in the spring of 1994. In the fall of 1994 and 1995, the higher periphyton biomass levels in riffles and runs in the Gunnison were found to be significantly greater than the Colorado River (Figure 24). The detrital component of the primary producer trophic level was determined by quantifying the coarse particulate organic material (detritus dry weight) within the river substrate. Figure 23