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Temporal Variations (River-Wide) Between 1994 and 1995, the trophic structure of the Colorado River was described during four time periods. Samples were collected in the spring (March) and the fall (September) during baseflow conditions of each year. The Gunnison River was sampled in the spring and fall of 1994 and the fall of 1995 at only one location (one stratum, three reaches). Figure 2 shows the sample periods and the corresponding hydrographs for the Colorado and Gunnison Rivers. It should be noted that spring flows in both rivers were below normal in 1994 and above normal in 1995. Stratum sampled for each time period can be seen in Table 3. COLORADO AND GUNNISON RIVERS Physical Parameters-The major DAILY MEAN FLOWS physical parameters measured at each 80000 sample location involved the _. quantitative description of the - - streambed materials. The four 070009 ---- physical parameters measured LL - - characterize the bed structure. 19990 Wolman pebble counts, in D -- combination with size fractions of Jan94 Apr-94 Jul-94 Oct-94 Fe"S May-95 Aug-95 Dee-95 DATE materials less than 2 cm, Colorado River at Cisco - Gunnison River at Grand Juncti n quantitatively determined the size distribution of bed materials. The Figure 2. The 1994 and 1995 hydrographs for the Colorado and Gunnison rivers. Solid bars denote sampling periods. second physical parameter, depth to embeddedness (DTE) is a measure of the distance from the top of the substrate to the top of the layer of fines in which the substrate is embedded. It represents an index of the available depth for aquatic organisms. The third parameter, the percent of surface area embedded (PAE), is a measure of the two-dimensional surface area (% surface area) covered by fine material whereas the fourth parameter (interstitial void volume) is a measure of the three-dimensional volume (cm3/m2) between substrate particles above the embedded layer. The average cumulative distribution of bed substrate sizes for the Colorado and Gunnison rivers can be seen in Figure 3. Substrates were divided into 15 size categories ranging from 0.063 mm to 512 mm. The initial conditions (size distributions) observed in the spring of 1994 appeared to be uniformly distributed in the size classes less than 1 mm. In total, bed materials less than or equal to 1 mm made up only 4 percent of the total material. Bed substrates between 1 and 10 mm were not very abundant while size classes between 10 and 500 mm made up over 90 percent of the bed material. The distribution of material sampled in the fall of 1994 was very similar to the spring 1994 data with the exception of materials less than or equal to 1 mm which increased to 9 percent of the total. This pattern continued in the spring and fall of 1995 with the substrates less than or equal to 1 mm, increasing to 20 percent of the total. It is interesting to note that the materials less than 0.25 mm markedly decreased in the fall of 1995 when compared to the spring of 1995. 9