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",0 ~ r-.. '~reliminary data on primary and secondary productivity and detrital abundance in run and riffle 2:pabitats was presented for the geomorphic reaches (Bliesner and Lamarra 1996). In both run and riffle habitats, highest periphyton biomass (primary productivity) was observed in Reaches 4 and 6, while invertebrate biomass (secondary productivity) was highest in Reach 8 and steadily decreased with downstream distance. Benthic detritus concentrations were significantly lower in Reaches 1-3 than in upper reaches (4-8). Other detailed habitat data currently being used by Bliesner and Lamarra (1996) to evaluate differences in depth, velocity, and embeddedness between various habitat types have not yet been analyzed based on their geomorphic reach designations. Completion of these analyses should provide additional information on habitat characteristics associated with the geomorphic reaches. Bliesner and Lamarra (1996) also presented results of a study on cobble substrate characterization where they compared substrate characteristics of suspected spawning locations in the San Juan River, near RM 131-132, with substrate characteristics of known and suspected spawning areas in the Yampa and Colorado rivers. They also compared substrate characteristics of suspected spawning locations in the San Juan River to other locales within the system, both upstream and downstream of RM 131-132. Results of comparisons between sites in the San Juan River and the Yampa and Colorado rivers indicated that substrate characteristics were similar in suspected or known spawning areas from all three systems, except that cobble size at the Colorado River sites was slightly larger, suggesting a general suitability of the sites in the San Juan. They also found that substrate characteristics of other cobble areas in the San Juan River study area, particularly upstream of RM 132, also appeared to be suitable for spawning, primarily due to similarities in the size of cobble and depth of open interstitial space. These sites were all located in geomorphic Reach 6 between RM 163 and 174. At sites below RM 132, in Reach 3 between RM 76 and 89, suitability of cobble areas for spawning appeared to be reduced based on less open interstitial space at the sites sampled. Summary Studies by Bliesner and Lamarra (1995, 1996) have provided substantial data on habitat characteristics by georn<?rphic reach in the San Juan River. Their information has provided thorough descriptions of macrohabitat composition, substrate characteristics, availability of low. , velocity habitat, and water temperature. Some aspects of habitat quality and quantity have also been related to specific geomorphic characteristics of each reach, such as gradient or landform. Other habitat characteristics appear to be more related to distance downstream of Navajo Dam than to the geomorphic characteristics of each reach, indicating the importance of longitudinal effects in the system. Results of ongoing studies and further analysis of existing data should provide additional information that can be used to assess habitat quality within each reach and refine the relationship between habitat characteristics and the geomorphologic characteristics of each reach. This, in addition to integration of data from other researchers useful in describing habitat characteristics of the geomorphic reaches, should lead to successful completion of this objective. 5.2.2. Determine Changes in Channel Morphology from Historical Conditions with Changes in Flow Regime. Changes in channel morphology from historic conditions were investigated by Bliesner and Lamarra (1994). Their investigation used cross-sectional survey data of the San Juan River floodplain collected by the New Mexico Interstate Stream Commission (NMISC) in 1962. Of the 23 transects San Juan River Recovery Implementation Program' Summary Report 12 17 January 1997 PR-576-2