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,'^~ '."',~ .i :J ",";:. .... .~; ,',','~ .,-.", ,; ~. t ~::. ; ') ~>i f'-. to C'- ....... C') Co; surveyed by NMISC, eight were resurveyed in the summer of 1993, all located below Hogback Diversion. Criteria for resurveying a cross-section was that at least one monument (transect end point) had to be intact and that a bearing was determinable. One limitation of the data provided by NMISC was that much of the channel bottom of the river was not surveyed. Consequently, assessing detailed changes in the river channel based on changes in bathymetry was not feasible in many cases, and evaluations of changes were "somewhat qualitative." However, by utilizing water surface elevations during ,both surveys and the flowrate at the time, some quantitative assessment of degradation or aggradation was made. Results of the resurveys by Bliesner and Lamarra (1994) indicated that only minor changes in channel bottom elevations occurred since Navajo Dam was completed. Five of the cross-sections showed slight narrowing and deepening. Two of the transects showed only slight narrowing, one possibly resulting from construction of a levee to protect an oil well. The remaining cross-section showed that most changes in position of the channel were due to the river changing course in a meander area, although channel bed elevations showed little change. The most significant finding of the resurvey by Bliesner and Lamarra (1994) was the change in riparian vegetation. They speculated that the lack of natural flooding events and introduction of exotic vegetation, particularly Russian olive, promoted channel stabilization, with the net result being a narrowing of the channel resulting in a reduced flood capacity. They also noted that during high flow events, Russian olives were being eroded from the bank and being deposited in the channel, thereby contributing to channel complexity. Further studies of changes in channel morphology from historic conditions are being based on interpretations and comparisons with historic aerial photography from the mid 1930s, early 1950s, early 1960s, and mid 1980s. These analyses are ongoing and scheduled for completion in 1997 (R. Bliesner 1997, pers. comm.). Evaluations of changes in channel morphology associated with recent changes in flow regime are being assessed based on changes in river chan.riel geometry over time and in association with specific flow conditions (Bliesner and Lamarra 1996). A total of 26 representative river cross-sectionS or transects were established during 1992 and 1993. Comparison of yearly survey data of pre- and post-runoff bed elevations along these transects is ongoing and has provided useful information on changes in channel morphology since 1992. Preliminary results indicate that changes in channel geometry are highly variable in response to annual hydrographs. However, a general trend of net scour has been observed since 1992, with an average elevation loss in the cross-sections of 0.3 m through 1995. This suggests that the channel may becoming progressively deeper and narrower within the study area. (Bliesner and Lamarra 1996). Summary The channel of the San Juan River has changed from what it was at about the time Navajo Dam was closed (1962). The channel is more stabilized, likely due to invasion of nonnative riparian vegetation and a reduction in natural flooding due to Navajo Dam. Data on recent changes in channel morphology indicate that the channel may be progressively becoming deeper and narrower in association with current flow regimes and conditions in the system. Further analysis of channel morphology changes from earlier time periods using historic aerial photography and should provide San Juan River Recovery Implementation Program Summary Report 13 17 January 1997 PR-576-2