FLOW OPTIMIZATION FOR GEOMORPHIC AND ECOLOGICAL IMPROVEMENTS IN THE WILD AND SCENIC REACH OF THE RIO CHAMA, NEW MEXICO

The Rio Chama in Northwestern New Mexico provides a unique opportunity for improvement of the river’s ecological function, since it currently conveys more water than it did historically. This is due to Reclamation’s San-Juan Chama trans-basin diversion project, which imports an annual average of 96,800 acre-feet of water from the San Juan River basin. The 33-mile long reach between El Vado Dam and Abiquiu Reservoir has also been protected as a National Wild and Scenic River. Construction of El Vado Dam and Reservoir on this snowmelt-dominated river in 1935 significantly reduced geomorphically and ecologically important peak flows. The 2- and 5-year recurrence interval flood peaks were reduced by 40-50 percent. The San Juan-Chama Project water has increased the median flow in the reach from 87 cfs to 230 cfs. The primary sources of sediment downstream of El Vado Dam are tributaries, erosion of hill slopes and bank erosion, most notably during July-September monsoons. The, Rio Chama is still a geomorphically-functioning river system, but a smaller one; the pre-dam floodplain has been converted to a terrace that is being colonized by upland vegetation species; a lower elevation, narrower floodplain, primarily vegetated by native willows and sedges, has formed in the former river bed. The current trout fishery within the reach appears to be impaired by a lack of a suitable foodbase and possibly by a lack of suitable spawning substrate; both of these deficiencies are probably related to sedimentation (fine-grained silts and clays). An unexpected, but controlled, 6,000 cfs release from El Vado Dam in 2009, which was the highest flow since 1984, clearly demonstrated the benefits of the high flow by inundating the pre-El Vado floodplain and accomplishing a significant amount of geomorphic work including bank/terrace erosion, fossilized bar dissection, gravel transport and bar formation and introduction of large woody debris.The objectives of the Rio Chama optimization project are to modify the existing post-dam flow regime between El Vado Dam and Abiquiu Reservoir, to meet geomorphic, ecological as well as recreational goals while preserving hydropower generation and downstream water storage and user rights.