Change Middle Rio Grande Channel Morphology Bosque Del Apache National Wildlife Refuge To Elephant Butte Reservoir

CHANGING MIDDLE RIO GRANDE CHANNEL MORPHOLOGY BOSQUE DEL APACHE NATIONAL WILDLIFE REFUGE TO ELEPHANT BUTTE RESERVOIR

Drew C. Baird, Ph.D., P.E., D.WRE1, Joshua Sperry2, Andrew Schied2, Ari Posner Ph.D3., and Nathan Holste M.S., P.E.1, and Pierre Y Julien Ph.D., P.E.4 1Sedimentation and River Hydraulics Group, Technical Service Center, Bureau of Reclamation, P.O. Box 25007, Denver, Colorado 80225, 2Graduate Student, Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80521, 3River Analysis Group, Technical Services Division, Albuquerque Area Office, Bureau of Reclamation, 555 Broadway Blvd, NE, Albuquerque, NM 87102, 4Professor, Department of Civil and Environmental Engineering, Engineering Research Center Colorado State University, Fort Collins, CO 80521.

ABSTRACT The Elephant Butte Reach of the Middle Rio Grande spans about 39 miles from the southern boundary of Bosque Del Apache National Wildlife Refuge to Elephant Butte Reservoir in New Mexico. Analyses of spatial and temporal trends in channel geometry and morphology are reported. Hydrology, hydraulics, and water surface elevation changes in Elephant Butte Reservoir caused significant morphological changes of the Middle Rio Grande over the past century. In recent years, since 2000, mean annual discharge and the magnitude of peak flow events have been lower than the long-term average due to a regional drought. Suspended sediment discharge has been lower than the historical average since about 1980 due to effects from upstream dams and land use changes. However, the main influence on channel morphology in this reach has been the reservoir water surface elevation. Between 1979 and 1985 water levels in the reservoir rose about 115 ft, resulting in channel sediment deposition throughout the entire reach. Since 2000, the reservoir elevation lowered by about the same amount, which resulted in main channel degradation with higher banks due to previous aggradation. The GIS analysis of aerial photographs and maps dating as far back as 1935 shows geomorphic changes, with a current channel width less than one-sixth of what it was in 1935. There was an increase in channel top width during the channel aggradation period until 2002, and subsequent reduction in top width due to channel degradation as the reservoir stage dropped after 2002. Median bed grain sizes remained about the same throughout the entire period. The hydraulic depth decreased during the river aggradation period and increased thereafter during the degradation period. During the period of increased reservoir stage, the channel slope decreased even though the channel was aggradational, and the channel slope increased while the channel experienced degradation when the reservoir stage decreased. Sinuosity has generally decreased since 1935. The geomorphic conceptual model of Massong et al. (2010) was considered and expanded to describe delta morphological processes associated with rising and falling reservoir stages. The model was also expanded from the elevation perspective to include cross section changes associated with each geomorphic stage.