Application of Shallow, Riparian Groundwater Models to Evaluate Maintenance and Alignment Alternatives for the Rio Grande Channel, Central New Mexico

Over the past century, the Rio Grande channel in a reach above the Elephant Butte Reservoir in Central New Mexico has become perched above its floodplain; therefore, the channel loses water at high rates to the groundwater system, and often dries during summer months. At the same time, ponding and shallow groundwater conditions occur within the floodplain.  The Middle Rio Grande Endangered Species Collaborative Program is evaluating the feasibility of relocating the river channel in the northern portion of the Bosque del Apache National Wildlife Refuge to enhance downstream water delivery and improve habitat for threatened and endangered species, including the Rio Grande silvery minnow and the southwestern willow flycatcher.  As part of this feasibility evaluation, a groundwater study is being conducted that includes the collection of site groundwater and surface-water data and application of a high-resolution groundwater model of the riparian zone to characterize changes in shallow groundwater conditions, evapotranspiration, and river seepage associated with alternative channel configurations.  The groundwater model simulates changes in the river location, width, depth, and extent of overbank flooding under various seasonal flow conditions for each of three alternatives, including conditions resulting from sediment build-up in the existing river channel; conditions with regular channel maintenance reducing sediment build-up; and, a realigned channel.  The results provide a basis for evaluation of trade-offs among management goals, including river channel efficiency and ability to remain wet during dry periods, maintenance of wetlands, and maintenance of desired riparian vegetative areas, as these relate to endangered species and water deliveries under the Rio Grande Compact.