CHANGES IN THE MIDDLE RIO GRANDE DUE TO THE ALTERED HYDROLOGY

After large-scale flooding in the 1940s, a series of dams were constructed within the Middle Rio Grande reach (Cochiti dam to Elephant Butte Reservoir, ~175 river miles) to protect the public from the Rio Grande: Jemez (River) Dam (1953, permanent pool added in 1979); Galisteo (Creek) Dam (1970); and Cochiti Dam on the mainstem Rio Grande (1973). These three dams were constructed primarily to reduce peak flood flows through temporary water storage but they also permanently store sediment and organic debris. Along with these large dams are smaller irrigation diversion dams and a vast canal system that conveys water for agricultural consumption, drains wet soils, and captures groundwater for reuse. As with other rivers, the changed hydrology and sediment load associated with Cochiti dam operations appears to have been the most significant alteration to the Middle Rio Grande. Initially, the channel bed downstream of Cochiti dam coarsened and stabilized (Lagasse 1980) with operations, however, the most notable change throughout the reach has been a reversal from an aggrading river system with well connected floodplains and wetlands to a degrading channel. Near Cochiti dam the coarsening and degradation has created an easily identifiable transition zone that is at present 30 miles downstream of Cochiti dam (Ortiz 2003). Other notable channel changes have been planform changes in conjunction with narrowing, mixed sand/gravel appearance, incision, and channel location stability. As a consequence to similar physical changes throughout the entire watershed, the endangered Rio Grande silvery minnow, now resides only in the Middle Rio Grande reach. Channel incision and water/sediment storage appear to be the foremost changes contributing to this species decline. Although extensive water consumption has exacerbated the already ephemeral nature of the Rio Grande and directly affects the fish, water storage appears to be more influential for the fish through habitat change. Significant efforts are underway to create habitats suitable for the silvery minnow in the Middle Rio Grande; however, nearly the entire reach is currently in geomorphic transition.