The Isleta Diversion Dam (IDD) spans a reach of the Rio Grande located approximately 10 miles south of Albuquerque, New Mexico. The Middle Rio Grande Conservancy District (MRGCD) built the dam in 1934, while the U.S. Bureau of Reclamation (Reclamation) rehabilitated the dam in 1954. Presently, additional modifications are planned for the dam’s east sluiceway as well as to the dam’s fish passage capabilities (Baird et al. 2023). While the sluiceway modifications aim to improve sediment management in the irrigation canals to which IDD diverts water, the fish passage modifications target the Rio Grande Silvery Minnow (Hybognathus amarus, hereafter RGSM). The IDD fishway design concept consists of an adaptation of the Price-Stubb baffled cylinder fishway constructed within sheet pile walls (Baird et al. 2023). To meet the fish passage guidelines established in the 2016 Biological Opinion (U.S. Fish and Wildlife Service), work is underway to construct a fishway connected through one of the IDD gates bays. This fishway provides opportunity for upstream dispersive passage of the RGSM during the irrigation season. During periods of high flow that occur during spring runoff, sediment deposition is very likely to occur around the fishway. Sediment transport and deposition alongside and downstream of the fishway is influenced by gate operations. Baird and Sixta (2014) showed flow separation zones (eddies) effected by various gate operations that correspond to zones of sediment deposition. Reclamation’s Sedimentation and River Hydraulics Group two-dimensional numerical model (SRH-2D) was used to evaluate various fishway designs to determine: (1) configuration and orientation that minimizes the number of gates affected by the fish passage structure, (2) equalizes sediment deposition and transport along each side of the fish passage structure and (3) provides optimum flow conditions that will most likely reduce or eliminate the need for mechanical sediment management. This paper includes SRH-2D hydraulic modeling performed for different configurations of the fishway. It outlines the rational for selection of fishway orientation and dimensions tested by SRH-2D numerical model, presents results of 2D hydraulic model simulations in relation to the fishway design and concludes with the recommended fishway configuration and orientation for future design.
