The importance of riparian ecosystems in semiarid and arid regions has generated interest in understanding processes that drive the distribution and abundance of dominant riparian plants. Changes in streamflow patterns downstream of dams have profoundly affected riparian vegetation composition and structure. For example, in the southwestern United States, flow regulation has contributed to the replacement of many riparian forests historically dominated by the native Populus fremontii (Fremont Cottonwood) and Salix gooddingii (Goodding’s Willow) by the exotic species Tamarix spp. (Salt Cedar). The proposed project will help guide reservoir release decision making to enhance downstream recruitment of native cottonwood and willow stands below Alamo Dam on the Bill Williams River (BWR), AZ and will inform water management along other desert river systems. In addition, the proposed project builds on more than 20 years of partnership-supported flow-biota science, and represents a significant contribution to management-driven questions associated with how dam operations can be optimized to benefit water-dependent ecological resources while meeting other project purposes such as flood control, and water quantity and quality requirements. The proposed investigation employs hydraulic and ecosystem function models pioneered by the U.S. Army Corps of Engineers, such as the Hydrologic Engineering Center’s Ecosystem Functions Model (EFM), River Analysis System (RAS), and Reservoir System Simulation (ResSim) fitted with bare Earth and vegetation data derived from advanced Light Detection and Ranging (LiDAR) remote sensing techniques. Simulated and observed recruitment of Fremont Cottonwood, Gooding’s Willow, and Salt Cedar will be evaluated for a series of flow events occurring over the last decade on the BWR. Results will be used to formulate operational guidance for water managers on this river system and other similar arid-river systems. The products generated by this investigation will be explicitly analyzed and applied to address challenges associated with management of southwestern river systems in the context of a changing climate.
