At the time of this report, the Bureau of Reclamation (Reclamation) is writing two supplemental Environmental Impact Statements (sEIS ) and a new Environmental Impact Statement (EIS) that will analyze the effects of changing water flow out of Glen Canyon Dam (GCD) (U.S. Department of Interior, 2024). These actions have the potential to affect downstream resources, including threatened and endangered species, in the Grand Canyon. This report covers modeling support provided on the two sEIS by the USGS Grand Canyon Monitoring and Research Center (GCMRC; U.S. Geological Survey, Southwest Biological Science Center). The first sEIS (Interim Guidelines sEIS) modifies Reclamation’s 2007 Colorado River Interim Guidelines for Lower Basin Shortages and Coordinated Operations for Lake Powell and Lake Mead (USBR, 2007) that determines annual water releases from GCD based on inflow to Lake Powell, the power generating requirements of GCD, and relative lake levels of Lake Powell and Lake Mead. Drought conditions have lowered the level of Lake Powell and may require GCD to release less water than was analyzed in the Interim EIS (7-million-acre feet/year). The effect of less water released, as well as lower lake levels and associated water quality concerns, on downstream resources was not analyzed in the 2007 Interim Guidelines EIS. Reclamation requested GCMRC support to provide models predicting the effects to resources of water releases lower than 7M acre feet/year, including models predicting effects to threatened and endangered species for use in a Biological Assessment.