Modeled preservation risk to paleontological and cultural resources within Lake Powell and Lake Mead in support of resource impact analysis for Post-2026 reservoir operational alternatives (ver. 2.0 June 2026)

These data were compiled in support of the Post-2026 Operational Guidelines and Strategies for Lake Powell and Lake Mead (Post 2026) Environmental Impact Statement (EIS). Specifically, these data present modeled risks to paleontological and cultural resource preservation based on projected reservoir water level fluctuations between December 2026 and December 2060 within Lake Powell and Lake Mead, located in the southwestern United States. The condition and preservation of paleontological resources, representing evidence of ancient life such as fossils, trackways, and burrows within geological contexts, and cultural resources, representing evidence of human activity such as archaeological sites, historic land uses, and artifacts, are significantly influenced by reservoir development and management. Land inundation with reservoir filling can result in changes in the mechanical and chemical weathering processes that affect resources, and these changes are particularly destructive along reservoir shorelines. The preservation risk model presented here was developed with the assumption that processes altering or eroding the surface along the fluctuating shorelines can result in reduced information potential, degraded surface conditions, and impacted resource integrity. As such, the model incorporates an estimate of where resources are likely to be located (resource distribution model) and generalized topographic characteristics and lake dimension changes with water level fluctuation that represent impacts to those resources (preservation hazard model). Based on these understandings, the two preservation risk models (paleontological and cultural) provide a categorical system that spatially ranks the full pool reservoir area from 1 – 5, with 1 having the least potential for resource changes and 5 the highest potential for resource changes. In December 2025, we released tabular data representing the spatial distribution of each modeled preservation risk as an area percentage for each rank and statistical rank summaries (mean values) within equal interval elevation bin from the full pool elevation to an elevation near empty for both Lake Powell and Lake Mead. The elevation-risk comparison was then used to assess potential risks to resource preservation for eight proposed dam operational alternative strategies under 1200 possible hydrological futures at monthly time step. In June 2026, we revised the dataset to include a ninth operational strategy under 400 of the 1200 possible hydrological futures.

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