Climatic Shifts in the Availability of Contested Waters: A Long-Term Perspective from the Headwaters of the North Platte River.

Early summer snowmelt from mountains in northern Colorado and southeastern Wyoming supplies the North Platte River, supporting nationally important agriculture, energy production, and urban development. Repeated decisions from the U.S. Supreme Court have fully apportioned Platte River waters among Colorado, Wyoming, and Nebraska, underscoring societal strains on this system. Now, climate change threatens the regional allocation of water. Tree-ring records indicate that past centuries contained multidecadal “megadroughts” far more severe than those of the historic period. However, the potential for even more persistent droughts, as the result of climate change, is poorly known. We document and evaluate the severity of recent and prehistoric droughts via a combination of data sources: modern temperature, precipitation, and stream gauge data; evidence of low lake-level stands; and related estimates of past hydroclimate change. Modern climate and stream data show an increase in spring temperatures of 2.21°C since 1916, an increase in the frequency of peak spring runoff before 1 May, and a reduction in winter precipitation. Lakes, however, that have only experienced minor hydrologic changes historically were desiccated during prehistoric dry periods during the past 12,000 years. Prehistoric lake shorelines indicate that water supplies were substantially reduced over centuries and millennia, such as from > 8,000 to < 5,000 years before present. The magnitude of these droughts likely also resulted in ephemeral river flows and thus indicates the potential for persistent shifts in regional hydrology. Such shifts should, therefore, be considered as part of long-term economic and legal planning. [ABSTRACT FROM AUTHOR]