Water Resources Research Act Program

Principal Investigators: Bryan Swindell

Abstract: Climate variability affects the delivery of water to river systems, leading to potential conflicts between human demands and requirements of the ecosystem. For this reason, determining the pattern of natural variability of river flow is critical to understanding the vulnerability of water resources to climate fluctuations. Unfortunately, the instrumental record of river flow rarely reflects the long-term natural variability in basin hydrology. Several of the climate patterns that drive precipitation delivery to the western United States occur on multi-decadal scales and are therefore not reflected in the instrumental record. Indeed, there is evidence that the instrumental record of Western rivers does not provide a comprehensive representation of hydrologic variability during the previous several centuries (Graumlich 2003; Woodhouse and Lukas, 2006). Tree-ring analysis has emerged as a way to provide information on long-term hydrological conditions needed to make better water management decisions and ensure ecosystem sustainability.

The Bighorn River and its tributaries drain approximately 59,272 km2 (22,885 mi2) of Montana and Wyoming and provide roughly 2.6 million acre-feet of water annually to the state of Montana. In fact, in an average year about one-third of the total Yellowstone River flow comes from the Bighorn. Increasing and competing demands on water supply in the basin make the economy of the region more vulnerable to drought. Land use patterns are changing from traditional industries such as ranching and logging to rural-residential, and the economy is increasingly driven by water-dependent recreation such as fishing, boating, snowmobiling and skiing (Gray et al. 2004). This study will use the tree-ring record to evaluate the range of precipitation experienced in the basin over the last millennium and will examine the frequency and duration of past droughts, which will help water managers plan for the future.