TRENDS IN STREAMFLOW, TEMPERATURE, AND PRECIPITATION on TRIBAL LANDS IN THE NORTHERN GREAT PLAINS

Tribal members of the Northern Great Plains, with their intimate connection to the land, are substantially influenced by changes to their environment. The U.S. Geological Survey is examining climate change on tribal lands as expressed in historical streamflow records for selected rivers on the Standing Rock Sioux Reservation and Cheyenne River Sioux Reservations in the Great Plains of North and South Dakota. Selected rivers include the Grand River on the Standing Rock Sioux Reservation, and the Moreau and Cheyenne Rivers on the Cheyenne River Sioux Reservation, all of which are located in the north-central Missouri River Basin. Previous studies of streamflow trends in the northern Great Plains indicated upward trends in annual flows for streams in the northeast Missouri River Basin (James River, Big Sioux River) and downward trends in annual flows in the northwest Missouri River Basin. The Grand, Moreau, and Cheyenne Rivers had no significant trends in annual streamflow (based on Kendall-Tau tests of significance) for the period 1957-2008. Trends in volumes of flows exceeding the 2-year recurrence interval for this time period indicate upward trends for the Grand and Moreau Rivers, and no significant trends for the Cheyenne River. The timing of spring runoff showed statistically significant trends toward earlier runoff (α = 0.20) for the Cheyenne River (12 days earlier) and Moreau River (17-22 days earlier). Trends in precipitation and temperature, based on gridded climate data (PRISM; ~4 km grid resolution) from 1957-2008 for these watersheds show increasing minimum temperature for all seasons and increasing maximum temperatures for winter and spring for the Grand, Moreau and Cheyenne River Basins, while precipitation shows no significant trends. Future trends for the Missouri River Basin based on the Community Climate Systems Model (CCSM) for the period 2000-2070 indicate warming of 3-4° C and a 16% increase in precipitation.