GROUNDWATER AND SURFACE-WATER FLOW MODELING OF CHAMOKANE CREEK BASIN, STEVENS COUNTY, WASHINGTON

Chamokane Creek basin is a 179-mi² area that borders and partially overlaps the Spokane Indian Reservation in southern Stevens County in northeastern Washington State. Primary aquifers within the basin are part of a sequence of glaciofluvial and glaciolacustrine fill within an ancient paleochannel eroded into Miocene basalt and Cretaceous to Eocene granite. The mean annual discharge of Chamokane Creek is 62.5 cubic feet per second (ft³/s), and the creek is a tributary of the Spokane River.

In 1979, all water rights in the Chamokane Creek basin were adjudicated by the United States District Court requiring regulation in favor of the Spokane Tribe of Indian’s senior water right. A court-appointed Water Master regulates junior water rights when the mean daily 7-day low flow in Chamokane Creek is less than 24 ft³/s; regulation has been necessary in 2 of the past 8 years (2001 and 2008). Additionally, the basin is closed to further groundwater or surface-water appropriation, with the exception of permit-exempt uses of groundwater.

The Spokane Tribe is concerned about the effects of future groundwater development in the basin and the potential effects of this growth on Chamokane Creek. To evaluate these concerns, the U.S. Geological Survey (USGS) is conducting a study with the primary goals of describing the groundwater and surface-water system of the valley-fill deposits of the basin and assessing the effects of potential increases in groundwater withdrawals on groundwater and surface-water resources.

The USGS coupled groundwater and surface-water flow model (GSFLOW) is being used to investigate the aquifer¯creek interactions and simulate the effects of potential groundwater withdrawals and climate scenarios on Chamokane Creek. In addition to measured streamflow and water levels, the model is constrained by snow course data, measured evapotranspiration, and estimates of actual evapotranspiration derived from a coupled remote sensing and Simplified Surface Energy Balance approach.