LONG-TERM STREAM-FLOW TRENDS IN IN THE BITTERROOT RIVER WATERSHED, MONTANA

Development of groundwater disrupts the natural flow of water into and out of an aquifer. Groundwater removed from the system must be balanced by a decrease in groundwater storage and some combination of increased recharge and/or decreased groundwater discharge. The combination of increased recharge and decreased discharge in response to pumping is termed capture. Usually groundwater discharge (baseflow) is captured during development (Bredehoeft, 2005).

Most of the municipalities and residences in the Bitterroot watershed rely on groundwater. Between 1970 and 2012 the number of wells in the watershed showed an eight-fold increase (GWIC, http://mbmggwic.mtech.edu/). The well density on non-federal land is approximately 30 wells per square mile, the highest in the state.

To assess the response of the Bitterroot River to groundwater development, long-term streamflow data at Missoula (USGS gauge 12352500) were analyzed using the Exploration and Graphics for River Trends (EGRET) (Hirsch and De Cisso, 2012). Seasonal trend analysis and Loess smoothing were employed to compute flow statistics (daily maximum, daily median, daily mean, and 7-day minimum) and long-term trends on the period of record from 1935 – 2013. The winter, baseflow season (Dec-Feb) should be the most sensitive to impacts from groundwater depletion

Although there are more than 20,000 wells in the Bitterroot Valley, groundwater use has not produced measurable impacts, on a basin-wide scale, to Bitterroot River baseflow. The timing of increased groundwater development (1970-2012) does not coincide with an overall decrease in streamflow in the Bitterroot River. Rather, streamflow trends are highly correlated to precipitation records and relatively uncorrelated with increased groundwater development.

Long-term groundwater levels show a slight declining trend in 20 of 27 shallow monitor wells in the watershed. The declines may reflect: 1) decreased groundwater storage due to pumping, 2) decreased recharge due to changing irrigation or other land–use practices, or 3) decreased recharge due to climate variability. The decline in groundwater levels may eventually affect streamflow in the Bitterroot River.