ScholarWorks at University of Montana

In 2008, the U.S. Geological Survey Global Change Program funded a study to examine integrated watershed scale response to global change in selected watersheds across the United States. Fourteen watersheds for which hydrologic models had been created using the Precipitation Runoff Modeling System (PRMS) were selected as study sites. PRMS is a deterministic, distributed-parameter, watershed model developed to evaluate the effects of various combinations of precipitation, temperature, and land use on streamflow and hydrology. The portion of the South Fork Flathead River watershed located upstream from Hungry Horse Dam in northwestern Montana is 1 of the 14 study sites.

Results from six General Circulation Models (GCMs), each using three GCM scenarios, were used to develop climate change scenarios for 2001-2099 for input to the existing PRMS model for the South Fork Flathead River. These PRMS simulations using the GCM scenarios were compared to PRMS simulations for current (1988-2000) conditions. All GCM simulations project an overall increase in temperature. Projected changes in precipitation for the South Fork Flathead River watershed were variable, with a slight tendency towards an increase in precipitation in the latter half of the 21st century.

PRMS simulations using the GCM scenarios project slightly increased mean annual streamflow in the South Fork Flathead River from about 2020-2099. However, these simulations project that less precipitation falls as snow, resulting in increased mean monthly streamflow January through April and decreased mean monthly streamflow June through September. Information from these climate-change simulations could be useful for management of Hungry Horse Reservoir.

The reported decline of native bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarkii lewisi populations west of the Continental Divide in Glacier National Park (GNP) prompted research to identify critical habitats and assess the abiotic and biotic factors influencing distribution and abundance. We evaluated the association of habitat metrics (stream width, elevation, gradient and woody debris density, mean August stream temperature) and biotic factors (the presence of non-native lake trout Salvelinus namaycush) with the distribution and abundance of bull trout and westslope cutthroat trout in 78 stream reaches in five sub-drainages of the North Fork Flathead River in GNP. Logistic and linear regression models were used to quantify the influence of these independent variables on species occurrence (presence/absence) and fish density. An information theoretic approach (AIC) was used to determine the most plausible combinations of variables in each case. The presence of westslope cutthroat trout was negatively associated with the presence of lake trout and positively associated with large woody debris abundance and water temperature. Westslope cutthroat were detected throughout a wide range of temperatures (8.5-16^oC), but were most abundant in small, complex streams that were not connected to lakes supporting nonnative lake trout populations. Bull trout occurrence was positively related to stream width and negatively related to channel gradient and water temperature. The highest bull trout densities were observed in streams with relatively cold mean August water temperatures (8-10^oC) and in areas isolated from lake trout by dispersal barriers. The low numbers of bull trout detected in this study likely reflect the imperiled status of lacustrine-adfluvial populations in GNP, owing to the invasion and establishment of nonnative lake trout from Flathead Lake. These data may be used to monitor and protect critical habitats and populations, inform conservation and recovery programs, and suppression/eradication efforts to reduce the impacts of nonnative invasive species.

Incidental recharge occurs when normal irrigation operations result in infiltration of water to an underlying aquifer. The effect (groundwater recharge) is generally unintended but has profound impact. In the Bitterroot Valley there are more than 500 miles of canals and laterals that divert about 374,000 ac-ft/yr of surface water to irrigate about 85,000 acres. Most of the diverted water, as much as 267,000 ac-ft/yr, either evaporates, runs off to the river, or infiltrates (as conveyance losses and irrigation returns). The component that infiltrates becomes incidental groundwater recharge.

Surface-water diversions, and the associated groundwater recharge, have been taking place for more than 100 years in Bitterroot Valley, creating artificial hydrologic conditions that many consider to be “normal” but are not necessarily natural. The results are observable throughout the valley and include creation of aquifers and wetlands in arid upland areas, formation of lush riparian areas, and enhanced late-season stream flows.

Understanding the nature and magnitude of incidental recharge in the Bitterroot Valley is important because rapid population growth and associated land-use changes have the potential to stress and/or alter the hydrologic system. Population growth has increased the demand on groundwater resources as demonstrated by the proliferation of private domestic wells—so called “exempt wells”. The number of domestic wells more than doubled between 1990 and 2010; well densities exceed 300 per square mile in places, the highest in the state. The population growth is also driving landuse change from agricultural to residential, reducing irrigated acreage, and potentially reducing the amount of recharge to the shallow groundwater system.

Despite the increase in groundwater use and land-use change from agricultural to rural residential, long-term groundwater level and stream flow data do not show measurable impacts, on a basin-wide scale, to groundwater storage or Bitterroot River baseflows.

The Flathead Basin Commission is one of the last intact ecosystems in North America, with the full complement of carnivores that were in the Basin when Europeans arrived, and with vascular plant diversity exceeding that of the Serengeti. However, the headwaters of the transboundary Flathead is threatened with large scale mining and energy development projects that would forever change the pristine nature of this basin. In 2008, studies were undertaken to compare water quality, aquatic biodiversity and fisheries in the Flathead Basin against the adjacent Elk Basin, which is heavily impacted by five open pit coal mines. To date, the results of the studies have shown statistically significant values with respect to heavy metals, particularly selenium, nitrates and sulphates in the Elk River versus the Flathead River.

Water right markets are defined by a competition for water resources from agricultural, environmental, industrial and urban sectors. All these demands are present in the Clark Fork River Basin. The emergence of an active water market will provide significant benefits to water management on the Clark Fork River, including

• More efficient use of water.
• Mechanism to facilitate future water transfers.
• A cost-effective, timely, voluntary, and transparent marketplace.
• Increased instream flows and other ecosystems gains.
• Encouraged land and water use planning.

Even so, criticisms of speculation, price collusion, and a drive for power continue to arise in market based discussions. Assessing these pros and cons is necessary to water market development. An ongoing project in the heart of the rapidly growing Missoula County provides an example of how water marketing benefits the basin and river. Grass Valley French Ditch Company desires to reallocate a portion of its irrigation shares to new ecosystem, domestic, and industrial demands within its water distribution system. Achieving this objective through water right marketing provides a win-win solution for the company and the community.