Crown of the Continent Hydrologic Observatory: Flathead Basin

 

BASIN

 

The proposed Hydrologic Observatory is 280 km on a north-south axis and 141 km on an east-west axis. Topographically, the watershed extends from the 3,092 m Mt Stimson to 753 m at its mouth near Perma, Montana. Within the 22,515 km2 basin over 80% of the basin is forested-alpine areas, 8% rangeland, 7% cropland and 1% urban. Four per cent of the basin is covered by water.

 

The Flathead River Basin is a tributary of the Columbia River that extends from headwaters watersheds located in British Columbia, Glacier National Park, the Great Bear Wilderness, the Bob Marshall Wilderness, the Mission Mountain Wilderness and the Rattlesnake Wilderness to the semi-arid rolling intermountain valleys near its mouth (Figure 1).

 

About 1,560 km2 of the North Fork of the Flathead River watershed lies in British Columbia, Canada. This portion of the watershed is managed by the BC Ministry of Environment, Lands and Parks and contains only a small amount of private holdings. Land management in the larger U.S. part of the Flathead basin involves the U.S. Forest Service, National Park Service, State of Montana, Flathead and Lake Counties, the Confederated Salish and Kootenai Tribes, private industrial and private non- industrial land owners.

 

 

The annual discharge of the Flathead River system is 10.6 billion cubic meters (Figure 4). The

North Fork, Middle Fork and South Fork of the Flathead river generate 80% of the annual stream discharge at the river mouth. The Swan River basin provides 9% of the annual discharge, and the Whitefish and Stillwater rivers, draining the northwestern portion of the

basin, generate 5% of the annual flow. Portions of the watershed regulated by dams include the South Fork of the Flathead River that contains the 95 km2 Hungry Horse Reservoir, the outlet of Flathead lake regulated by Kerr dam and a small dam on the Swan River near Big Fork, MT.

 

 

 

 

 

 

Metasedimentary rocks of the Precambrian Belt Super Group dominate the tectonic history of

the region as the Rocky Mountains formed through compressional deformation about 80

million years ago. Subsequent extensional forces forced apart large parallel bocks

of bedrock with sediment- filled intermountain valleys. Most recently, continental and alpine

glaciations carved the landscape and deposited glacial sediments in the intermountain valleys.

This landscape was also dominated by glacial lakes including glacial Lake Missoula that filled

and emptied leaving an unparalleled record of glacial lake sedimentation. The mountain

arÍtes, u-shaped valleys, and hummocky terrain of the intermountain valleys reflect the glacial

influence on the basin landscape.

 

Soil development and depth are dependent upon the type of deposition and physiographic

position. Soils of intermontaine valleys are primarily formed in either fine textured lake bed sediments or floodplain sediments over coarse alluvial gravels. Depth of development and suitability for various crops is dependent upon the short growing season and the availability of irrigation diversions. Soils of uplands are formed in deep tertiary alluvial deposits or in Belt colluvium or residuum. Soil development is limited by the rugged topography (surface drainage), limited rainfall, and recent deposition or exposure. The upland soils are vegetated by intermontane prairies or coniferous forests. Forest soils and forest productivity are heavily influenced by ash accumulations from Cascade volcanoes

 

The climate is dominated by the Pacific coastal systems in the winter that are occasionally overridden by the continental air masses originating north and east of the area. The average winter temperature in Kalispell is 7C and summer temperatures typically range from 20C to 27C. The large variation in topography results in high mountain areas receiving 200 to 300 cm/y of precipitation with snow packs reaching 6 m. In contrast the valleys receive 39 to 50 cm/y (FHEIS, 1983). The growing season is estimated at 120 to 130 days in the main Flathead Valley with an extended growing season of 140 days along the east shore of Flathead Lake.

 

Studies of glaciers in the Glacier National Park region recorded 150 glaciers in 1850 and 37 in

2004. Forecasts suggest all glaciers will be gone within 40 years. These studies are associated

with sets of long term climatic records collected by the park and USGS researchers.

 

Mountain vegetation below the 2,450 m treeline is dominated by Englemann spruce, Whitebark pine and subalpine fir (Figure 2). At lower elevations Douglas- fir and zones of ponderosa pine are present. In some wet areas red cedar and western hemlock are observed. Wildfires allow lodge pole pine and western larch to dominate burned areas. Grasses such as rough fescue, Idaho fescue and bluebunch wheat grass dominate the dry valley floors and hills in the southern portion of the basin. Riparian zones are dominated by black cotton wood, some paper birch and shrubs (FHEIS, 1983).

 

The basin contains 12 hydrologic landscape regions (USGS). It is dominated by semi arid mountains with impermeable bedrock (15, 17,18). Other major landscapes include sub-humid plans with impermeable soils and permeable bedrock (3), humid plains with permeable soils and impermeable bedrock (9), and semiarid plains with impermeable soils and bedrock (8). Valley floors are classified as arid to semiarid plans and plateaus with permeable soils and bedrock (5, 10, 12, 13, and14). A portion of the mountain region is classified as humid mountains with permeable soils and impermeable bedrock (20).

 

 

 

 

The basin contains over 500 lakes ranging from glacial tarns to the 510 km2 oligotrophic

Flathead Lake, the largest naturally occurring freshwater lake west of the Mississippi River

(Figure 1). It occupies an extensional half-graben that is bounded on the east by the

seismically-active Mission Mountain normal fault. Flathead Lake is located at the former ice

margin of the Cordilleran Ice Sheet during the most recent (Pinedale) glaciation. Sediments

within the lake contain a very well preserved record of deglaciation associated with retreat of

the Cordilleran Ice Sheet.

 

Water management practices include water diversions for irrigation, power generation, flood control, flows for endangered, threatened and listed species, and recreation. Federal, state, tribal and local units are involved in water management. Management actions include local needs and national mandates.

 

Stream flows are snow-melt dominated and underpinned by groundwater baseflow. The site headwaters contain 37 glaciers and thousands of square kilometers of watersheds in which fire and disease are the only disturbances.In contrast, the HO also contains watersheds at multiple scales that were dominated by glaciers within the last 100 years but are now glacier free, impacted by timber harvests and fires of varying ages to varying degrees, modified by water management practices including irrigation diversion and dams, and altered by development for homes, cities and agriculture.

 

 

 

For several thousand years this system has been dominated by snow-melt runoff and moderated by large quantities of water stored in Glacial ice. However, the timing and magnitude of droughts and summer flows have changed dramatically. With the information that can be gleaned from sediment cores and landscape records at different scales, this HO will provide scientists with opportunities to establish baseline watershed conditions and data on natural hydrologic variability. Such a context frames the current and further observations and assists with translating measured changes into links with the varied HO ecosystems.

 

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The HO watersheds are some of the only pristine watersheds left in the contiguous U.S.. They provide critical habitat for key species including the native threaten bull trout and lynx, and the listed western cutthroat trout, bald eagle, gray wolf and the grizzly bear.

 

Crown HO

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