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A Valley Shaped Over Time

A Valley Shaped Over Time 495x329  "Glacial Lake Missoula...at its maximum during the last ice age, the lake level reached an elevation of about 4350 feet. The water was then at least 2000 feet deep at the ice dam, and the volume of the lake was about 500 cubic miles-comparable to that of modern Lake Ontario" from Roadside Geology of Montana, Alt & Hyndman 1986.

 

 Hydrogeomorphic Changes

The ice age glaciers retreated thousands of years ago. And yet the snow melt, precipitation and the Bitterroot River drainage mimic (on a much smaller scale) the more ancient processes (hydrogeomorphology) that Glacial Lake Missoula had on the Bitterroot Valley. The following is taken from the Refuge Comprehensive Conservation Plan; the references are here.

Photo of Car Body Rip RapThe Bitterroot River stretch at the Lee Metcalf Refuge lies near the geomorphic threshold between a highly braided river channel pattern from Hamilton to Stevensville and a straight or sinuous channel pattern immediately downstream (figure 10). Consequently, the river channel pattern for the area is changing and highly sensitive to perturbation (for example, inputs of sediment, changes to shading or discharge) (Gaeuman 1997). The combination of irrigation development and land use changes, mainly in the 1900s, significantly altered hydrology and river channel morphology and movement in the Bitterroot Valley and its floodplains and facilitated degradation and loss of wetlands in this ecosystem (for example, Kudray and Schemm 2008). The extensive irrigation network of the Bitterroot Irrigation District led to construction of reservoirs, ditches, water diversion structures, and modified natural drainage routes. Stream channel networks, common in the Bitterroot Valley near the refuge, were altered by culvert and bridge crossings, railroad levees and beds, and extensive channelization of tributaries. Many stream channels, including sections of the Bitterroot River, were lined with riprap rock and car bodies to slow stream migration and in-channel bank erosion (photo at beginning of paragraph). In addition to local physical disruptions to topography and hydraulics, the entire fluvial system of the Bitterroot River has been altered by historical land use changes. The valleys and lower hill slopes have been grazed and farmed, while the upper valleys and mountains have been partly deforested. Overgrazing was common on many valley terraces and, when coupled with deforestation in neighboring mountains and slope areas, led to erosion and increased sediment loading in the Bitterroot River (Briar and Dutton 2000). Subsequently, extensive sedimentation has occurred in drainages and floodplain depressions on the refuge (USFWS 1988–93).

Photo of Bitterroot River ErosionThe channel morphology and discharge of the Bitterroot River has also been affected by land and water use in the valley (Gaeuman 1997). From 1936 to 1972, the Bitterroot River underwent significant adjustments in sinuosity and braided character causing a nearly 4-percent reduction in channel length between Darby and Missoula (Cartier 1984). Other data suggest that in the last decade, increased instability, channel migration, and overall widening of the river’s braided area from Hamilton to Stevensville has occurred compared to other reaches of the Bitterroot River both above and below (Gaeuman 1997). This instability has caused rapid erosion of riverbanks on the refuge (photo at beginning of paragraph) and increased physical dynamics of sediment and waterflow that facilitate rapid lateral channel migration across the refuge floodplain. In contrast to the highly active river migration physics from Hamilton to Stevensville, substantial narrowing of the Bitterroot River occurred near Stevensville and the refuge lands after 1937 in part because of artificial control structures. Part of the river has been channelized immediately upstream of riprap bank stabilization structures near the railroad embankment on the refuge. This artificial narrowing of the Bitterroot River to control river migration and bank erosion has actually heightened river migration tendencies immediately upstream of structures and has the potential to carve new channels across the refuge floodplain.

Aerial photograph maps of a 2.5-mile stretch of the Bitterroot River on the north end of the refuge from 1937 to 2009 show the highly unstable channel location of the river (figure 14). Three key points (labeled A, B, and C on figure 14) of river migration are apparent through the time-series of photographs, and typical movements of the outer riverbanks average about 8 feet per year. During more active periods of river channel bank migration, the rate of erosion is greater than 32 feet per year. The 1955 photograph reveals that the river migrated significantly to the south and was deemed a threat to the existing railroad bed and trestle. Subsequently, actions were taken by the railroad company to stop river migration by placing car bodies (figure 12) along the riverbank to act as riprap and cut off the river, which created an oxbow that is still present. The most active area of river migration in 2009–2010 is at point C. Between 2004 and 2009, the river migrated about 197 feet east, or about 39 feet per year. If this rate of river migration continues, then the river may reach the refuge’s main road in about 15 years and effectively remove about 10.5 acres of current floodplain land.

The Bitterroot River Irrigation District’s Main Supply Canal continues to transport water to most of the eastern benches in the Bitterroot Valley, including those next to the refuge. This canal facilitates a net transfer of about 75,000 acre-feet per year of water from the west side of the valley to the eastern benches and terraces. During summer, irrigation withdrawals significantly reduce flow in the Bitterroot River and some of its tributaries. Part of the diverted flow eventually drains back into the river system; this irrigation return flow is about 280,000 acre-feet per year in normal precipitation years. This includes well water and other canals used for irrigation. Average discharge of the Bitterroot River near Florence is 1,540,000 acre-feet per year, and at this point there is about a 13 percent current loss of discharge from irrigation use, other consumptive uses, and evapotranspiration. More than 10,000 wells are now in the valley, and the extraction of water from these wells, coupled with irrigation diversion, may be affecting ground water levels, recharge to floodplain wetlands, ground and surface water quality, and the connections of branches (anastomosis) of the Bitterroot River (Briar and Dutton 2000).
 

 

Last Updated: Feb 21, 2014
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