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Geology

Gravel Bar on Bitterroot River 512 x 219 "Collectively, the many landscape and hydrological changes in the Bitterroot Valley since the Presettlement period have dramatically altered the physical nature, hydrology, and vegetation communities of the Lee Metcalf NWR." (Heitmeyer et al 2010)

 

Topography and Elevation

Bitterroot MountainsElevations on the Lee Metcalf National Wildlife Refuge range from about 3,230 feet on its north end to about 3,260 feet on its south end at the river (figure 11). The topographic variation within the refuge is related to the historical channel migrations of the Bitterroot River and its tributaries, scouring and natural levee deposition along minor floodplain channels, and alluvial deposition. A large southeast portion of the refuge contains higher, more uniform elevations while north and west portions of the refuge have lower, more diverse elevations. Alluvial fans are present in many locations along the Qafy geomorphic surfaces on the east side of the refuge. A larger tributary fan is present where North Burnt Fort Creek enters the Bitterroot River floodplain; this fan is much larger than the alluvial fans along the floodplain margin that grade into the Sapphire Mountains.    

Geology and Physiography

Bitterroot RiverThe following is taken from a report by Heitmeyer, Artmann and Frederickson (2010) describing the geology and physiography of the Refuge: "The Bitterroot Valley, where the Lee Metcalf Refuge is located, is a north-trending basin bounded by the Bitterroot Mountains on the west and the Sapphire Mountains on the east. These mountains and the rich montane Bitterroot Valley date to nearly 90 million years before the present (B.P.) (Hodges and Applegate 1993). The Bitterroot Valley extends about 120 miles from the confluence of the east and west forks of the Bitterroot River south of Darby to its junction with the Missoula Valley and Clark Fork River 5 miles south of Missoula. The elevation of the valley floor ranges from about 3,900 feet above mean sea level (amsl) in the south to about 3,200 feet amsl near Missoula. Summit elevations of surrounding mountains range from 6,000 to 8,000 feet amsl in the Sapphire Range and exceed 9,500 feet amsl in the Bitterroot Range.

The Bitterroot Mountains are composed of granitic rocks, metamorphic materials, and remnants of pre-Cambrian sediments of the Belt series. The Sapphire Mountains are mostly Belt rocks with localized occurrences of granitic stocks. The unusually straight front of the Bitterroot Range is a zone of large-scale faulting (Langton 1935, Pardee 1950); however, the Bitterroot Valley shows little sign of recent tectonic activity (Hyndman et al. 1975). Undisturbed valley fill shows that tectonic movement since the early Pliocene has been slight or that the entire valley floor has moved as a single unit. The structural basin of the Bitterroot Valley has accumulated a considerable thickness of Tertiary sediments capped in most places by a layer of Quaternary materials. Surficial geology evidence suggests Tertiary fill in the Bitterroot Valley may be up to 4,000 feet thick in some locations (Lankston 1975). Sediment is coarse colluviums near the fronts of mountains with finer-grain alluvial fill deposits that interfinger with floodplain silts and clays. Channel deposits of the ancestral Bitterroot River lie beneath the valley center.

Alluvial fan of Kenai Nature Trail Low terrace alluvium occurs as outwash, or alluvial fans, below the mouths of tributaries on both sides of the valley (Lonn and Sears 2001). Floodplain alluvium is mostly well-rounded gravel and sand with a minor amount of silt and clay derived from the edges of the neighboring terraces and fans. Most of the refuge is mapped as Qal alluvial deposits of recently active channels and floodplains. These deposits are well-rounded, and sorted gravel and sand with a minor amount of silt and clay. Minor amounts of Qaty (younger alluvial outwash terrace and fan complex deposits from the late Pleistocene) occur next to the Bitterroot Valley alluvium on the north end of the refuge. Materials in these terraces are well-rounded and sorted gravel of predominantly granitic, gneissic, and Belt sedimentary origin (Lonn and Sears 2001). Qafy surfaces extend along the Bitterroot Valley on both sides of the refuge. These surfaces are younger (late Pleistocene) alluvial outwash terrace and fan complexes of well-rounded cobbles and boulders in a matrix of sand and gravel deposited in braided-stream environments that formed between and below the dissected remnants of older fans. These surfaces appear to have been at least partly shaped by glacial Lake Missoula, which reached an elevation of 4,200 feet and covered the Bitterroot Valley near the refuge 15,000–20,000 years B.P. during the last glacial advance (Weber 1972).

The Bitterroot River has an inherently unstable hydraulic configuration and high channel instability, particularly between the towns of Hamilton and Stevensville (Cartier 1984, Gaeuman 1997). The river reach immediately upstream from the refuge has a complex pattern that is characterized by numerous braided channels that spread over a wide area of the valley bottom. The zone of non-vegetated gravels associated with this main braided channel system has widened and straightened since 1937 (Gaeuman 1997). In addition to this widening, severe bank erosion is common, but numerous cutoff chutes counteract some lateral bend displacement. Together, active river movements and a braided river channel pattern create low riverbanks and natural levees that encourage chutes and other avenues of river overflow. A complex network of minor channels occurs in the valley floor including the floodplain lands on the refuge (figure 10). These minor channels appear to flow from ground water discharge, which promotes erosion at slope bases and headwater retreat of small channel head cuts on the floodplain. Channel fragmentation appears to be controlled by irregularities in the respective elevation gradients of the valley.

Sapphire MountainsAbout 10–15 miles north of Stevensville, the Bitterroot River channel is more confined, compared to its highly braided form farther south. Despite limited changes in river shape north of Stevensville, the river stretch along the refuge has maintained a highly dynamic, instable channel form due to its geological, topographic, and hydraulic position. The historical floodplain at the refuge was characterized by the following: (1) multiple abandoned channels (for example, Barn and Francois Sloughs) that were connected with the main river channel during high-flow events; (2) small within-floodplain channels (for example, Rogmans and Swamp Creeks) that received water from ground water discharge and occasional overbank backwater flooding during high-flow events; (3) entry of two mountain- or terrace-derived major tributaries to the Bitterroot River (for example, North Burnt Fork Creek and Three Mile Creek); (4) slightly higher elevation inter-drainage point bars, natural levees, and terraces; and (5) alluvial fans (figure 7)."
References for citations above are here.
 

Last Updated: Feb 22, 2014
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