Mitigation for the Construction and Operation of Libby Dam

Young Creek State Lands Restoration Project
Young Creek is one of the most important westslope cutthroat trout spawning
tributaries to Libby Reservoir because it represents one of the last known genetically pure
populations of westslope cutthroat trout in the region and it is also one of the most
potentially productive tributary streams to Libby Reservoir. Although bull trout (Salvelinus
confluentus) do not routinely spawn in Young Creek, juvenile bull trout commonly enter
Young Creek from the reservoir and rear for extended periods. This stream also provides
water for agriculture, and other riparian-dependent resources. During the 1950’s,
approximately 1,200 feet of the channel located on the state owned section (DNRC School
Trust Land; Figures 8 and 9) on Young Creek was straightened, diked, and the stream
channel moved near the toe of the hill slope. This channelization compromised the stream’s
ability to effectively transport sediment through the channelized area, which caused the
channel to aggrade (deposit bedload materials) and exacerbate flood conditions. Ironically,
these were presumably the conditions that the original channel modification aimed to
alleviate. Consequently, the aggradation caused numerous problems with the stream, such
as; poor aquatic habitat, increased flood potential, lateral bank scour and increased sediment
supply. Additionally, livestock grazing and timber management in the upper reaches of
Young Creek likely contributed to the instability of the channel. The degraded condition of
this section of Young Creek has contributed to the stream’s inability to adequately transport
stream flow and bedload supply and still maintain a stable channel. The project site is a
1,200-foot, over-widened reach of Young Creek containing several mid-channel gravel bars
and eroding stream banks. In order to improve the function and stability of this section of
Young Creek, Montana FWP reconstructed the stream channel in the fall of 2003.
The intent of the project is to: 1) reduce the sediment sources and bank erosion
throughout the project area by incorporating stabilization techniques that function naturally
with the stream and which decrease the amount of stress on the stream banks; 2) convert the
channelized portions of stream into a channel type that is self-maintaining and will
accommodate floods without major changes in channel pattern or profile; 3) use natural
stream stabilization techniques that will allow the stream to adjust slowly over time and be
representative of a natural stream system; and 4) improve fish habitat, particularly for
westslope cutthroat trout, and improve the function and aesthetics of the stream and adjacent
riparian ecosystem.
The Young Creek State Lands Restoration Project changed the dimension, pattern
and longitudinal profile of this section of Young Creek. Prior to project implementation, the
stream consisted of multiple channels throughout much of the project reach with lateral
channel migration common between and within years. We completed cross-sectional
surveys in 4 riffles prior to project construction and 10 after the project was completed. We
measured cross sectional area, mean bankfull width, depth, maximum depth, and width to
depth ratio at each transect, and compared mean values for each parameter using a t-test.
The existing conditions were typified by an over widened and shallow channel with a mean
bankfull width of approximately 28 feet, a mean bankfull depth of 0.6 feet, and a mean width
to depth ratio of approximately 48 (Table 7). The designed channel significantly (p < 0.05)
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