Evaluation of Historical Sediment Deposition 



Lake McDonald 



As in the other study lakes, the lowest 

 sedimentation rate in Lake McDonald was re- 

 corded at the beginning of the period of record. 

 Between 1880 and 1910, the sedimentation rate 

 was 7 mg/cm7yr (Figure B-3). The sedimenta- 

 tion rate increased to 10 mg/cmVyr between 

 1910 and 1935, and then increased substantially 

 to 29 mg/cm7yr over the period between 1935 

 and 1945. These increases follow construction 

 of a two lane highway, called the Going to the 

 Sun Road, which passes through a large part of 

 the Lake McDonald watershed. This major road 

 runs along the south shoreline of Lake McDonald 

 for much of its 9 mile length, and continues up 

 the drainage along McDonald Creek for 11 

 miles, closely bordering the creek in numerous 

 locations. The road then switches back up the 

 along steep exposed terrain within the Lake 

 McDonald catchment leading up to Logan Pass 

 on the Continental Divide. Roadbed prepara- 

 tion included construction of numerous em- 

 bankments along steep areas, blasting tunnels 

 through bedrock, and considerable earth mov- 

 ing activities, all of which undoubtedly contrib- 

 uted sediments to surface waters. 



Although the initial road was completed in 

 the early 1930s, the sedimentation rate in Lake 

 McDonald did not peak until the late 1930s. 

 This apparent lag in lake sedimentation may 

 have resulted from a delay in transport of sedi- 

 ments from the upper portion of the watershed 

 down into the lake. Such delays could be due to 

 the long distance between road building activi- 

 ties on the erosive slopes near the continental 

 divide and Lake McDonald. In addition, the 

 heavily forested streams in the McDonald Creek 

 Basin, may have higher sediment retention rates 

 compared to logged watersheds. Natural down- 

 fall in the stream bed serve as stream sediment 

 traps. Furthermore, unlogged watersheds may 

 have smaller maximum stream flows compared 



to logged watersheds, which could result in 

 reduced sediment flushing capacity in undevel- 

 oped watersheds (see Hauer, module A). Thus 

 there are a number of factors which may have 

 contributed to the apparent delay in sediment 

 deposition in Lake McDonald. 



The sedimentation rate declined rapidly in 

 the 1960s. Revegetation and stabilization of the 

 original road cuts likely reduced sediment de- 

 livery to surface waters along the road. This 

 reduction also may have been partially due to 

 stabilization of the road surface by paving in the 

 1950s. It is possible that periodic regrading of 

 the road together with road dust stirred up by 

 cars along this heavily travelled road may have 

 contributed to elevated sedimentation levels in 

 Lake McDonald. After the road was paved, the 

 potential contribution of road dust to lake sedi- 

 mentation would have been greatly reduced. 



The mean sedimentation rate for the 29 year 

 period from 1961 to the present declined to 14 

 mg/cm7yr. This rate is roughly twice the rate 

 estimated for the late 18(X)s. Reasons for the 

 continued existence of sedimentation levels 

 above background are speculative. The 1980 

 eruption of Mt. St. Helens left no visible band of 

 sediments in the core. However, it is possible 

 that this eruption caused increased sediment 

 deposition in Lake McDonald. Thus, it is pos- 

 sible that in absence of this eruption, the most 

 recent sedimentation rate may have been closer 

 to levels measured in the late 1 8(X)s. In addition, 

 there have been limited human activities in the 

 basin which could have contributed to recent 

 sedimentation rates. These activities include 

 ongoing maintenance on the Going to the Sun 

 road, as well as limited construction projects 

 around the lake. However, future plans call for 

 significant "improvement" along much of the 

 Going to the Sun road. 



Although the Going to the Sun Road repre- 

 sents the largest human land disturbance activ- 

 ity in the Basin, there has been some other 



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Flathead Basin Cooperative Program Final Report 



