Evaluation of Historical Sediment Deposition 



limited development around the lake. This in- 

 cludes construction of several lodges along the 

 lakeshore as well as small lakeshore cabins 

 limited to the extreme east and west ends of the 

 lake. Much of this activity occurred in the early 

 part of this century, and does not appear to have 

 noticeably influenced mean lake sedimentation 

 rates shown in Figure B-3. 



Natural disturbance events did not appear to 

 be closely correlated with major changes in 

 sedimentation rate in Lake McDonald. As in the 

 other study lakes, there were no large fires in the 

 basin during the period of record. One of the 

 larger fires in the watershed occurred in 1967, 

 when approximately 5% of the watershed 

 burned. In addition, the 1964 flood occurred 

 during this interval. Sedimentation rates during 

 the time interval following the flood and fire 

 remained well below those achieved during the 

 1 940s and 1 950s. Sedimentation rates may have 

 increased for a short time after these events, 

 however any such increases were dampened 

 out by subsequent sedimentation rates. Smaller 

 fires in 1926, 1937 may have contributed to 

 increased sedimentation in Lake McDonald; 

 however, the impact of these natural distur- 

 bances are likely masked by the road building 

 activities. 



None of the McDonald Basin fires or floods 

 left visible bands in the sediment core during 

 the 110 year period of this study. However, a 

 thick black band is visible in the Lake McDonald 

 core at a depth of 1 8- 19 cm, which is below the 

 oldest strata dated in this study (see complete 

 report for details and photograph). This band 

 may correspond to extensive fires which burned 

 during the year 1 735. Several large fires burned 

 portions of the Lake McDonald Basin that year, 

 including much of the steep landscape leading 

 down to the shoreline of Lake McDonald (Barrett 

 1988). Sedimentation rates must have increased 

 dramatically in Lake McDonald following this 

 large fire, given the appearance of this thick (1 



cm) ash layer in the lake sediments. Unfortu- 

 nately, I was unable to estimate the actual 

 sedimentation rate during this period since 

 current sediment dating techniques are not 

 available for close estimation of dates over 1 50 

 years old. 



Management Implications 



This study shows that past land disturbance 

 activities are correlated with increased sedi- 

 ment deposition rates in all three study lakes. 

 Sedimentation rates increased 3 to 10 fold over 

 background levels, corresponding with logging, 

 road building, and/or railline construction in the 

 watersheds. Results of the BMP audits revealed 

 that the greatest number of BMP deviations 

 were related to road drainage and road 

 maintenance(Potts, Module F). Lake sediment 

 analyses suggest that over time, the sediment 

 contribution from roads (such as the Going to 

 the Sun Highway) is greatly reduced. Recent 

 sedimentation rates in Whitefish Lake provide 

 equivocal evidence for reduced sedimentation 

 rates using newer forest practices. However, 

 sedimentation rates in Swan Lake reached their 

 highest estimated level within the last 15-20 

 years concurrent with a doubling of timber 

 harvest activities in the basin in the last 10 

 years. Results from Potts (Module H) indicate 

 that a few areas in the Swan drainage had an 

 unusually high concentration of harvest activi- 

 ties. Water quality violations resulting from 

 recent timber harvest activities also have oc- 

 curred in the Swan Basin. Therefore, if recent 

 forest practices employed in the 1970s and 

 1980s do indeed reduce sediment delivery 

 compared to older practices, then any such 

 improvement appears to have been offset by the 

 recent large expansion of harvest activities, at 

 least in the Swan Lake Basin. In addition, sedi- 

 ments resulting from past activities may still be 

 in transit in the river systems above Swan and 



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Flathead Basin Cooperative Program Final Report 



