Evaluation of Historical Sediment Deposition 



The 1964 flood occurred during the latter 

 part of this trend. The sedimentation rate did not 

 increase dramatically following this flood, the 

 second largest on record. Rather, the sedimen- 

 tation rate continued an increasing trend which 

 was initiated in the 1950s. Although the 1964 

 flood likely contributed to increased erosion 

 and sediment transport in the basin, the impact 

 of this flood on lake sedimentation appears 

 small in comparison to previous land distur- 

 bance activities. 



From 1967-1971, the sedimentation rate 

 declined to 72.6 mg/cm7yr, concurrent with a 

 decline in logging activity in the late 1960s. 

 From 1971 to 1983, the lake sedimentation rate 

 was reduced to 54.7 mg/cm7yr. Although tim- 

 ber harvest was elevated at the beginning of this 

 period, harvest declined to low levels during the 

 latter part of this period. The 1974 flood also 

 occurred during this period. Foresters report 

 that the 1964 and 1974 floods did not cause 

 extensive erosion and gullying throughout the 

 Whitefish Lake watershed. Rather, these floods 

 resulted in considerable erosion and channel 

 destabilization in the main stream courses and 

 floodplains. Although the flood of 1894 sug- 

 gests that flooding of undisturbed watersheds 

 may not result in greatly increased sedimenta- 

 tion rates compared to human disturbance ac- 

 tivities, it is unclear whether the same may be 

 said for floods occurring in watersheds dis- 

 turbed by timber harvest. For example, it is 

 possible that the floods of 1 964 and 1 974 flushed 

 out sediments from the larger streams which 

 had accumulated there as a result of past activi- 

 ties. 



Between 1983 and 1990, the mean sedi- 

 mentation rate was 52.2 mg/cm7yr. Timber 

 harvest activities increased during this period; 

 however, as in the 1970s, this increased harvest 

 was not accompanied by comparable increases 

 in sedimentation as in previous years. In addi- 

 tion to timber harvest activities, two other fac- 



tors may have contributed to changes in lake 

 sedimentation in recent years. First, the 1980 

 eruption of Mt. St. Helens produced a fallout of 

 volcanic ash across western Montana. There is 

 no visible ash layer in the study cores, and lake 

 sedimentation rates did not appear to increase 

 during this time period. Nevertheless, sedi- 

 ments were deposited in the basin as a result of 

 this eruption. Second, recent increases in lake- 

 shore housing and other developments along 

 Whitefish Lake may have contributed sedi- 

 ments to the lake. Considerable areas of the 

 shoreline still remain undeveloped. Although 

 lake sedimentation rates did not increase during 

 recent years, it is possible that observed sedi- 

 mentation rates would have been lower in the 

 absence of volcanic ash deposition and lake- 

 shore development. 



There are several factors which may ex- 

 plain the reduced sedimentation rate in White- 

 fish Lake during the 1970s and 1980s. First, this 

 time period coincides with significant efforts 

 on the part of government resource manage- 

 ment agencies and the timber industry to at- 

 tempt to reduce the impact of timber harvest 

 activities on erosion and sediment transport to 

 surface waters. A combination of mandatory 

 and voluntary standards were adopted in an 

 attempt to reduce the sedimentation risk. These 

 efforts focused on minimizing erosion associ- 

 ated with road construction, stream crossings, 

 and restricting harvest activities on the most 

 sensitive lands. The sedimentation data provide 

 evidence that these more recent logging prac- 

 tices may have reduced the rate of sediment 

 transport to Whitefish Lake, in comparison to 

 previous timber harvest activities in the basin. 



Although the recent reduction in lake sedi- 

 mentation may support the effectiveness of 

 newer logging practices in reducing sediment 

 transport, this observation must be tempered by 

 several important observations. First, the recent 

 sedimentation rates are still well above 



Flathead Basin Cooperative Program Final Report 



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