Evaluation of Historical Sediment Deposition 



background sedimentation rates estimated for 

 the period prior to human settlement. Second, 

 the 1980s have been characterized by a series of 

 relatively mild run-off years. It is possible that 

 subsequent flood events could dislodge sedi- 

 ments deposited in the flood plain and/or pools 

 in the stream channel during the 1980s, and 

 carry them into the lake. If this occurs, then lake 

 sedimentation rates would increase, as these 

 sediments, eroded during recent years, were 

 finally transported into the lake. 



Another factor may have contributed to the 

 reduced lake sedimentation rate observed since 

 the early 1970s. Recent timber harvest in the 

 Whitefish Lake watershed has been concen- 

 trated on bottom lands where the erosion poten- 

 tial is reduced compared to the steeper, more 

 erosive lands in the upper regions of the water- 

 shed which were logged during the 1960s. In 

 fact, a significant portion of the land logged in 

 the 1 980s lies on Plum Creek Timber Company 

 lands in the Lazy Creek drainage, and repre- 

 sents harvest of timber regrown since the area 

 was first logged in the 1930s. Reduced sedi- 

 mentation also may be due to the fact that new 

 road construction, a major source of sediments, 

 was reduced somewhat in the 1980s due to 

 availability of pre-existing roads in the drain- 

 age. Nevertheless, a portion of the recent log- 

 ging activity lies on previously unlogged, steeper 

 terrain (located on Stryker Ridge). 



It is clear that recent logging efforts concen- 

 trated in the lower portion of the Basin have had 

 less impact on sedimentation in Whitefish Lake, 

 compared to the original roading, logging, and 

 rail spur construction on many of the same areas 

 in the early 1930s which produced a 10- fold 

 increase in lake sedimentation. However, it is 

 not clear from these data whether the recent 

 logging practices including Best Management 

 Practices (BMPs) have reduced sediment load- 

 ings in comparison to activities in the 1 950s and 

 1960s. There is some evidence to suggest this; 



however before drawing definitive conclusions 

 one must sort out the relative importance of 

 improved practices together with other poten- 

 tial causal factors such as use of pre-existing 

 roads, a series of comparatively mild run-off 

 years, and other factors which may affect sedi- 

 mentation such as lakeshore housing develop- 

 ment and the eruption of Mt. St. Helens. Fur- 

 thermore, it is not clear if BMPs have been 

 utilized for a long enough period to see any 

 effect on lake sedimentation. 



Swan Lake 



The history of the Swan Lake Basin over the 

 last 120 years includes a number of natural and 

 human-related disturbance activities. There were 

 large floods in 1894 and 1964. As in the other 

 lakes, neither of these floods appeared to be 

 accompanied by large changes in lake sedimen- 

 tation. However definitive conclusions about 

 the 1894 flood are speculative in Swan Lake 

 since this flood occurred within the oldest dated 

 core section. There were no large fires in the 

 Swan Basin during the period of record. Rela- 

 tively small fires did occur in 1910, 1919, 1936 

 and 1963. The largest of these occurred in 1919 

 when several thousand acres burned, represent- 

 ing less than 1% of the Swan Lake drainage 

 area. There were no visible ash layers in the 

 sediment cores. Further discussion of natural 

 disturbances is incorporated in the following 

 chronological discussion of changes in the Swan 

 Lake Basin. 



The lowest sedimentation rate measured in 

 Swan Lake over the 120 year period of record 

 occurred from 1874 and 1899, prior to human 

 disturbance activities in the basin (Figure B-2). 

 The mean sedimentation rate during this period 

 was 16 mg/cmVyr. During the first two decades 

 of the 1900s, the sedimentation rate increased 

 slightly to 19 mg/cmVyr. There are a number of 

 factors which may have contributexl to this 



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Flathead Basin Cooperative Program Final Report 



