Evaluation of Historical Sediment Deposition 



the Swan Lake drainage in the 1940s. Although 

 I was unable to obtain accurate timber harvest 

 estimates for the 1940s, the level of harvest was 

 comparable to the 1950s (Art Whitney, per- 

 sonal communication). Logging roads and skid 

 trails were built in the lower portion of the basin 

 during this period. 



Timber harvest and associated road build- 

 ing activities accelerated during the 1960s and 

 lake sedimentation rates remained elevated 

 during this period, reaching a mean rate of 38 

 mg/cmVyr from 1957 to 1972 (Figure B-2). 

 The 1964 flood also occurred during this time 

 interval, and these flood waters may have accel- 

 erated sediment transport to the lake. However, 

 as in the other two study lakes, this large flood 

 did not appear to be correlated with a large 

 increase in lake sedimentation. A small fire in 

 1963 burned less than 1(XX) acres in the upper 

 portion of the Lost Creek basin and also may 

 have contributed to lake sedimentation. 



The lake sedimentation rate increased again 

 to 49.5 mg/cmVyr from 1972 to 1990. This 

 sedimentation rate was the highest recorded 

 over the 120 year period of record, some three- 

 fold higher than background rates estimated for 

 the late 18(X)s. This recent sedimentation in- 

 crease is correlated with a large increase in 

 logging activity and associated road building in 

 the Swan Basin (Figure B-2). During the 1980s, 

 timber harvest activities doubled over the levels 

 attained during the 1960s and 1970s. During 

 this period, timber harvest and associated road 

 building expanded on bottom lands as well as 

 on the flanks of the Mission and Swan mountain 

 ranges that enclose the Swan Valley. Results 

 from Module H (Potts 1991) indicated that a 

 few areas in the Swan River drainage had an 

 unusually high concentration of timber harvest 

 activities. Much of this increased harvest oc- 

 curred on private lands owned by Plum Creek 

 Timber Company. Timber harvest on State and 

 Federal lands declined slightly during this same 



time period. 



Although timber harvest and related road 

 building represent the largest human land dis- 

 turbance activity in the basin in recent years, 

 there has been an increase in construction of 

 lakeshore cabins and homes around Swan Lake. 

 This activity also may have contributed to in- 

 creased lake sedimentation. Nevertheless, large 

 areas of lake shoreline remain undeveloped. 



Comparisons between Swan Lake and 

 Whitefish Lake yield additional insight into the 

 impact of lakeshore development on lake sedi- 

 mentation rates. The lakeshore area around 

 Whitefish Lake also has experienced home and 

 recreational development. Recent development 

 around Whitefish Lake may even exceed that 

 around Swan Lake. However, in contrast to 

 Swan Lake, recent sedimentation rates in White- 

 fish Lake do not appear to have increased. Thus 

 it is unlikely that lakeshore development is 

 responsible for the recent large increase in 

 sedimentation in Swan Lake. Rather, the con- 

 trasting lake sedimentation responses appear 

 more closely related to differences in recent 

 timber harvest and road building activities in 

 the two basins. As described earlier, recent 

 timber harvest activities in the Swan Basin far 

 exceed past acreages subject to harvest in the 

 Basin. By contrast, recent timber harvest acre- 

 ages in the Whitefish Basin fall within the level 

 of activity reached several times over the last 60 

 years. 



As in Whitefish Lake, the lack of large 

 flushing flows in recent years may have re- 

 sulted in substantial accumulation of sediments 

 in the river system. This could be more of a 

 factor in Swan Lake than Whitefish Lake, given 

 the low gradient depositional area in the river 

 immediately above Swan Lake. If so, then 

 subsequent flushing flows could carry large 

 quantities of previously eroded material into 

 the lake. 



Flathead Basin Cooperative Program Final Report 



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