Evaluation of Historical Sediment Deposition 



unusually large spring flood events, one might 

 expect to observe increased sedimentation rates 

 in the study lakes. Three major spring floods 

 have been observed in the Flathead Basin dur- 

 ing the period of record. These occurred in 

 1894, 1964 and 1974. The largest of the three 

 occurred in 1894, when severe spring floods 

 produced the highest flood waters ever recorded 

 in the Rathead Basin. Whitefish Lake was 

 raised to its highest recorded level, 9.5 feet 

 above the low water mark (Schafer and Engel- 

 ter, 1973). The sedimentation rate estimated for 

 the core section spanning 1894, was 26 mg/cmV 

 yr compared to 20 mg/cmVyr during the previ- 

 ous period. For a flood of such magnitude, this 

 increase in sedimentation rate was relatively 

 small in comparison to subsequent changes. 

 Furthermore, the 1894 flood was not the only 

 land disturbance event in the watershed. Early 

 land clearing and timber harvest activities 

 commenced in the catchment in the late 1880s 

 and early 1890s. The first sawmill on on the 

 Whitefish River, just below the lake, was built 

 in 1891. Nevertheless, in 1894, the vast major- 

 ity of land in the Whitefish Lake watershed 

 remained undisturbed by human activities. Thus 

 it appears that major floods in undisturbed 

 watersheds have a relatively small impact on 

 deposition of fine sediments in lakes in com- 

 parison to extensive human disturbance activi- 

 ties (discussed later). 



More detailed analysis of the lake sediment 

 record over shorter time increments (<1 year) 

 would likely reveal increased sediment deposi- 

 tion for a short time following major flood 

 events such as 1894. However, such increases 

 were not sufficient to elevate the mean sedi- 

 mentation rates over the longer intervals meas- 

 ured in this study. Shorter water retention time 

 in lakes during flood events may lessen their 

 impact on lake sedimentation rates. Neverthe- 

 less, the majority of sediments entering White- 

 fish Lake are deposited in the lake, even during 



spring run-off events (Golnar 1985). 



The 1964 flood was the next most intense 

 flood event recorded in the valley. The sedi- 

 mentation rate in Whitefish Lake reached levels 

 up to 88 mg/cmVyr during this time period; 

 however, there were significant timber harvest 

 activities (described later) occurring in the basin 

 over the same time interval (Figure B-1). Thus 

 it is difficult to isolate the effects of the flood. 

 The smallest of the three floods occurred in 

 1974. As with the 1964 flood, it is difficult to 

 determine the effect of this flood on lake sedi- 

 mentation rates. The sedimentation rate was 

 well above background levels during this pe- 

 riod; however, signiflcant timber harvest ac- 

 tivities occurred during this time period as well. 



The 1964 and 1974 floods washed out 

 numerous culverts and several bridges in the 

 Swift Creek drainage. Observations by local 

 foresters (T. Vars, D. Klehm) indicate that 

 widespread surface erosion did not appear to 

 have taken place in the Whitefish Lake water- 

 shed during these fioods. Nevertheless, the 

 floodwaters were reported very muddy and 

 carried high levels of suspended sediment. These 

 foresters speculated that increased sediment 

 loads associated with these floods likely came 

 from transport of sediment previously depos- 

 ited in the stream channels, together with ero- 

 sion of unconsolidated banks which are promi- 

 nent along Swift Creek. More detailed discus- 

 sion of these floods will follow in subsequent 

 coverage of human disturbance activities. 



Another natural disturbance event which 

 may have influenced the sedimentation rate in 

 Whiteflsh Lake was the 1980 eruption of Mt. St. 

 Helens. This eruption 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, it is pos- 

 sible that the most recent sedimentation rates 

 would have been lower in the absence of 



Flathead Basin Cooperative Program Final Report 



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