Evaluation of Historical Sediment Deposition 



Whitefish Lakes. 



Since increased sedimentation may have 

 negative impacts on both stream and lake envi- 

 ronments the large increases in sediment load- 

 ings documented in this study represent an im- 

 portant environmental concern regarding both 

 past and projected activities. Hauer and Blum 

 (Module C) reported that increased timber har- 

 vest activities were correlated with increased 

 suspended sediments, nutrients, and algal growth 

 in streams. These results are consistent with 

 studies of timber harvest in other areas (Likens 

 et al. 1970, Lowe et al. 1986). Weaver and 

 Fraley (Module D) showed that increased lev- 

 els of fine sediments in fish spawning areas in 

 streams may substantially reduce the spawning 

 success and viability of bull trout and westslope 

 cutthroat trout. These species are native to the 

 Flathead Basin and have been designated sensi- 

 tive species by the U.S. Fish and Wildlife Serv- 

 ice and species of special concern by the state of 

 Montana. 



Unfortunately there are nocomparable tech- 

 niques available for quantifying historical 

 changes in the sediment composition in impor- 

 tant salmonid spawning and rearing areas of 

 streams. However, since the majority of lake 

 sediments transported to lakes enter via streams, 

 past large increases in erosion, transport, and 

 subsequent deposition of fine sediments in the 

 lake environment would likely have been ac- 

 companied by increased deposition of sedi- 

 ments in portions of the stream channels. Thus 

 I expect that past increases in sedimentation 

 documented in this study had negative impacts 

 on stream ecosystems in these watersheds. The 

 fact that estimated sedimentation rates in Swan 

 Lake reached their highest levels within the last 

 15-20 years raises concerns about the potential 

 negative impact of increased sedimentation in 

 important bull trout and Westslope cutthroat 

 trout streams above Swan lake, and the effect of 

 future land disturbance activities on these 



streams. 



Another concern regarding increased sedi- 

 mentation in surface waters is undesirable stimu- 

 lation of algal productivity and lake eutrophica- 

 tion. Sediments represent a major source of 

 nutrients to surface waters (Mortimer 1941; 

 Perry and Stanford 1982). Data from the Flathead 

 Basin show a close correlation between sus- 

 pended sediment concentrations in streams and 

 stream nutrient (phosphorus and nitrogen) 

 concentrations (Spencer and Hauer 1991, Ellis 

 and Stanford 1988, Stanford and Ellis 1988, 

 Stewart 1983, Golnar 1985). Detailed nutrient 

 budget analyses from Whitefish and Flathead 

 Lakes indicate that 60-70% of the annual phos- 

 phorus and nitrogen loadings come from stream 

 inputs, with the bulk of this input associated 

 with turbid spring run-off and unrelated to point 

 source inputs (Stanford and Ellis 1988, Stewart 

 1983, Golnar 1985). 



Lakes serve not only as sediment traps, but 

 also nutrient traps. Golnar (1985) estimated that 

 74% ofthe phosphorus entering Whitefish Lake 

 was retained in the lake. While some nutrients 

 may become permanently buried in the lake 

 sediments, a portion of the nitrogen and phos- 

 phorus pool entering the lake environment 

 remains in the water column. In studies on 

 Flathead Lake, Dodds, Priscu, and Ellis (1991) 

 showed that phosphorus and nitrogen could be 

 recycled in the water column in a matter of 

 hours or less. Thus, past increases in nutrient 

 loadings are still likely affecting the lake eco- 

 systems. 



Phosphorus and nitrogen availability have 

 been shown to be the primary factors limiting 

 algal production in lakes in the Flathead Basin 

 (Dodds and others 1989, Spencer and Ellis 

 1990). Stanford and Potter (1976) and Perry and 

 Stanford (1982) hypothesized that stream sedi- 

 ments in the Flathead Basin, upon entering the 

 lake environment, may settle out and strip 

 phosphorus and algae from the lake water 



Flathead Basin Cooperative Program Final Report 



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