Evaluation of Historical Sediment Deposition 



interface. If this happens, the eutrophication 

 process would accelerate rapidly, leading to a 

 deleterious cycle of further oxygen depletion, 

 which in turn stimulates more widespread re- 

 lease of sediment-bound phosphorus into the 

 lake water. Such changes would likely lead to 

 serious declines in water quality, including 

 nuisance algal blooms, poor water clarity, and 

 degradation of fisheries habitat. This negative 

 scenario of events has been documented in 

 numerous lakes in other parts of the country, 

 frequently fueled by increased human activities 

 and development in the lake basin. 



The USEPA (1976) estimated that 99.7% of 

 the total phosphorus load to Swan Lake was 

 from non-point sources. Unfortunately, there is 

 insufficient data available at present to allow 

 compilation of a comprehensive nutrient budget 

 for Swan Lake in 1990, or to fully explain the 

 cause of the dissolved oxygen depletion in the 

 lake. There may be other important sources of 

 nutrients and/or organic loadings to the lake 

 that also have contributed to current water quality 

 conditions (e.g. shoreline homes, upstream 

 development, and natural sources). However, 

 regardless of the actual cause, the reduced 

 oxygen levels in Swan Lake raises serious 

 concerns about any future increases in nutrient 

 loadings to the lake. . 



Available data provide evidence that in- 

 creased sediment loadings have negatively 

 impacted stream and lake resources in the 

 Flathead Basin. Symptoms of aquatic resource 

 degradation include elevated fine sediment 

 levels in key bull trout and westslope cutthroat 

 trout spawning streams (see Weaver and Fraley , 

 Module D) and increased sediment loadings to 

 lakes. The cause-and-effect relationship be- 

 tween increased fine sediment loading (from 

 whatever source) and spawning habitat degra- 

 dation is well understood. However, given the 

 limited data available on nutrient budgets from 

 the study lakes, it is more difficult to assign a 



direct causal relationship between increased 

 sediment loadings and lake water quality. 

 Nevertheless, there is no question that sedi- 

 ments represent a significant source of nutrients 

 to lakes in the Flathead Basin. Increased nutri- 

 ent loadings to lakes typically lead to increased 

 productivity, and given sufficient productivity, 

 water quality problems such as periodic oxygen 

 depletion and algal blooms. 



The present study provides evidence for a 

 link between past human land disturbance ac- 

 tivities (primarily related to timber harvest and 

 road building) and increased fine sediment (and 

 nutrient) loadings to lakes. These data provide 

 evidence from the Flathead Basin that human 

 disturbance activities increase fine sediment 

 deposition to a greater extent than natural dis- 

 turbance events. Similar conclusions have been 

 drawn from studies in other regions (Hutchin- 

 son and others 1970, Davis 1975, Batterbee and 

 others 1985, see reviews in Berglund, 1986). 



Considerable efforts are presently being 

 made to reduce the input of nutrients to lakes in 

 the Flathead B asin, in an attempt to maintain the 

 high water quality which characterizes many of 

 its lakes. Examples of nutrient control measures 

 being employed include a ban on the sale of 

 phosphate detergents in 1985 and nearly $20 

 million dollars towards construction and/or 

 expansion of wastewater treatments plants and 

 collection facilities for phosphorus removal. 

 New, or upgraded treatment plants, are either in 

 place or under construction for all major com- 

 munities upstream from Flathead Lake. Similar 

 efforts should be directed at other controllable 

 nutrient sources in the basin such as timber 

 harvest and related road building and road 

 maintenance. 



Flathead Basin Cooperative Program Final Report 



Page 37 



