Evaluation of Historical Sediment Deposition 



column. However Stewart (1983) disproved 

 this hypothesis and concluded that algal pro- 

 duction in Flathead Lake appeared to be stimu- 

 lated by sediment additions. Furthermore, con- 

 trolled bioassay experiments demonstrate that 

 addition of sediments from a variety of loca- 

 tions in the Flathead Basin stimulate algal growth 

 (Perry and Stanford 1982, Ellis and Stanford 

 1988). Ahhough the bioavailability of phos- 

 phorus contained in turbid spring run-off in the 

 basin may be only 6% (Ellis and Stanford 1988), 

 this still represents the largest single nutrient 

 source to Flathead Lake (Stanford and Ellis 

 1988). Thus, enhanced erosion and sediment 

 transport, as documented in this study, have 

 undoubtedly contributed to lake eutrophica- 

 tion, an undesirable process resulting from 

 stimulation of algal growth, reduced water clar- 

 ity, oxygen depletion and other related prob- 

 lems (Wetzel 1988). 



At present, Whitefish Lake is in a transi- 

 tional state between oligotrophy and mesotro- 

 phy (Golnar 1985). Late-summer hypolimnetic 

 oxygen depletion already is occurring in the 

 lake. Golnar (1985) concluded that Whitefish 

 Lake lies near a critical threshold of "exces- 

 sive" phosphorus loading, as determined from 

 the nutrient loading model of VoUenweider and 

 Kerekes (1980). Other lakes in the Flathead 

 Basin are threatened by increased nutrient load- 

 ings. Flathead Lake also is undergoing eutro- 

 phication as evidenced by increased algal pro- 

 duction (Stanford and Ellis 1988). As with 

 Whitefish Lake, scientists have described 

 Flathead Lake as being on a threshold, such that 

 increased nutrient loadings seriously threaten 

 water quality in the lake (Bahls 1986, Stanford 

 and Ellis 1988). Other lakes in the Flathead 

 Basin including Ashley Lake and Lake Mary 

 Ronan develop summer hypolimnetic oxygen 

 depletion and are also at risk from increased 

 nutrient loadings. 



During 1990, dissolved oxygen levels in 



Swan Lake declined to 0.5 mg/L near the bot- 

 tom of the south basin (see data in the complete 

 report). This appears to be the lowest dissolved 

 oxygen measurement recorded in any of the 

 large lakes in the Flathead Basin which are 

 noted for their high water quality. There are 

 smaller seepage-type lakes in the basin (for 

 example. Echo, Loon, and Foy's Lakes) which 

 have more severe dissolved oxygen depletion, 

 together with algal blooms and reduced water 

 quality which characterize these more produc- 

 tive lakes. Two limnological studies of Swan 

 lake conducted in the mid-1970s also reported 

 dissolved oxygen depletion in Swan Lake but 

 not nearly to the extent of the 1990. Unfortu- 

 nately, the measurements made in the 1970s 

 were taken from shallower depths than in 1990. 

 Although the 0.5 mg/L measurment made in 

 1990 is significantly lower than any previously 

 recorded level, there is insufficient data to say 

 with any certaintly that DO concentrations 

 have declined significantly since the 1970s. 

 Nevertheless, reduction of hypolimnetic oxy- 

 gen levels to near anaerobic conditions in Swan 

 Lake (regardless of the past history) is surpris- 

 ing and alarming, especially given the short 

 water residence time in the lake. 



The existing data documenting substantial 

 oxygen depletion in Swan Lake have led to the 

 lake being described as an impaired lake (Loren 

 Bahls, Montana Water Quality Bureau, per- 

 sonal communication). Reduced oxygen con- 

 centrations undoubtedly exclude trout and other 

 aquatic organisms from portions of the lake in 

 late summer and fall. Evidence from numerous 

 scientific studies indicate that if oxygen con- 

 centrations decline just a little bit more in Swan 

 Lake, than one can expect a rapid increase in 

 available phosphorus in Swan Lake (Mortimer 

 1941, Wetzel 1988). This would be caused by 

 the release of sediment-bound phosphorus into 

 the lake water when oxygen concentrations 

 decline to mgA- at the sediment-water 



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Flathead Basin Cooperative Program Final Report 



