Alkali Lake 



Climatic setting. The net annual precipitation balance for Alkali 

 Lake (from the MAPS database) is approximately -13 inches. This 

 places the site toward the less evaporative end of the climatic 

 spectrum for closed basin sites, but near the more evaporative 

 extreme for Rocky Mountain sample sites. Monthly average 

 precipitation for the Fortine climatic station shows relatively 

 even distribution throughout the year, with a moderate peak in May 

 and June. Annual records from the Fortine station show the 3-year 

 running average precipitation has been below the long-term mean 

 during most of the last 30 years. Annual precipitation during the 

 sample year of 1993 exceeded the average for the period of record 

 by approximately 5 inches. 



Geologic setting. The Alkali Lake basin is underlain by calcareous 

 till and possibly other glacial sediment types of late(?) Wisconsin 

 age. Shorelines are composed of coarse-grained sediments in high 

 energy areas, marly mud in deeper water and sheltered littoral 

 areas. Bedrock geologic units are of the Proterozoic Belt Series, 

 probably including the calcareous and dolomitic Helena Formation. 



Hydrologic type. Alkali Lake is a glacial depression lacking 

 surface drainage at current water levels. Strandlines represented 

 by tree kills indicate historic water levels within the past few 

 decades which would have allowed surface discharge. Groundwater 

 outflow is inferred from moderate salinity of this topographically 

 closed basin. 



Basin characteristics. Alkali Lake has a small surface catchment 

 with respect to lake area and volume. The basin is linear and 

 shallow at both ends, with a maximum measured depth in mid-basin 

 of 37 feet. The cluster of lakes in the vicinity display disjunct 

 water quality; Thirsty Lake, nearby and at a lower elevation, 

 exceeds Alkali Lake in specific conductance by more than an order 

 of magnitude. Road construction has partially isolated the eastern 

 arm of the lake during high-water conditions. 



Water chemistry- Alkali Lake is proportionately high in magnesium 

 and is almost completely depleted in calcium; low calcium 

 concentrations appear to be maintained by solubility controls, 

 evidenced by deposition of authigenic carbonate minerals. High 

 relative magnesium concentrations are typical of hydrologically 

 similar sample sites. Equilibrium calculations indicate 

 supersaturation with calcium and magnesium carbonate species. 



In comparison to hydrologically similar sample sites, Alkali 

 Lake exhibits relatively low water-column concentrations of trace 

 elements. Total organic carbon concentrations are at the high end 

 for the region and for hydrologically similar sites. 



Chemical history. Specific conductance measurements from 1968 were 



