Lcihrity Lake 



Climatic setting. The net annual precipitation balance for the 

 Lahrity Lake site (from the MAPS database) is -17 inches, placing 

 the site toward the middle of the range for hydrologically and 

 regionally similar sites. Monthly average precipitation for the 

 Ovando 9 SSE climatic station shows a bimodal distribution, with 

 a primary June peak and a secondary January peak. The 3-year 

 moving average of total annual precipitation remained below (often 

 considerably below) the long-term average from the mid-1970s 

 through 1993. The total precipitation for 1993 (the year prior to 

 sampling) was very near the long-term average. 



Geologic setting. Lahrity Lake occupies a glacial ice-block 

 depression (kettle) in till (and other glacial sediments?) 

 deposited by ice advancing into the Ovando Valley from the Monture 

 Creek drainage. The adjacent bedrock outcropping of Monture Hill 

 is a prominent resistant knob underlain by the Snowslip Formation 

 of the Proterozoic Belt series. The Monture Creek source area for 

 the glacial deposits in the area includes large exposures of the 

 calcareous and dolomitic Helena Formation. 



Inspection of the glacial sediments surrounding Lahrity Lake shows 

 abundant cobbles and boulders in a calcareous silty matrix. 



Hydrologic type. Lahrity Lake is a glacial depression lacking 

 surface drainage at present water levels. Strandlines represented 

 by beach erosion features indicate water levels which may have 

 allowed surface water discharge. High salinity indicates that 

 ground-water outflow is restricted, allowing accumulation of 

 solutes. 



Basin characteristics. Lahrity Lake is a steep-sided basin with 

 minimal surface catchment area relative to the size of the lake; 

 the ratio of catchment to wetland area ranks near the bottom of the 

 range for the entire sample set. Additional areas of small closed 

 basins surrounding Lahrity Lake may contribute ground-water inflow. 

 Irregular knobs of till within the lake basin suggest deposition 

 as ablation debris in a region of stagnant ice. 



Measured water depths at the time of sampling ranged up to 11.5 

 feet; average depth was on the order of 6 feet and lake volume was 

 estimated to be about 150 acre-feet (or 6.5 million cubic feet.) 

 The sampled lake level was approximately 10 feet below the base of 

 a prominent erosional shoreline feature and approximately 13.5 feet 

 below the stage which would allow lake outflow to the southwest. 

 At these lake stages, lake volume probably was several-fold greater 

 than at the time of sampling, as low-gradient areas at the north 

 end of the lake flood. 



Water chemistry. Presently, Lahrity Lake contains highly alkaline, 

 "polysaline" sodium/potassium-sulf ate brine virtually depleted in 

 calcium; the proportional potassium concentration is uniquely high 



