Evans Lake 



Climatic setting. The net annual precipitation balance for Evans 

 Lake (from the MAPS database) is -16 inches, placing the site 

 toward the more evaporative end of the range for it's hydrologic 

 classification. Monthly average precipitation for the Ovando 9SSE 

 station shows a bimodal distribution, with a primary May-June peak 

 and a secondary winter peak. Since the mid-1970 's, the 3-year 

 running average of annual total precipitation has been below (often 

 well below) the average for the period of record. Total 

 precipitation in 1993 (the year of DHES sampling) was very near the 

 long-terra average. 



Geologic setting. Evans Lake is a steep-sided glacial kettle 

 developed in till deposited by ice advancing into the Ovando Valley 

 from the North Fork of the Blackfoot River. Surficial sediments 

 in the Evans Lake basin are notably cobbly in texture, and may 

 include glaciof luvial deposits as well as till. Littoral sediments 

 typically feature gravel and cobbles in a matrix of marly mud. 



Bedrock geologic units underlying Evans Lake are not exposed in the 

 immediate area; generally the Ovando Valley is underlain by a 

 seguence of Tertiary valley-fill sediments overlying Proterozoic 

 metasediments of the Belt series. Belt sediments exposed in the 

 North Fork drainage (inferred to contribute to the North Fork) 

 include large outcrop areas of the Helena Formation and other 

 calcareous and dolomitic units. 



Hydrologic type. Evans 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 nay have allowed surface discharge. Ground water 

 is inferred to be an important contributor to Evans Lake; based on 

 the relatively high salinity observed in the lake, ground-water 

 outflow is probably restricted. 



Basin characteristics. Evans Lake has a small surface catchment 

 with respect to lake area and volume. The basin is steep-sided and 

 relatively deep, with a maximum measured depth of 25.5 feet in 

 September 1994. High-water drainage, if any, would occur through 

 a gap along the northeast quadrant of the lake. 



Water chemistry. Evans Lake contains "mesosaline" sodium sulfate 

 water nearly depleted in calcium. Very low calcium concentrations 

 are probably maintained by solubility controls; equilibrium 

 calculations indicate supersaturation with carbonate species. 



In comparison to hydrologically similar sites, Evans Lake (at the 

 time of sampling) was relatively low in orthophosphorous 

 concentration. Nitrate and ammonium concentrations were mid-range 

 among hydrologically similar sites. (Ammonium concentrations 

 measured during winter conditions were considerably higher - see 

 below.) The total organic carbon concentration was near the middle 

 of the range for hydrologically similar sites. 



