Brush Lake 



Clinatic setting. The net annual precipitation balance for Brush 

 Lake derived from the MAPS database is -23 inches, toward the less 

 evaporative end for sites in this class. Based on analyses of 

 evaporation pan data, this is a considerable underestimate Brush 

 Lake's water deficit, and points out the need to view the MAPS 

 estimates only as relative indicators of net site precipitation. 

 Mean monthly precipitation for the Medicine Lake climatic station 

 shows a pronounced June maximum. Annual totals for this station 

 show apparent periodicity with a 5 to 7 year cycle. Throughout the 

 1980s, the 3-year running average remained below (often well below) 

 the long-terra average, recovering in the early 1990s. In 1993 (the 

 year before sampling) precipitation exceeded the long-term average 

 by more than 1 inch. 



Geologic setting. Brush Lake occupies an ice-block depression 

 (kettle) in the sequence of late Pleistocene outwash sand and 

 gravel forming the Clear Lake aquifer. Surficial sediments within 

 the lake basin are dominated by authigenic carbonate minerals 

 precipitated from in-flowing ground water. Glacial sediments are 

 underlain at depth by the early Tertiary Fort Union Formation. 



Hydrologic type. Brush Lake is a glacial depression lacking 

 surface water outflow or significant surface water inflow. Flow- 

 through ground water dynamics in an evaporative setting maintain 

 water chemistry within a limited range. Brush Lake is believed to 

 have been internally drained (with respect to surface water) and 

 perennial throughout postglacial time. In-flowing ground-water is 

 supersaturated with carbonate phases upon loss of C02 to the 

 atmosphere, resulting in the deposition of extensive tufa and marl 

 deposits. 



Basin characteristics. Brush Lake has a minimal surface catchment, 

 exhibiting the lowest surface catchment to wetland area ratio of 

 any sample site. The lake basin is relatively steep-sided and 

 deep; maximum lake depth is approximately 60 feet. Regional 

 ground-water flow through the Clear Lake aquifer and it's tributary 

 Grenora channel flows through Brush Lake, controlled by local 

 stratigraphy, aquifer head distribution and evaporative 

 withdrawals. 



Water chenistry. Brush Lake contains alkaline, "oligosaline" 

 sodium-sulf ate/carbonate water almost completely depleted in 

 calcium. Major-ion aqueous chemistry is controlled by the 

 precipitation of calcium and magnesium carbonates. At the time of 

 sampling, nitrogen (nitrate and ammonia) concentrations were below 

 detection limits, and phosphorous concentrations (total and 

 orthophosphorous) were near the lower end of the range shown by 

 sample sites in this class. 



In arsenic and boron concentrations , Brush Lake was among the upper 

 25% among it's classification. The reported iron concentration was 



