Lake Mason National Wildlife Refuge 



Climatic setting. The net annual precipitation balance (from the 

 MAPS database) is -30 inches, placing Lake Mason at the extreme of 

 the range of evaporative intensity shown by the sampling sites. 

 Mean monthly precipitation for the Roundup climatic station shows 

 a pronounced June maximum. Annual total precipitation in Roundup 

 shows the 3-year running average meeting or exceeding the long-term 

 average since the mid-1980s. Total precipitation in 1992 (the year 

 prior to sampling) was about 1.5 inches above the long-term 

 average, and in 1993 precipitation exceeded the average by about 

 5 inches. 



Geologic setting. The Lake Mason basin is underlain by surficial 

 deposits of inferred lacustrine and paludal origins, apparently of 

 low permeability and up to 40 feet in thickness. The lake lies 

 within a structurally controlled basin situated near the axis of 

 a syncline; in most of the basin, the Lance Formation (a 

 dependable low-yield sandstone aguifer) underlies surficial 

 sediments or is exposed at the surface. Peripheral areas of the 

 basin are underlain by Bearpaw Shale and Judith River Formation; 

 dissected areas of permeable terrace gravels lie elevated along 

 drainage divides. 



Hydrologic type. Lake Mason is a natural, structurally controlled, 

 unglaciated basin altered by the construction of a dam and outlet 

 control structure. The lake has surface water inflow and outflow; 

 the record of discharge through the control structure is not known. 

 Recent outflow may be inferred, however, from the dilute water 

 quality at the time of sampling. 



Basin characteristics. Lake Mason occupies a low-gradient basin 

 receiving controlled inflow from the Willow Creek drainage and 

 overflow from upgradient intermittent lakes. The catchment to 

 wetland area ratio ranks toward the middle of the range for sites 

 of the unglaciated plains. Mapped stream patterns suggest a 

 shallow ground-water system may influence Lake Mason. Upstream 

 water storage, irrigation diversions and (probably) return flows 

 affect the quantity and timing of the runoff received by the lake. 

 Permeable upland terrace gravels in the Willow Creek drainage may 

 support streamflows with ground-water storage. 



Water chemistry. At the time of sampling. Lake Mason water was of 

 an alkaline, "oligosaline" magnesium/sodium-bicarbonate character. 

 Mineral equilibria indicate substantial oversaturation with 

 carbonate phases. Ammonium, phosphorous and total organic carbon 

 concentrations all ranked mid-range for sites in this 

 classification. Arsenic and boron concentrations were toward the 

 lower end of the ranges shown by sites of the unglaciated plains. 

 Lake Mason was among a cluster of 5 sites from the region with much 

 higher chromium concentrations than the rest of the sample set. The 

 copper concentration was also very high, matched or exceeded only 

 by nearby Warhorse National Wildlife Refuge and by western Montana 



