Benton Lake National Wildlife Refuge 



Climatic setting. The net annual precipitation balance for Benton 

 Lake (from the MAPS database) is -27 inches, placing the site 

 toward the more evaporative end of the range for it's class. 

 Average monthly precipitation for the Great Falls Airport climatic 

 station shows a strong May-June peak. Between 1980 and 1993, the 

 three-year running average of annual precipitation remained near 

 the mean for the period of record, despite several extreme years. 

 In 1993 (the year of sampling), annual precipitation exceeded the 

 average by more than 7 inches. The 1992 total was about 2.5 inches 

 below average. 



Geologic setting. Benton Lake is underlain by a thick sequence of 

 glaciolacustrine sediments deposited in Glacial Lake Great Falls 

 during late Pleistocene time. Glacial Lake Great Falls sediments 

 in the Benton Lake basin are described as composed dominantly of 

 plastic clay. Till and other surficial materials underlie other 

 portions of the catchment. Underlying pre-Quaternary formations 

 include the Marias River Shale and the Blackleaf Formation. 



Hydrologic type. Benton Lake is a highly managed system relying on 

 pumped importation of water from Muddy Creek. There is no surface 

 water outflow and due to topographic position and geologic 

 characteristics, no likelihood of significant ground-water outflow. 

 A large fraction of the Muddy Creek water delivered to Benton Lake 

 consists of irrigation return flows from the Greenfields Division 

 of the Sun River Irrigation Project. 



Basin cheiracteristics . The catchment to wetland area ratio for 

 Benton Lake is toward the low end of the range for the sample set. 

 However, Benton Lake inflows are managed by manipulation of imports 

 from Muddy Creek. Benton Lake itself is subdivided by dikes into 

 six separate pools with sequentially lower interconnecting 

 spillways from the inlet pool to the southeastern end of the lake. 

 Salinity and disease management involves flooding only some of the 

 pools during any given year, while others stand dry. 



Water chemistry. At the time of sampling in August 1993, Benton 

 Lake Pool #1 contained alkaline, "oligosaline" water of mixed 

 cation/sulfate chemistry. Mineral equilibria indicate 

 oversaturation with carbonate phases and moderate undersaturation 

 with respect to gypsum. The nitrate concentration was very high, 

 while phosphorous (total and orthophosphate) and total organic 

 carbon concentrations were toward the low end of the range for the 

 wetland type and region. The water column selenium concentration 

 was the highest of the five sample sites for which detectable 

 selenium was reported. WET05 was also among the five sites with 

 detectable nickel reported from the water column. The reported 

 arsenic concentration was among the lowest for the sample sites. 



Chemical history. Monitoring programs of the U.S. Fish and 

 Wildlife Service and the USGS have generated an intensive (if 



