Lake Bowdoin Outlet 



Climatic setting. The net annual precipitation balance for Lake 

 Bowdoin (from the MAPS database) is -28 inches, placing the Bowdoin 

 sites at the more evaporative end of the range shown by sites in 

 this classification. Monthly mean precipitation for the Malta 7NE 

 climatic station shows a broad summer peak with a maximum in May; 

 the Malta climatic station, with a longer period of record but 

 farther from Lake Bowdoin, shows a pronounced June maximum. Annual 

 totals for the Malta 7NE station, shows the years 1988 to 1992 

 below the average for the period of record. The total for 1993 

 (the year prior to sampling) exceeded the average for the period 

 of record by about 2.5 inches. 



Geologic setting. The natural catchment of Lake Bowdoin is 

 underlain primarily by the Late Cretaceous Claggett Shale, with the 

 overlying Judith River Formation exposed in some higher-elevation 

 areas. The apex of the Bowdoin Dome lies a few miles to the north; 

 this structural dome is a significant hydrocarbon trap and supports 

 an active natural gas field. Surficial sediments underlying the 

 Lake Bowdoin basin are generally described as "alluvium", mapped 

 as a broad east-west band underlying the lake. These deposits are 

 apparently continuous with permeable gravels underlying lower 

 Beaver Creek to the east, which are believed to be glacial in 

 origin. 



Hydrologic type. Lake Bowdoin is an area of natural internal 

 drainage augmented and greatly expanded by a complex system of 

 dikes, water control structures and water imports. Under favorable 

 conditions, managed outflow can occur both to the Dry Lake Unit (a 

 satellite basin to the east of Lake Bowdoin) and directly to Beaver 

 Creek. Outflows are possible only during high water levels, 

 however. Discharge to Beaver Creek did not occur for 7 years prior 

 to the 1994 DHES sampling, during which time the Lake Bowdoin 

 system was effectively closed to surface water outflows. 



Basin characteristics. The natural catchment of the Lake Bowdoin 

 Basin is comparatively small, consisting mainly of intermittent 

 drainages from the south which are routed to peripheral wetland 

 areas and ultimately to Lake Bowdoin when flows allow. Since most 

 of the inflow is diverted from the Milk River Irrigation Project, 

 the entire upper Milk River above the Dodson diversion is 

 effectively contributing water and solutes to the Lake Bowdoin 

 system. Delineated contributing areas and tabulated acreage 

 figures for the Bowdoin sites thus include the Milk River basin 

 above Dodson. The WET38 sampling site is adjacent to the outlet 

 structure conveying water to the Dry Lake Unit. 



Water chemistry. At the time of sampling, the WET36 site was 

 characterized by highly alkaline "oligosaline" sodium sulfate 

 water. Mineral equilibria indicate oversaturation with carbonate 

 phases. Ammonium, phosphorous (total and orthophosphate) and total 

 organic carbon concentrations all rank toward the mid-range of 



