Bent Flat Fen Proposed RNA 



Climatic setting. The net annual precipitation balance for the 

 Bent Flat fen (from the MAPS database) is -4 inches, placing this 

 wetland toward the middle of the range shown by Rocky Mountain 

 sites. Mean monthly precipitation values for the Hungry Horse Dam 

 climatic station shows a bimodal distribution with June and January 

 peaks. The recent record for this climatic station is fragmentary; 

 annual totals for 1991 and 1992 were below average for the period 

 of record. 



Geologic setting. Bent Flat is a calcareous fen underlain by 2 

 meters of less of peat and organic-rich marl. The fen sediments 

 are deposited over a surface of dense, clay-rich till. Active 

 precipitation of carbonate minerals occurs in a complex of pools, 

 channels and vegetation mats forming string-and-f lark patterning 

 approximately along the topographic contour of the fen surface. 

 The fen is supported by ground-water discharge from upland springs 

 within a few hundred meters upgradient. Adjacent upland slopes are 

 underlain by tufa and travertine deposited by these springs. 



The springs are believed to represent ground-water discharge from 

 Cambrian limestones underlying slopes and peaks to the east and 

 northeast. 



Hydrologic type. Bent Flat is a patterned fen (unusual in Montana) 

 with inflow provided by short streams fed by nearby ground-water 

 discharge. Outflow occurs by a peripheral stream and by 

 channelized outflow from the toe of the wetland. 



Basin characteristics. Ground-water supporting the Bent Flat fen 

 is recharged over an unknown area. Flow within the fen is complex, 

 following convoluted surface pathways through interconnected pools 

 (f larks) and moving as shallow groundwater flow through the 

 vegetation mat. The fen is developed on a low-gradient slope at the 

 base of a steep glaciated ridge. Where examined, the hydraulic 

 conductivity of the underlying glacial materials appears to be low. 



Water chemistry. Bent Flat Fen contains relatively dilute, mildly 

 alkaline calcium-bicarbonate water which actively precipitates 

 calcite as it moves from the ground-water environment to the fen. 

 Saturation with calcium carbonate phases is shown by the ubiquitous 

 deposition of authigenic calcite (and/or aragonite?) around the 

 outlet springs and within the fen. Deposition may be seasonal, as 

 equilibrium calculations indicate slight calcite undersaturation 

 under cold (3 degrees C) temperatures. 



Nitrogen (nitrate plus ammonium) concentrations rank in the upper 

 third among hydrologically similar sites; phosphorous 

 concentrations are near the lower end of the range. The total 

 organic carbon concentration at the time of sampling was toward the 

 upper end of the range shown by hydrologically similar sample 

 sites. 



