McKillop Creek 



Climatic setting. The net annual precipitation balance for the 

 McKillop Creek site (from the MAPS database) is -3 inches, ranking 

 the site mid-range in moisture regime among Rocky Mountain sites. 

 Mean monthly precipitation for the Libby Ranger Station climatic 

 station shows a bimodal distribution, with a wintertime (November- 

 January) maximum and a secondary June peak. Annual total 

 precipitation for the Libby RS station shows the 3-year running 

 average at or above the long-term annual average from 1983 through 

 1993. The annual total in 1993 (the year prior to sampling) was 

 egual to the long-term average. 



Geologic setting. The McKillop Creek wetland is underlain by 

 glacial (?) deposits of undescribed character. Bedrock formations 

 underlying the site and the contributing drainage basin belong to 

 the Proterozoic-aged Belt series, including the dominantly 

 calcareous and dolomitic Wallace Formation. The wetland site 

 closely overlies the intersection of two mapped normal faults, one 

 trending northwest along lower McKillop Creek and it's unnamed 

 south fork, and the other trending northeast across the two forks 

 of McKillop Creek. The McKillop Creek wetland's peculiar water 

 chemistry (see below) suggests that the wetland may receive a 

 component of inflow from deeply circulating bedrock ground-water. 



Hydrologic type. The McKillop Creek wetland occupies one of a pair 

 of basins of uncertain (glacial?) origin, lacking a surface water 

 outlet at the observed water levels and possessing a surface water 

 inlet stream mapped as intermittent in flow. The relatively moist 

 setting and dilute water chemistry imply outflow from the site 

 either by ground-water recharge or by unmapped surface water 

 outflow. 



Basin characteristics. The McKillop Creek wetland has a notably 

 steep catchment and a relatively large catchment area to wetland 

 area ratio, implying rapid hydrologic flux through the wetland. 

 The northeastward orientation of the catchment may favor retention 

 of winter snowpack. The depth of the wetland water body is unknown 

 but probably quite shallow. Bedrock structural controls may 

 influence basin topography and wetland water chemistry. 



Water chemistry. The McKillop Creek wetland exhibits a mildly 

 acidic, dilute sodium/potassium-bicarbonate/chloride composition 

 which is unique among the sample sites. The high ratios of sodium 

 to total cations and chloride to total anions suggests either 

 contamination with sodium chloride road salt (an uncommon deicing 

 salt) or inflow from dilute but deeply circulating ground water. 

 The latter explanation is favored on the basis of other chemical 

 characteristics described below. Mineral equilibria indicate 

 substantial undersaturation with respect to carbonate phases 

 despite the presence of calcareous and dolomitic rocks within the 

 basin. 



