Swan River Research Natural Area 



Climatic setting. The net annual precipitation balance for the 

 Swan River site (from the MAPS database) is -8 inches, placing the 

 site toward the more evaporative end of the range shown by Rocky 

 Mountain sample sites. Mean monthly precipitation for the Swan 

 Lake climatic station shows a bimodal distribution, with a major 

 winter maximum and a secondary June peak. Annual totals for this 

 station were not available for years after 1990; the late 1980s 

 experienced annual precipitation below the average for the period 

 of record. 



Geologic setting. The WET65 site lies near the contact between 

 Recent alluvium of the Swan River and upland slopes composed of 

 late Pleistocene-aged glacial till described as relatively loose 

 and sandy. Ground-water discharge (from glacial sediments?) occurs 

 nearby to the west, supporting a peat dome centered around the 

 discharge area. Low hills immediately to the east are interpreted 

 as remnants of a kame terrace, supporting a glacial origin for 

 sediments immediately underlying the wetland. The surficial till 

 mantle of upland slopes is underlain by the Helena Formation, a 

 calcareous and dolomitic unit of the Proterozoic-aged Belt series. 



Hydrologic type. The WET65 site is in a flow-through setting with 

 surface water inflow and outflow; the site may also receive some 

 ground-water inflow discharging from glacial deposits underlying 

 upland slopes to the east and west or from the alluvial sediments 

 underlying the site, which appear to narrow and thin in the 

 (inferred) downgradient direction. 



Basin characteristics. The surface catchment contributing to this 

 site includes upland areas in the Mission Mountain foothills, and 

 (apparently) the western third of the Swan River floodplain for 

 several miles upstream. Ground-water inflow from a larger area may 

 contribute to the wetland, depending on flow paths within the Swan 

 River alluvium. The geometry of the valley-margin channel along 

 which the wetland lies may contribute to local ground-water 

 discharge. 



Based strictly on map relationships from the 1965 topographic 

 coverage of the area, the WET65 wetland appears to be at some risk 

 of losing it's main source of surface-water inflow. The unnamed 

 stream flowing through the wetland bifurcates (according to the 

 1965 mapping) a half mile upstream from WET65. The eastern channel 

 bypasses the wetland site, and according to the 1965 topographic 

 mapping appears to be establishing a lower base level than the 

 channel supporting the wetland. Stream piracy could change water 

 availability at WET65 if this process continues. 



Water chemistry. The water sampled at WET65 is dilute, 

 circumneutral in pH and of calcium-bicarbonate composition. Mineral 

 equilibria indicate approximate saturation with calcite and 

 undersaturation with respect to dolomite. The nitrate 



