Mill-Willow Silver Bow wetland. 



Climatic setting. The net annual precipitation balance for the 

 Mill-Willow site (from the MAPS database) is -22 inches, placing 

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

 sample sites in the western intermontane valleys. Mean monthly 

 precipitation for the Anaconda climatic station shows a broad 

 summertime peak. Annual precipitation totals for the Anaconda 

 station show the 3-year running average falling below the long-term 

 average from 1987 through 1992, the year of the DHES sampling. The 

 1992 total was about 1 inch below the long-term average. Water 

 supply at this site is probably more dependent on the condition of 

 mountain snowpack than on the low-elevation precipitation measured 

 at the Anaconda station. 



Geologic setting. The WET51 site is underlain by Quaternary-aged 

 unconsolidated alluvial deposits, probably to a thickness of a few 

 tens of feet. Tertiary-aged valley-fill sediments underlie the 

 Quaternary alluvium; these sediments may be near their local 

 maximum in thickness beneath the sampling site, thinning rapidly 

 to the east and west. Recent alluvial sediments and shallow 

 ground-water in the Mill-Willow-Silver Bow area are pervasively 

 contaminated with mineral processing wastes. 



Hydrologic type. The WET51 site is in a riparian wetland in an 

 area of apparent local ground-water discharge, with perennial 

 surface water inflow and outflow and close surface water-ground 

 water linkages. The site has been modified to some degree by 

 ditching, with unknown effects on wetland hydrology. 



Basin characteristics. The mountain basin of Willow Creek provides 

 a relatively large, high-elevation catchment for the WET51 site; 

 the catchment to wetland area ratio ranks toward the upper end of 

 the range shown by sites in this classification. Willow Creek and 

 other local streams provide recharge to shallow ground-water 

 systems; ground-water discharge occurs along gaining stream reaches 

 and in wetland areas such as that sampled at WET51. Historic mine 

 wastes are broadly distributed in the Silver Bow and Mill Creek 

 drainages. Contamination with mineral processing waste is also 

 suspected to have caused soil and ground-water contamination in the 

 Willow Creek drainage. Low stream gradients, overbank 

 sedimentation and probable channel migration in the vicinity of the 

 WET51 site suggest that the sample site (located near the present 

 Willow Creek channel) may be underlain by contaminated sediments 

 derived from Silver Bow Creek and Mill Creek, as well as Willow 

 Creek sediments. 



Water chemistry. The WET51 site is characterized by dilute 

 calcium-bicarbonate water of near-neutral pH. The sulfate 

 concentration appears to be proportionately higher than for many 

 other dilute sample sites in western Montana. Mineral equilibria 

 indicate approximate saturation with carbonate phases. The nitrate 

 concentration ranked mid-range for sample sites of this class; 



