Upper Stillwater River Wetland 



Climatic setting. This site is in a high elevation subalpine 

 setting with a high precipitation surplus. The net annual 

 precipitation balance (from the MAPS database) is 44 inches, one 

 of the two highest values for the sample sites. Mean monthly 

 precipitation for the Cooke City climatic station shows relatively 

 even distribution throughout the year, with low summer and winter 

 peaks. Total annual precipitation for this station shows 

 comparatively low values throughout the later 1980s and early 

 1990s. Precipitation in 1993 (the year prior to sampling) was near 

 the average for the period of record. 



Geologic setting. The WET72 site is underlain by Holocene-aged 

 alluvium and Pleistocene glacial deposits of undescribed character. 

 The surficial deposits cover bedrock lithologies including 

 Tertiary-aged volcanic rocks of silicic to intermediate composition 

 and Cambrian-aged limestone and shale. The upper basin of Daisy 

 Creek, tributary to the upper Stillwater River above the WET72 

 site, is characterized by extensive hydrothermal mineralization 

 related to the intrusion of the Henderson Mountain Stock and 

 associated dike complex. 



Hydrologic type. The Upper Stillwater wetland is a riparian meadow 

 with perennial surface water inflow and outflow during the ice-free 

 season. 



Basin characteristics. The alpine and subalpine basin of the Upper 

 Stillwater basin provides a relatively low catchment to wetland 

 area ratio due to the large estimated wetland area. Stream 

 gradients within the wetland are very low. Mine wastes and acid 

 mine drainage (AMD) from the historic McLaren Mine have resulted 

 in severe contamination of upper Daisy Creek, a tributary joining 

 the Stillwater River upstream from the WET72 site. Metal-laden AMD 

 encounters calcareous geologic formations (the Meaghar Limestone) 

 a short distance below the McLaren Mine, depositing a hydroxide 

 precipitate as pH rises and removing most dissolved metals from 

 solution. High flows scour the hydroxide solids from Daisy Creek 

 and redeposit them in the stream channel and as overbank sediments 

 in the low-gradient wetland area. 



Water chemistry. The WET72 water sample exhibited a dilute, mildly 

 alkaline calcium-bicarbonate character. Eguilibrium calculations 

 indicate slight undersaturation with carbonate minerals. 

 Phosphorous (total and orthophosphate) and total organic carbon 

 concentrations were low in comparison to most other Rocky Mountain 

 sites. The ammonium concentration ^as below reporting limits and 

 the nitrate concentratioTi ranked mid-range in comparison to other 

 Rocky Mountain sites. 



With the exception of iron (ranking mid-range in comparison to 

 other Rocky Mountain sample sites) , trace element concentrations 

 were below reporting limits in the water column- 



