To«msend Valley Wetland 



Climatic setting. The net annual precipitation balance for the 

 Townsend Valley wetland is -28 inches, placing the site at the more 

 intensely evaporative extreme of the range shown by western 

 sampling sites. Mean monthly precipitation for the Townsend 

 climatic station shows a broad summer peak with a June maximum. 

 During most of the 1980s and early 1990s, annual precipitation for 

 the Townsend station was below the average for the period of 

 record. Precipitation in 1993 (the year prior to sampling) 

 exceeded the average by more than 5 inches; regionally, 1994 

 precipitation was below average at many stations. 



Geologic setting. The WET69 site is underlain by Recent alluvium 

 deposited by the Missouri River; tributary alluvium and older 

 Quaternary terrace deposits bound the Recent alluvium, which 

 overlies Tertiary-aged valley fill sediments. Shallow water table 

 depths apparently result in ground-water discharge to depressions 

 such as the WET69 wetland. 



Hydrologic type. The WET69 site occupies a depression of unknown 

 origins lacking apparent surface-water outflow or significant 

 surface-water inflow. Ground-water discharge to the wetland is 

 inferred from shallow local water table depths, chemical 

 characteristics and the lack of a significant surface catchment in 

 an area of net annual evaporation. Ground-water outflow also 

 probably occurs, based on the relatively dilute character of the 

 WET69 sample. 



Basin characteristics. The WET69 site has a very small surface 

 catchment substantially modified by human activities. Two 

 transportation corridors (State Highway 12 and an active railroad 

 line) closely bracket the site. Irrigation may contribute to the 

 shallow water table depths supporting the wetland. 



Water chemistry. At the time of sampling, water in the WET69 

 wetland was near-neutral, "oligosaline" , and sodium-calciura- 

 magnesium/bicarbonate in character. The pH of the sample was 

 unstable, rising from 7.6 in the field to a lab value of 8.7 . 

 Mineral eguilibria indicate approximate saturation with calcium 

 carbonate phases. The proportional chloride concentration (>11% 

 of anion equivalency) is higher than most sample sites of the 

 intermontane valleys. The cation distribution is distinct from 

 most ground-water analyses from the Missouri River alluvial 

 gravels, which tend to have calcium as the proportionately highest 

 cation. 



The ammonium and total organic carbon concentrations at this site 

 ranked toward the upper end of the ranges shown by western sample 

 sites. Phosphorous concentrations ranked mid-range or lower. 



