Yellowstone River Oxbow 



Clinatic setting. The net annual precipitation balance for WET80 

 (from the MAPS database) is -30 inches, placing it at the more 

 intensely evaporative extreme of the data set. Mean monthly 

 precipitation for the Miles City climatic station shows a 

 pronounced June maximum. Annual total precipitation was below the 

 long-term average most years during the 1980s, while precipitation 

 in the 1991-1993 period was at or above the long-term average. 

 Water supply to the WET80 wetland is, however, probably tied 

 closely to the stage of the Yellowstone River, which is influenced 

 by precipitation and other climatic conditions throughout the 

 basin. 



Geologic setting. The WET80 site occupies an abandoned side 

 channel of the Yellowstone River, developed along the edge of the 

 modern floodplain and just below the edge of the older alluvial 

 terrace underlying most of Miles City. The adjacent and 

 underlying(?) bedrock is the Tullock member of the early Tertiary- 

 aged Fort Union Formation. The alluvium is described as relatively 

 coarse-grained and permeable; water table elevations are shallow, 

 and are apparently intersected by the old channel swale containing 

 the wetland. Geologic structures to the east and west generate 

 pressure head in the Fort Union and underlying Hell Creek aquifers. 

 Flowing wells (and by inference, discharge to the shallow alluvial 

 aquifer system) occur in the vicinity of Miles City. 



Hydrologic type. The Yellowstone River "oxbow" (sic) occupies a 

 modified natural channel originating near the confluence of the 

 Yellowstone and Tongue Rivers and passing through Miles City before 

 rejoining the Yellowstone channel. The channel appears to 

 intersect the water table of the alluvial aquifer in places. The 

 wetland receives intermittent surface water inflow and, probably, 

 ground-water discharge from the Yellowstone River alluvium. 

 Outflow occurs via a high-water channel to the Yellowstone River. 

 Rates of flux through the wetland probably depend on the stage of 

 the Yellowstone and Tongue Rivers and on related water levels in 

 the alluvial aquifer underlying the floodplain. 



Basin characteristics. WET80 has a small local surface catchment 

 heavily modified by urban activities and structures. Ground water 

 inferred to support the wetland is probably recharged in part by 

 the mainstem Yellowstone River. The depth and volume of the 

 wetland water body and the frequency of surface inflow and outflow 

 are not known. 



The WET80 site is the only one of the sample sites where urban land 

 uses dominate the wetland catchment. 



Water chemistry. At the time of sampling, the WET80 site displayed 

 near-neutral, "oligosaline" sodiura-sulf ate/carbonate water with a 

 depressed dissolved oxygen concentration and relatively high total 



