Documentation of Existing Conditions in the Central Platte Valley

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I SECTIOHTWO Environmental Setting either side of the floodplain, in areas that are now cropland. Subirrigation supports a complex of aquatic, wetland, and lowland plant communities along elevational contours within the floodplain. Groundwater may seasonally, or intermittently, intersect surface depressions. Occasional surface overflows also occur. Groundwater elevations in wet meadows and other bottomland grasslands are influenced by a combination of river stage, precipitation, and evapotranspiration (Henszey and Wesche 1993). For the February through June time period, river stage is often the dominant factor. The influence of river stage decreases with distance from the river and decreases when the stage is sufficient to maintain the groundwater level at or above the surface. Groundwater levels rapidly respond to changes in river stage (up to 2,500 feet of the channel within 24 hours) (Hurr 1983). Precipitation is the next most dominant influence on wet meadows and other bottomland grasslands, and evapotranspiration is only important from May through September ( Wesche et al. 1994). The approximate hydrologic threshold to support a wetland is a water table of less than 1 foot for a continuous period of at least 14 days during the growing season, with a mean interannual frequency of 1 to 2 years (National Research Council 1995). Bottomland areas that do not meet the components of this threshold are not likely to develop wetland conditions, although they may still have high value as wildlife habitat depending on the habitat component being assessed. Detailed information on groundwater depths and durations has been collected for only limited areas as part of specific studies. Wesche et al. 1994 presents maps of depths to groundwater on four dates at three sites ranging in size from about 26 acres to 80 acres. Depths to groundwater during the study ranged from >0 (surface water) to < 6 feet. At the Elm Creek Site, groundwater depths of >0 to 1 foot only occurred on about 15 percent of the study area when groundwater was highest. More than 80 percent of the Crane Meadows study site had groundwater at depths of >0 to 1 foot range on the date with the highest measurement. Hydrographs for depth to groundwater presented by Wesche et al. 1994 suggest that the wetland hydrology threshold was met at the Elm Creek and Rowe Sanctuary only at a limited number of sites and only some of the years. Wetland hydrological conditions were more prevalent at Crane Meadows. Wetland hydrology was often present only during May, but it extended into June or July at some of the Crane Meadows wells. Goldowitz and Whiles (1999) and Whiles and Goldowitz (1998) measured surface water hydroperiods at five slough and backwater wetlands for 2 years and found that hydroperiods varied from 41 to 274 days. The two sites with the shortest hydroperiods were filled with water less than one -third of the time (15 -32 percent). They contained water in the early spring, dried by the summer, and filled again in late fall or early winter. Two intermediate sites were water - filled more than two- thirds of the time but were dry in the summer; one site was perennial. Goldowitz and Whiles (1999) did not report the lengths of time that wetland hydrology was present. Pulses of relatively high flows, which historically occurred during spring months, may have been the primary hydrological influence on wet meadow establishment and survival along the Platte River before flows were regulated by impoundments. The smaller fluctuations in river stage that tW r Wd0dW rd q#* 1 2 -10 68F0D9728 W,1.doc 6/2/19W9:52AMYURSGWCFS/2 l