Ground Water and River Flow Analysis

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t"I P'i L� TRENDS Long -term trends in ground water levels in central Nebraska can be divided into four categories using 1950 as the pre - development benchmark. Based on data retrieved from the Nebraska Department of Natural Resource website (figure 1), there are areas of accretion, areas of depletion, areas of depletion and recovery, and areas of no change. Much of Kearney, Phelps, and Gosper Counties is in an area of accretion where covered ground water mound has developed under lands irrigated by water supplied by the Central Nebraska Public Power and Irrigation District. The mound has reached elevations as high as 100 feet above historic levels, creating a distinct ground water divide between the Platte and Republican Rivers. Ground water supplies under large parts of Dawson, Buffalo, Hall, and Adams Counties were depleted by 10 to 20 feet during the 1950 to 1970 decades. A large part of Adams County and a 40- thousand -acre area near the middle of Buffalo County have not recovered although readings since 1990 generally show an upward trend. Most of Hall, Dawson, and Buffalo Counties began recovery in about 1980 and have reached or exceeded pre - development levels. The flood plain area of the central Platte River has not changed within the recorded water level period. These areas typically are subject to water tables of less than 5 or 10 feet. CURRENT LOCAL GROUND WATER LEVELS Figure 2 shows the direction of ground water flow between May 25th and 27th, 1999, through the central Platte Valley (Stanton, 2000). Ground water tends to move toward the river, except on the south side of the Platte River in the upper Little Blue River drainage basin to the southeast of Grand Island. Monitoring of water levels in the 28 wells shows ground water changes from March to September. From March through early May, the water level in most of the wells was 1 to 2 feet lower in elevation than when the snapshot was taken on May 25th and 27th. After June 1st, nearly all of the wells that were monitored through the summer of 1999 rose to higher levels, increasing the gradient toward the Platte. Because the ground water level is generally higher than the river and sloping toward the river, the flood plain lands have very shallow water tables. The primary flood plain generally has ground water levels that are from 1 to 3 feet above the water level in the Ground Water and River Flow Analyses 3