Ground Water and River Flow Analysis

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1 it i Under the Program flow types analyzed, the only measurable changes from rises in the Platte River would be in ground water levels near the river. Ground water levels more than 3,000 feet from the river would not be impacted. Basements and farmlands located within 3,000 feet of the river may experience somewhat higher ground water levels during Program augmentation flows. periods, given the volume of water expected to be allocated to a program. Each pulse flow event would require 30,000 to 40,000 acre -feet of water. Pulse flows would raise the river level at most 10 -12 inches for the duration of the higher releases. A 1 -foot rise in the river for a 3 -day period would cause a measurable rise in ground water only within ' /z mile of the river. Figure 9 shows what the effects would be from a 12 -inch rise in river depth on the 3rd and 6th days. Effects would diminish rapidly after the 6th day. BASE FLOW AUGMENTATION Base flow augmentation would provide flows between 500 and 1,000 cfs. These increases would be provided several times during the average year to meet various species' needs, such as spring crane roosting flows. These flows could last from 10 days to 30 days, and would raise river levels about 5 inches. Figure 10 shows the effects of these flows after 10, 20, and 30 days. Effects would diminish as soon as the increased flows ended. CONCLUSIONS 23 Ground Water and River Flow Analyses In the Platte River Valley, ground water is generally higher than the river and moves ' toward the river under natural conditions. The river is the ultimate outlet for ground water that builds up in the aquifer from precipitation. While irrigation practices have modified the natural ground water regime to some extent, the natural conditions still generally prevail. Changes in river levels have a strong influence on ground water levels very near the river, but the influence diminishes rapidly with distance from the river and ' is essentially nonexistent beyond 2 miles from the river. This is borne out by the theoretical analyses (appendix A), the daily analysis of well hydrographs with river flow hydrographs (appendix B), the statistical analyses (appendix C), and the snapshot of ground water conditions (appendix D). Regional ground water fluctuates more closely with precipitation than with river levels. The Program may raise water levels in the river by as much as 1 foot for periods of 3 or 4 days and by as much as 1/2 foot for periods up to a month. These rises will have some effect on ground water levels within a half mile of the river while they are taking place. 23 Ground Water and River Flow Analyses