Ground Water and River Flow Analysis

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15 iAquifer characteristics. —The characteristics of the aquifer determine how quickly the water table will respond to changes in river elevation and to what distance from the river significant effects will be seen. • Distance from the river.— The farther away from the river, the less influence changes in river elevation will have on the ground water level. The influence drops off geometrically rather than linearly, so the effect diminishes rapidly as distance from the river increased. F-I 1 LI i Elevation of the river compared to the local groundwater.— Water seeks its lowest level. If the water table is higher than the river, water moves toward the river. If the river rises suddenly but ground water levels are higher than the river, ground water further from the river will continue to move toward the river while water in the river will begin moving into bank storage. Figure 7 shows this relationship. Time since any rise in river level occurred. —Bank storage continues until the next change in the river elevation, or until the new river level reaches a balance with the water table elevation. Ground water levels in the Central Platte Valley outside the flood plain are typically higher than the river elevation. Therefore, water movement is toward the river. Ground water, just like surface water, always flows toward the lowest point. When the water table is higher than the river, ground water moves downward toward the river's lower elevation. The ground water movement can change somewhat when the river rises. This effect can be analyzed by Glover's Bank Storage method, which models the interaction between the river and ground water. Figure 7A -D depicts a conceptual model of this movement. When the river rises, the water surface of the river becomes higher than the ground water surface just outside the bank (figure 7B). Water begins to move out from the river into the bank, creating bank storage. A few hundred feet away from the river where the ground water surface is still higher than the river water surface, ground water continues to move toward the river. As these flows meet, the ground water surface must rise to accommodate the added volume of water (figure 7C). When the river returns to its previous level, the water that was stored in and near the bank begins to return to the river, until the ground water levels return to their original elevations (figure 7D). If the river stayed at the higher elevation for 3 or 4 weeks, the ground water surface very near the bank would rise to the new level of the river, but at a distance of /z mile or more from the river the change would be too small to measure. Note that a rise in the level of the river can never cause a rise in ground water levels to an elevation higher than the river. Ground Water and River Flow Analyses