Effects of ground-water development on the proposed Palmetto Bend Dam and Reservoir in southeast Texas

Ground water continues to discharge into the Navidad and Lavaca Rivers by seepage outflow even though large amounts of ground water are pumped for irrigation. Although a reduction in streamflow probably has occurred, a complete loss of the low flow of the streams by infiltration to a lowered water table seems remote. The large ground-waterwithdrawals will continue to cause land-surface subsidence, which will range from 0.012 foot to more than 0.026 foot per year. A minimum of about 0.013 to 0.015 foot of annual subsidence at the upper ends of the proposed reservoir and a maximum of about 0.019 foot near the dam site can be expected. Structural failures of manmade features have not occurred from the subsidence, but regional changes in the land slope have occurred and will continue. Numerous fault traces have been mapped in the area, but most of the faults are inactive. If movement along the fault planes should occur, the vertical displacement probably will not exceed the total subsidence.