Subsidence is the permanent and irreversible lowering of the elevation of the land surface in relation to mean sea level. Subsidence in this area is caused by the compaction of subsurface clay layers due to the loss of support pressure resulting from water level declines. Prior to the District’s creation, prolonged and increased pumping of groundwater for municipal, industrial, and irrigation supplies in Harris and Galveston counties had caused the water level of the aquifer system beneath the area to decline over a broad region, which resulted in one of the largest subsidence bowls in the United States. Approximately 4,700 square miles of land subsided more than 0.5 feet from 1943 to 1973, with the maximum subsidence in the area approximated at 10 feet. More than 31 square miles of low lying coastal land was permanently inundated, and the potential for tidal flooding, particularly from storm surges, increased in the coastal region because of loss of elevation.