Texas, the second-largest state in the United States, possesses a diverse landscape that spans from coastal wetlands to mountain peaks. This vast geographical range makes the question of whether any part of Texas is below sea level complex. While the state’s overall average elevation is far above the ocean’s surface, certain localized areas along the coast are indeed at or below sea level due to a combination of natural and human-induced factors.
The Geography of Low Elevation Coastal Areas
The regions of Texas that sit near or below sea level are all part of the Gulf Coastal Plain, a flat, low-lying area that gently slopes toward the Gulf of Mexico. This plain is naturally vulnerable to tidal surges and flooding because of its minimal elevation above the ocean. The Houston-Galveston region, in particular, contains numerous pockets where the land surface has fallen below sea level.
The primary reason for this lowering is a process called land subsidence. This geological phenomenon is largely caused by the compaction of fine-grained aquifer sediments, such as silts and clays, following the withdrawal of vast amounts of groundwater. As water is pumped out for municipal and industrial use, the pressure supporting the sediment layers is reduced, causing the land above to sink irreversibly.
Between 1906 and 1979, the Houston-Galveston area experienced land subsidence of up to 10 feet in some locations, greatly increasing the risk of inundation from normal tides and hurricane storm surges. For example, the San Jacinto Battleground State Historical Park has seen approximately 100 acres become permanently submerged due to this sinking.
Efforts by regulatory bodies, such as the Harris-Galveston Subsidence District, have focused on slowing this process by switching from groundwater to surface water sources. While these measures have helped to arrest subsidence along the western margins of Galveston Bay, the land that has already sunk cannot be recovered.
Understanding Mean Sea Level and Elevation Data
Determining if a point is “below sea level” relies on understanding how elevation is measured. The reference point for all land elevations is a complex concept known as a vertical datum. Surveyors and cartographers reference a specific, fixed surface rather than the actual, fluctuating level of the ocean.
In North America, the standard reference surface for elevation measurement is the North American Vertical Datum of 1988, or NAVD 88. This fixed reference was established by adjusting a large network of leveling observations across the continent. Therefore, an elevation of zero feet or a negative value measured against NAVD 88 means the location is below this specific reference point, not necessarily below the instantaneous water level of the Gulf.
The actual average height of the ocean is defined by Mean Sea Level (MSL), which is the arithmetic mean of hourly water heights observed over a specific National Tidal Datum Epoch (a 19-year period). Because the sea surface is constantly changing due to tides, ocean currents, and gravity variations, MSL is highly localized and is not the same as the fixed NAVD 88 datum. Coastal areas can be technically below the geodetic reference point even if they are not permanently under water.
Topographical Contrast: High Altitude Regions
Contrasting sharply with the low-lying coast, the vast majority of Texas maintains a significantly higher elevation. The state’s topography features a dramatic elevation change, rising from the Gulf Coast to its western boundary. The High Plains in the Panhandle region, for instance, sit at an average elevation over 3,000 feet above sea level.
The most extreme elevation contrast is found in the Trans-Pecos region of West Texas, characterized by desert basins, plateaus, and rugged mountain ranges. This area includes the Guadalupe Mountains, which represent the uplifted portion of an ancient marine fossil reef. The state’s highest natural point, Guadalupe Peak, is located within Guadalupe Mountains National Park.
Guadalupe Peak reaches an elevation of 8,751 feet above sea level, rising over 3,000 feet above the Chihuahuan Desert floor. This immense height difference underscores that the coastal areas below sea level are geologically isolated anomalies, making them an exception to the state’s generally high average elevation.