The Ogallala Aquifer is a vast underground water reservoir beneath the Great Plains in the United States. It is one of the world’s largest aquifers, providing fresh water for millions of people and various economic activities. Composed of sand, silt, clay, and gravel, it holds ancient water accumulated over geological timescales.
Geographical Reach
The Ogallala Aquifer spans approximately 174,000 square miles (450,000 square kilometers) across the central United States. It underlies portions of eight states: South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas. The aquifer stretches roughly 800 miles from north to south and about 400 miles from east to west, forming a significant subterranean feature of the High Plains region.
Subsurface Dimensions and Water Volume
Beneath the surface, the Ogallala Aquifer exhibits varying depths and thicknesses. Its average depth is estimated to be between 100 and 300 feet, though it can reach depths of up to 1,000 feet in some areas, particularly in Nebraska. The water within the aquifer is stored in the pore spaces of unconsolidated sediments.
The total water volume is estimated to be around 3 billion acre-feet, which translates to roughly 3 trillion gallons. This volume is enough to cover the entire state of Texas with one foot of water.
Current Status of Water Levels
The Ogallala Aquifer is experiencing significant water level declines due to human activity. Since 1950, agricultural irrigation has been the primary driver of water withdrawal, with about 27% of all irrigated land in the United States situated over the aquifer, supplying approximately 30% of the groundwater used for irrigation nationally.
The rate of water extraction often far exceeds the natural recharge rate, leading to substantial net depletion. In some areas, water levels have dropped by as much as 100 to 200 feet since predevelopment times. It would take over 6,000 years for the aquifer to naturally replenish once fully depleted.
The Aquifer’s Geological Formation
The formation of the Ogallala Aquifer is a testament to geological processes over millions of years. It began to take shape during the late Miocene and early Pliocene epochs, approximately two to six million years ago. During this period, the actively uplifting Rocky Mountains to the west provided a vast source of eroded material.
Ancient rivers and streams transported these sediments eastward across the Great Plains. These materials were deposited as extensive alluvial and eolian fans, gradually filling existing valleys and eventually covering the entire region to form the Ogallala Formation. The water saturating these sediments is largely “fossil water,” accumulated from precipitation and ancient meltwaters, slowly percolating into the porous geological layers.