The Texas Water Development Board has completed the 200th survey in its Hydrographic Survey Program, a milestone in the agency’s decades-long effort to document how much water Texas reservoirs can actually hold today. The agency detailed the program in an Aug. 26 article published by the Texas Water Newsroom, a communications project of the TWDB.
Sediment settles on reservoir floors over time and steadily reduces storage, the agency said, which means the number that matters to water managers is not a reservoir’s original design capacity but its capacity now. The TWDB identifies current reservoir capacity estimates as an important component of statewide water planning as the state’s population and water needs continue to grow.
The 200 completed surveys cover 116 unique Texas reservoirs, 57 of which have been surveyed more than once, according to the agency. Of the 122 reservoirs featured on WaterDataforTexas.org, the program has surveyed 94, and of the 184 existing major water supply reservoirs identified in the 2027 State Water Plan, it has surveyed 106.
The Texas Legislature authorized the program in 1991, and surveys began in 1992, the agency said. The Texas Water Code allows the TWDB to conduct surveys at the request of political subdivisions or state or federal agencies to determine storage capacity, sedimentation levels and rates, and projected water supply availability. The program receives no dedicated funding; fees collected for the work recover its costs, a model the agency said keeps surveys cost-effective while ensuring the resulting data remains available to the public.
Surveys fall into two categories, the agency said. Volumetric surveys measure the reservoir’s current bottom surface and produce updated elevation-area-capacity tables and bathymetric contour maps. Sedimentation surveys pair acoustic data with sediment core samples to identify the pre-impoundment surface, estimating sediment volume and annual sedimentation rates. Survey lines are generally spaced about 500 feet apart and run perpendicular to the river channels that existed before impoundment, with readings collected by a single-beam, multi-frequency depth sounder operating at 200, 50 and 12 kHz. The TWDB also offers multi-beam surveys for high-resolution views of structures such as dams and water intakes.
Elevation-area-capacity tables are provided at 0.1-foot increments, the agency said, and its survey archive makes reports, contour maps, shapefiles and tables publicly accessible. One survey establishes a snapshot, the TWDB noted, while repeat surveys record change across decades.