TWDB hydrographic surveys provide data needed to understand our water supply

TWDB hydrographic surveys provide data needed to understand our water supply Posted on August 26, 2026

A reservoir's surface often hides changes happening out of sight beneath it. Over time, sediment accumulates on the reservoir floor and the underwater landscape changes, reducing the amount of water a reservoir can store. For water managers, the important question is not simply how much water a reservoir was designed to hold, but how much storage capacity it has today. A reservoir's capacity is an estimate at a certain point in time and is a number that can change over time.

The Texas Water Development Board (TWDB) Hydrographic Survey Program exists to provide that snapshot as well as a better understanding of the rates of change since the reservoir was first filled.

The TWDB recently completed its 200th survey-including 116 unique Texas reservoirs, 57 of which have been surveyed more than once-marking a major milestone in the agency's efforts to document reservoir capacity and sedimentation rates. Of the 122 reservoirs on WaterDataforTexas.org, the TWDB Hydrographic Survey Program has surveyed 94. Of the 184 existing major water supply reservoirs identified in the 2027 State Water Plan, the TWDB has surveyed 106.

The Hydrographic Survey Program represents a long-running source of physical data describing some of Texas' most important water supply reservoirs. That historical data matters because the real value of hydrographic surveying emerges over time. One measurement tells you how the reservoir bottom looks. Repeated measurements tell you how it is changing and support improved estimates of available water supplies. The TWDB identifies current reservoir capacity estimates as an important component of statewide water planning as Texas' population and water needs continue to grow.

Hydrographic Survey Program overview

The Texas Legislature authorized the TWDB to develop the reservoir survey program in 1991, and the agency began conducting surveys in 1992. The Texas Water Code authorizes the agency to conduct surveys at the request of political subdivisions or state or federal agencies to determine reservoir storage capacity, sedimentation levels and rates, and projected water-supply availability. There is no dedicated funding for the program, so fees collected for the work performed are used to recover the costs to conduct the surveys. This model, established by the Texas legislature, provides cost-effective surveys of the state's reservoirs and a means for ensuring the availability of this data to the public.

The surveys that provide essential data for reservoir owners, water planners, and engineers fall into two categories.

Volumetric surveys measure the reservoir's current bottom surface. The TWDB uses that data to calculate current storage capacity and to produce updated elevation-area-capacity tables and bathymetric contour maps.

Sedimentation surveys determine how much of the reservoir's original storage has been reduced by sediments settling on the bottom. These surveys identify both the current bottom and the pre-impoundment surface. The TWDB combines acoustic data with sediment core samples to estimate sediment volume and annual sedimentation rates.

In addition to these standard services, the TWDB can provide multi-beam surveys for a high-resolution view of underwater structures, such as dams and water intakes.

How a TWDB hydrographic survey works

A hydrographic survey starts with a deceptively simple question: Where is the bottom of the reservoir?

Answering that question accurately takes more than a boat and a depth finder. The TWDB first establishes the survey boundary outlining the perimeter of the reservoir, and then survey lines are planned across the reservoir. For standard volumetric and sedimentation surveys, survey lines are generally spaced about 500 feet apart and run perpendicular to the river channels that existed prior to impoundment.

A survey boat then collects acoustic measurements of the reservoir floor. The standard TWDB approach uses a single-beam, multi-frequency sub-bottom profiling depth sounder operating at 200, 50 and 12 kHz. For a volumetric survey, the 200-kHz signal is primarily used to identify the current bottom surface.

Sedimentation surveys require another layer of information. The TWDB analyzes the different acoustic frequencies and correlates those measurements with samples of the sediment pulled from the bottom of the reservoir. These sediment-core samples help identify the pre-impoundment surface beneath accumulated sediment. That allows data analysts to distinguish the reservoir's current bottom from the landscape that existed before the reservoir was filled.

The acoustic measurements are then converted into a bathymetric model. Because survey lines do not cover every square foot of the reservoir, the TWDB uses spatial interpolation to estimate conditions between them-and this is where the skills of the TWDB hydrosurvey team play a huge part. Modeling the reservoir bottom surface is as much an art as science, and the hydrosurvey team's knowledge and expertise enable them to turn the raw data into a useful model.

Since the creation of the program in 1991, data collection techniques, innovations such as lidar, and analytical methods have improved tremendously-resulting in the most accurate products for our customers. The hydrosurvey team delivers more than a collection of depth readings. The three-dimensional picture of the reservoir floor can be used to estimate how much water the reservoir can hold-and, when a sediment survey is performed, reservoir specific sedimentation rates.

What is produced by the survey?

A hydrographic survey produces a set of measurements, models, and maps that are useful products for reservoir owners, engineers, planners, and the general public-including elevation-area-capacity tables, bathymetric contour maps, depth-range maps, and elevation-relief maps. Sedimentation surveys provide additional deliverables: sediment-thickness maps, sediment volumes, and estimates of annual sedimentation rates.

The elevation-area-capacity table is particularly important. The TWDB provides these tables at 0.1-foot elevation increments, allowing reservoir managers and engineers to relate a water-surface elevation to the corresponding surface area of the reservoir and its storage volume at that elevation. The agency's survey archive makes all hydrosurvey results accessible to everyone, and depending on the survey, interested parties can find reports, contour maps, shapefiles, and bathymetric elevation-area-capacity tables.

Why reservoir capacity matters

Water supply models, planning decisions, and infrastructure investments all depend on calculations about how much water can be stored and whether storage capacity is changing over time. That is why repeated surveys are so valuable. They establish a record of physical change and provide a way to test assumptions against the physical realities of a reservoir-and better data means better water planning.

The TWDB Hydrographic Survey Program provides a way to keep water supply estimates from Texas reservoirs grounded in current physical conditions. A single survey establishes a detailed snapshot of a reservoir. Repeat surveys record changes through time. And, the survey archive documents this history across decades and across reservoirs throughout Texas, which helps water planners ensure that water supplies across the state can meet their communities' needs for years to come.

For more information about the TWDB Hydrographic Survey Program, please send an email to [email protected] or call (512) 936-0815.

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