Streamflow and Springflow Data for Comal and San Marcos Rivers, Texas Now Available Online

Release Date:

A new U.S. Geological Survey website provides important information about streamflow in the Comal and San Marcos Rivers and springflow at Comal and San Marcos Springs. This website was developed in collaboration with the Edwards Aquifer Authority.

The Comal and San Marcos Springs, the two largest springs in Texas, provide water for supply and recreational uses, as well as habitat for endangered and threatened species. Understanding the amount of springflow is critical for water resource managers to make daily decisions on balancing multiple water needs, particularly during periods of low flow.

Historically, information on Comal and San Marcos springflow has been difficult to find, and the differences between streamflow and springflow have often been confusing to stakeholders. Now, streamflow and springflow measurements can be found at one location online. The new website also includes an explanation of how springflow is calculated from streamflow, and why the flows might appear to change over time.

“The new USGS website clarifies how the springflow records of Comal and San Marcos Springs are calculated,” said Marcus Gary, a hydrogeologist at the Edwards Aquifer Authority. “This information greatly improves the ability of the EAA and the many stakeholders of the Edwards Aquifer to understand, report and apply the published flow rates from these critical natural resources.”

Springflow cannot always be measured directly at Comal or San Marcos Springs. Instead, the USGS measures streamflow at two gauges located downstream from the spring networks, Comal River at New Braunfels and San Marcos River at San Marcos. Streamflow is made up of springflow and surface-water runoff (from storm events). During non-storm periods, all streamflow at these two gauges comes from springflow. During storm events, the relative contributions of springflow and surface water runoff are estimated by USGS staff. After the storm events, springflow at Comal Springs and San Marcos Springs is recalculated using a computer program following standard springflow separation methods and the previous estimates are updated with the newer values. The computed springflow values are calculated and checked each weekday before being released online.

“This new website provides up-to-date springflow estimates, even during storm events, and helps communicate what those numbers mean,” said USGS supervisory hydrologist Ryan Banta. “To provide the most accurate data, scientists routinely go into the field to verify the streamflow measurements used in the springflow computations.”

For more than 100 years, the USGS has monitored flow in selected streams and rivers in Texas. The information is routinely used for water supply and management, monitoring floods and droughts, bridge and road design, determination of flood risk and for many recreational activities.

Keep up with the latest USGS Texas news by following @USGS_Texas on Twitter. View the USGS Texas Water Dashboard to see more information about current water conditions in Texas.

Photo of USGS scientist measuring flow downstream of Comal Springs, Texas.
USGS scientist Vidal Mendoza measures flow downstream of the Comal Springs, Texas.(Credit: Alec MacDonald, USGS. Public domain.)
Photo of Spring Lake in San Marcos, Texas
Spring Lake in San Marcos, Texas, has hundreds of small springs underneath it.(Credit: Cassi Crow, USGS. Public domain.)
Photo of two of the main spring discharge points of Comal springs.
Two of the main spring discharge points of Comal springs.(Credit: Mike Nyman, USGS. Public domain.)