Streamflow and Springflow at Comal and San Marcos Rivers

San Marcos River and San Marcos Springs at San Marcos

Daily Average Streamflow and Springflow

The interactive graph shows both the daily average streamflow and the computed springflow for the past 1,000 days.

Common Data Questions

or ... Why is springflow the same as streamflow most of the time, but not all of the time?

Streamflow is the combination of surface-water runoff and springflow. Surface-water runoff, as its name suggests, is the overland flow resulting from rainfall that is not absorbed into the ground. Springflow is the groundwater contribution to streamflow, often in the form of springs or seeps.

During dry weather, springflow makes up the majority of the streamflow.

or ... I checked streamflow earlier today and now I see that it has changed. Why is that?

The USGS measures streamflow at gaging stations every 15 minutes, commonly referred to as real-time measurements. The daily average streamflow is the average of the 15-minute streamflow measurements over the course of that day. Springflow is based on the daily average streamflow, not on the 15-minute streamflow data.

or ...I downloaded last month's data, and now I see that the value for some of those past dates has changed. How can that be?

During storm events, the computer program is not able to separate streamflow into surface-water runoff and springflow; hence, the springflow is manually estimated by USGS staff. After the storm event, the computer program is rerun and the estimated values are replaced with the computer calculated values.

Historical values may also change when the USGS performs a series of quality-control checks as part of reviewing the data. For example, routine streamflow measurements are made at the site to ensure the reported values are accurate. The reported values may be updated slightly to match the field measurements more closely.

The reported springflow values undergo multiple quality control checks. As part of those checks, some springflow values may be temporarily removed until all of the QA/QC processes are performed and the data are finalized (e.g., when “shifts” are applied to historical data).