Report Title: Major and Catastrophic Storms and Floods in Texas  
Report Guide
Glossary of Terms
Introductory Materials
Substantial flood peaks
Links to related web resouces
Measuring and gaging streamflow
Measuring Precipitation
Recent storm reports
John Patton storm narratives
Texas Flood Safety
Dedication and Credits
Go to top of page
  Measuring and Gaging Streamflow
Measuring Streamflow
The USGS began measuring streamflow in 1888 as part of studies involving irrigation of public lands. Streamflow measurements are made by direct or indirect methods. Most direct measurements are made by sounding stream depths and measuring stream velocities with meters. Low-flow streamflow measurements are made with wading rods and pygmy meters (figs. 1, and 2). Most flood measurements are made using large meters and heavy depth-sounding weights suspended from steel cables (fig. 3). Flood measurements usually are made from bridges (fig. 4) or boats, but years ago many flood measurements were made from cars suspended from cables that traverse streams (fig. 5) or from equipment mounted on automobiles (fig. 6). Other direct measurements of streamflow are made by acoustic, optical, or radar equipment and by injection of dyes. Many flood-peak discharges are based on indirect measurements, which are made by applying open-channel hydraulic principles to surveyed peak-stage profiles along stream channels. Indirect measurements are used to compute peak discharges in open channels or at bridges, culverts, dams, and other hydraulic structures that constrict the peak water surface.

Streamflow-Gaging Stations
Streamflow generally cannot be continually sensed or recorded. The objective in operating a streamflow-gaging station is to obtain a continuous record of stage (water-surface elevation or gage height) in the stream from which a continuous record of discharge can be computed for the site. A continuous record of stage is obtained by installing instruments that sense and record the stage in the stream. Discharge measurements are made at various stages to define the relation between stage and discharge and are made at periodic intervals to verify the stage-discharge relation or define any change in the relation due to changes in channel geometry. Weirs and dams are constructed at some stations to stabilize the channel geometry and thus the stage-discharge relation. The stage-discharge relation is known as a rating curve. From the rating curve, a table of corresponding stage and discharge values is developed for each streamflow-gaging station and used to convert stage to discharge.

The first streamflow-gaging station was installed in 1889 on the Rio Grande near Embudo, N. Mex. Streamflow stations from then until about 1960 contained stilling wells on stream banks (fig. 7). A pipe from the well to the stream allowed the water level in the well to be the same as that in the stream. A float in the well was attached to a graphic recorder in the housing atop the well, thus the gage height of the stream was continuously sensed and recorded (fig. 8).

By about 1960, servo-control manometers and digital recorders were being installed to replace the stilling wells (fig. 9). The gage height was sensed by measuring the water pressure at the opening of a tube mounted near the bottom of the streambed and extending to the gaging-station shelter. This equipment could sense and record gage height from a shelter remote from the streambed.

Beginning about 1995, pressure transducers and data loggers were being installed (fig. 10). This equipment was smaller, less complicated, and more reliable than the servo-control equipment. Modem and satellite transmitters and antennas also were being installed (fig. 11) so that gage-height data could be transmitted, processed, and presented in near-real time on the World Wide Web (Web).

Historical and near-real-time gage heights and discharges for Texas streams are presented on the Web at


Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11