Stream gaging or the measurement of water discharge in rivers and streams is a major part of the work of the U. S. Geological Survey (USGS). Currently, USGS operates more than 7400 stream-gaging stations in the United States and the information developed from them is highly important.

A stream gaging station or gage house is a small structure usually located on or near a bridge. The gage house contains instrumentation to monitor the water level fluctuations of the river or stream. No doubt you have seen these structures at one time or another and wondered what they are. Usually, they are a metal structure about four feet square and eight feet high with an antenna on top. The equipment inside the gage house monitors the stream level or gage height and reports via satellite to a central USGS location on a frequent basis, often an interval of 15 minutes. When gaging stations are installed the equipment is surveyed to its height above mean sea level so that data from all gaging stations in the United States are comparable.

This is only part of the story, however. On a regular basis, USGS hydrographers make physical measurements of the water discharge of the river or stream at a gaging station. The hydrographers measure the area of the stream perpendicular to the flow of the stream. They measure the width of the stream and multiply it by the height of the water column from streambed to water surface to compute the area. Specifically, they measure the height at twenty or more verticals across the width of the stream. They also measure the velocity at each vertical. Using the area and velocity data they compute the streamflow for the specific time the measurement was made. The streamflow or discharge is usually reported in cubic feet per second. At the same time, they measure the elevation of the stream by reading the information provided by the equipment in the gaging station and checking the information mechanically.

The regular physical measurements of water discharge are used together with the information gathered by the equipment in the gage house to develop a rating table for the streamflow at the location. The rating table allows users to take the gage height provided by the instrumentation to determine the water discharge at any specific moment in time for that location.

So why is this information important and why are there more than 7400 stream gages in the United States? Probably the most important reason is that the information is useful is for disaster prediction and planning. Flooding is a regular occurrence throughout the United States. Streamflow information provides cities and areas downstream from floodwaters with the ability to predict the severity of anticipated flooding from upstream storm events and to make needed evacuations of people and property. Over the years, countless lives have been saved and property damage minimized because of the availability of streamflow information.

A few of the many other uses of the data are for navigation purposes, streamflow regulation and management, determination of the movement of spills of toxic material, and for planning for construction of bridges and culverts. I will describe the process of streamflow measurement in more detail in a future article.