About three miles north of Middleburg on Foxcroft Road, you cross Goose Creek. Have you noticed the corrugated metal structure off the right side of the bridge? Likewise, when you cross Goose Creek between Gilberts Corner and Leesburg, have you noticed the small building that looks like a porta-potty but has an antenna on its top? No, it's not a porta-potty with a television. Actually both the corrugated structure and the building are U. S. Geological Survey (USGS) stream gages.

Why is it important to measure streamflow? It must be because the USGS operates more than 9,000 stream gages nationwide. Let's leave the reasons for the end of the article and talk first about how the stream gages work.

Within each stream gage are sophisticated devices that measure the height of the water flowing in the stream, bounce the information off a satellite and make it available in almost real time to computers at the nearest USGS office. Based on this height measurement, the actual streamflow is computed.

About 18 times a year over each year the gage has been operated, USGS personnel using a device called a current meter physically measure the volume of water flowing in the stream, either by wading the stream at low flow or from the bridge. Multiplying the width of the stream times the average height of the water above the stream bed gives the cross-sectional area of the water. Multiplying the area times the average velocity gives the actual flow of the stream. The results are recorded in cubic feet per second.

Using the height measured by the gage and the physical measurement just described, USGS personnel plot the streamflow against the gage height on a graph. Each time a velocity measurement is made, it is plotted on the graph. After a number of points are plotted, they are connected by a line. The plot is called a rating table and the line is used to determine the streamflow each time the instruments send a gage height reading to the satellite and on to the USGS office.

Now, let's get back to the original question, why are gaging stations important? I'll give you a few of the many reasons. First, and probably most important, is public safety. NOAA and the Army Corps of Engineers use this information both for forecasting and managing floods. When you hear that River X is 3 feet above flood stage, it means that it's 3 feet above the gage height of the nearest gaging station on River X at which it overflows its banks. Countless lives are saved every year thanks to predictions developed from gaging station data.

A few of the many other reasons include power production scheduling at dams, designing bridges, culverts, and levees, determining loads of stream sediments and chemical constituents, and setting permit requirements for discharge of treated wastewater. In sum, streamflow data from gaging stations is valuable for both immediate decision-making like flood forecasts but also for long range decision making such as siting and size of a new bridge.

Perhaps now, when you see a porta-potty with an antenna by a stream or river, you'll have a new appreciation for how these simple structures impact our everyday lives.