To the average person, it seems to make sense to talk about the level in feet from the US Geological Survey's real time river gauges. They can easily picture 2.2 feet. But at the same time, unless you know what 2.2 feet really signifies on that particular gauge, the figure means nothing to you. It does not mean the river is 2.2 feet deep. It doesn't even mean that it's 2.2 feet deep in the riffles. All it really means is that on the gauge's arbitrary "ruler", the water level at the gauge is standing at the 2.2 foot mark. You have no idea what the normal level is. You have no idea whether the river is high, low, or in between, unless you have been floating the river for years, and checking what the level was every time you floated it, so that you can correlate that level to your floating experience on that day.
But there is another measurement on nearly all the gauges; they will show you the flow in cubic feet per second. It may be difficult for the average person to understand this measurement. It simply means that, if the river is flowing 100 cubic feet per second at the gauge (abbreviated as "cfs"), then 100 cubic feet of water are going by the gauge each second. But how is that measured?
First, most real time river gauges, to simplify, use a tube attached to a vertical or steep surface, like a bridge piling, with the bottom of the tube in the water below the lowest level the river is ever expected to reach. That lowest level is 0.0 feet on the gauge. There are openings below the waterline that allow water into the tube, and water pressure pushes the water within the tube up to the level of the stream's surface on the outside. There is a float inside the tube that records the level, which goes to a remote recording station, usually updating every 15 minutes. There are different mechanisms in some gauges, but they all give the level in feet at the gauge itself.
Measuring stream flow (volume) is more complicated, and thus impractical to measure constantly, so the measurement of level is assumed to signify a certain number of cfs. In other words, on a given gauge, 2.2 feet is the same as 100 cfs. Keep in mind, however, that each gauge is different, and 2.2 feet on a different gauge is likely to mean a totally different number of cfs. So how do they KNOW that a given level in feet corresponds to a certain number of cubic feet per second? Well, they periodically measure the flow, getting a wide range of gauge height to flow correlations.
The way flow is measured is by taking a line across the channel of the river, and at intervals along that line, measuring the depth and the current speed. If the gauge is at a bridge, the bridge itself makes a convenient line along which to measure. Usually, there are 25 to 30 measurements taken across the channel at equal intervals. The depth at each interval is recorded, and the current speed is taken using a current meter. At other times, a cable may be strung across the channel as the line by which the depth and current speed is measured. And there are now more high tech methods of measuring depth and current speed, rather than the old current meters and a measuring stick. But basically, those measurements are used to compute the number of cubic feet of water going by the measured line at a given time interval.
Usually these measurements are taken at certain time intervals, but they are also usually taken at the height or near the crest of floods, because hydrologists wish to know what the volume of water flowing in high water is. So the measurements are taken, and correlated with the level in feet on the gauge at that time to give one an accurate measurement of how many cfs are flowing at that given gauge height. Other than that, the shape of the river bed and valley in profile is measured, and other levels in feet are estimated. The estimates are usually surprisingly accurate. Here is a diagram of how it all works:
This diagram shows a bridge, with the river valley cross section delineated in brown. The gauge is the solid red line on the left bridge piling. The bottom of the gauge, level 0.0 feet, is about two feet below the river at normal flow (the lowest solid blue line). The pink lines are the intervals at which the river's flow is measured at the downstream edge of the bridge. At normal flow, the river's depth varies from 0 feet at the right bank to 6 feet at the deepest part of the channel, and the cross section of the river's flow at that point comes to 167 square feet. If the average of the current speed in all the measured intervals is 1 foot per second, then the river is flowing 167 cubic feet per second at the normal flow, which is 2.0 feet on the gauge. The middle blue line shows the river level at 10 feet on the gauge. The profile of the river and its valley at that level shows 523 square feet. Since the water probably flows faster in higher water, I've multiplied that by a current speed of 3 feet per second to come out with a flow of 1,569 cubic feet per second. The highest blue line is the level at a major flood, 18.0 feet. The cross section of the valley comes to 1,121 square feet, and at an average current speed of three feet per second, it would be flowing 3,363 cfs.
The results of these measurements show up on the web page for each gauge, and are recorded. With enough years of record, very accurate figures of mean and median flow for that gauge can be computed, along with record highs and lows, and 25th and 75th percentile figures. Mean flow is the average of all flows measured for that given date, one for each year of record. Median is the middle value in that list of numbers; there are just as many years where the flow was greater than the median as years when it was less than the median. The 25th percentile is the flow figure where the flow was only lower in 25% of the years of record, while the 75th percentile is the figure where flows were lower in 75% of the years.
Next: how to use the gauges and get the most information from them.
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