Using a Staff Gage
A staff gage is nothing more than a long ruler placed semi-permanently in a stream or lake and used to read water depth. Stream gages are the most common and useful measure and are therefore emphasized here. However, you also can put a staff gage in a lake to monitor changes in lake water level.
Why Use a Staff Gage?
Staff gage information can be used in an indirect way to estimate stream flow. If you place a staff gage near a section of stream for which you are collecting flow data, you can identify the relationship between stream depth and stream flow. Once you know this relationship, you can estimate flow from the stream depth without having to take the time and trouble to make a detailed flow measurement. Periodically, the staff gage will need to be recalibrated against measured flows since the streambed, and thus the relationship between stream height and flow, can be expected to change over time.
Setting Up a Staff Gage
As was the case with stream flow, site selection is very important. The criteria used to select a flow measuring site are also important to the selection of a gage site. In fact, most of the time you will want to place a staff gage wherever you monitor flow.
Once you have selected a good flow monitoring site, it still may be hard to find a proper place for the staff. If placed too near the side of the stream, the staff may be dry during summer months. If placed near mid-stream, it may be washed away by high winter flows. The staff should not be placed in very slow-moving water or in a pool because sediments will accumulate around its base. The resulting localized changes in water flow patterns could affect your readings. On the other hand, turbulent water can make it difficult to read the gage. If your station is located near a bridge that has pilings in mid-stream, the downstream side of the piling often provides a good location for a gage.
The simplest way to install a staff gage is to attach it to a permanent structure, such as the bridge piling mentioned above. Life is rarely so convenient. You may have to provide your own "permanent" structure, for example, by pounding a strong metal pipe into the stream bed. Be sure the pipe is strong and tall enough to last through high water conditions. A PVC pipe also works. However, since these pipes are light and hollow, numerous holes should be drilled through them to allow the water to flow freely through the pipe. This will alleviate much of the extra strain on the pipe caused by high, fast-moving water. (NOTE: Lakes are easy. Just attach the gage to the end of a dock or pier where you can easily lean over and read it. Be sure the dock is permanently anchored to the lake bottom. A floating dock or one that is removed every year wont work.)
You can also "gage" a stream without using a staff gage by measuring the distance between the water surface and a known fixed height. If a bridge crosses your stream near where you want to establish a staff gage, youre in luck. The distance between the bridge and the surface of the water changes in response to changes in the stream depth. Measuring the distance from the bridge to the water surface lets you determine stream height.
To use this method, make a permanent mark on the side of the bridge to be sure your measurement is always taken at the exact same spot. Drop a weighted tape measure until the weight touches the water and record the distance from the bridge to the water. This is a wonderfully simple method of obtaining stream height data, but it may not be highly precise. Even very small changes in stream depth or height may reflect large changes in stream discharge, particularly in a wide stream. When measuring the distance to the water surface from a bridge, you may have difficulty telling just exactly when the weight has reached the waters surface. On a windy day, the tape may not hang straight down to the water surface, or waves may make the tape bounce. Furthermore, different people who take the reading will likely interpret the point of contact with water a bit differently. The farther above the water the bridge is, the more these factors are likely to affect accuracy.
To reduce the error from this method, be sure your mark on the bridge is clear and has a line showing exactly where the tape should be read. Write down exactly how you are defining the surface of the stream. For example, is it when the weight touches the water, or when the weight is fully submerged? Taking care of these small details will improve consistency in the measurement.
The next section discusses forming a stage-discharge relationship.
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