by Brian Waldman
Here are 5 simple "rules" about the gauges which might help you to understand them better.
1) No two gauges are the same. The readings for any gauge are unique for that particular stretch of water. In other words, a flow of 1000 cubic feet per second (cfs) on one river will not look the same as a flow of 1000 cfs on another. Same for stage height. Three foot on a White R. gauge won't equal 3' on a Blue R. gauge. You literally do have to become familiar with each body of water as it relates to the particular gauge in question.
2) Height and Flow are the same for a particular gauge. It doesn't matter which one you go by. What I mean by this is that for a given height on a particular river gauge, the flow will always be the same. So if a particular gauge is showing a height of 3' and a flow of 1000 cfs, it will read those exact same 2 figures whether the river is rising, falling or steady. Every time the gauge reads 3' it will also read 1000 cfs. Doesn't matter whether you read it today, tomorrow or next week, with one exception. The USGS physically tests current flow for all stream gauges on a semi-frequent basis, making sure that the calculations presently used by gauge are accurate. As such, it is possible that over a period of months or years you might see a slight difference due to changes in the river physically that necessitate updating the program, but it won't happen over the short term. So go ahead and read whichever you like.
3) As height increases, flow increases more. See the chart above for a picture of this. Doesn't matter whether you graph a tiny creek or a large river, this same pattern will emerge. The two are not linear. Flow increases exponentially as height increases. This is because height is a static measurement above a fixed point on the gauge. Flow is a volume calculation of water moving past a fixed point. At some point, as the river height increases it eventually spreads out as it over takes lower ground adjacent to the main stem. So even though the height may have only increased by another foot, the volume of water has increased substantially due to this widening of the river. Additionally, water flows downhill, so as height increases and more water gets "stacked" on top of itself, pressure and gravity cause it to fall that much faster. That is why you typically have a really fast fall of the peak at first on a gauge and then a gradual sloping out of the trend as gauge height decreases.
4) There is no simple mathematical formula or calculation you can apply to the gauges to know when to go. All rivers rise and fall at different rates due to their morphology, their watershed and other unique characteristics. You can get a pretty good idea based on the little triangles on the flow graphs. Those triangles on the flow chart are the median daily streamflow for a given river for that particular day for however many years the data has been taken. It is the same as the 50% exceedance value in the table below the graph. What this means is that half of all recorded flows on that day have been heavier and half have been lighter, but it IS NOT the average. The average is almost always higher and not as good an indicator. If the chart is showing something of a level near the triangles, you are probably safe to go, regardless of the rivers location. As a rule of thumb, if you use a 25% factor over the medians or lower as an estimate of fishability, you should be fine.
5) Always check out how many years of record the median flow is based upon. The shorter the time frame, the less trustworthy the median values as relates to fishability. In other words, if a gauge only has medians based on 5 or 6 years of record, the less reliable the triangle "thingy" will be. If a gauge has 30 or 60 years of medians, then you have enough "normal" water records to base a decision on.
Hope these simple "rules" make sense.
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