Reading River Graphs

by Brian Waldman

Whether you are kayaking, wading or walking, at some point you've probably wondered about traveling to a river to recreate. But you're not sure if the recent rains have made the river unsafe for navigation, or if the flow is too high to be fishable. You can look at the river gauges from the USGS website: http://waterdata.usgs.gov/in/nwis/current?type=flow , but you're not sure how to read them and you don't want to risk a long drive, all for naught. Well, here are a few tips that might help explain things a little better the next time this occurs...

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 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, or at least within a tiny fraction depending upon scale. 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. Everytime the gauge reads 3' it will also read 1000 cfs. Doesn't matter whether you read it today, tomorrow or next week. Now it is possible that over a period of months or years you might get a slight difference due to changes in the river physically, but it won't happen over the short term. The only time these gauges stray from their set flow regimes is when the USGS has documented actual flow changes and gone in and updated the new calibration curves. So read whichever you like.
 
3) As height increases, flow increases more. See the chart below for a picture of this. Doesn't matter whether you graph a tiny creek or a large river, this same pattern will appear. 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. 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. For instance, if you use a 25% factor over the medians or lower, you should be fine, but without knowing what the median flows are (no flow chart for that station), you can't apply a formula to figure out if a given height is fishable. Either you know that stretch or not. You can take a flow gauge like B'ripple's and change the graph to 31 days. This will usually give you an idea of what the "normal" pool should be, but you can't apply a multiplying factor to that to get some number to base that on.  
 
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.

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