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Understanding USGS Streamflow Data and Flood Events

Updated: Apr 21, 2023

Probably the three most useful tools for anglers concerned about "is it fishable" are a thermometer, precipitation maps, and streamflow gaging station data from the US Geological Survey. Thermometers are pretty self explanatory. The one thing I will say is use them not only in the summer to find "fishable" water (under 68*F for trout) but in the winter they are valuable to find places that are slightly warmer. A difference of a few degrees can make all the difference in the winter. Camping on the West Fork of the Kickapoo at the West Fork Sports Club a number of times this summer has made it rather clear to me that more people need to understand the importance of carrying a thermometer!

The after effects of the 2018 flood. The Driftless had a number of waterfalls - new knickpoints on the streams.

Precipitation maps are not much more complicated but a little harder to find. The sources I tend to use are:

  1. iweathernet's radar estimated precipitation map which allows you to look at 1, 2, 12, 24, 48, and 72 hour precipitation estimates for the entire country. They are radar estimated, not measured on the ground, so the totals are not perfect but the map does tell you a lot about general patterns.

  2. Wisconsin State Climatology Office produces "Climate Watch" which has a series of maps with the precipitation, high and low temperatures, and snowfall and snowpack for the past 24 hours, and 7, 30, and 90 days. Maybe their most useful maps are the precipitation departure maps which tell a lot about areas that have been wetter or drier than average of the past week, month, or about three months. (see image below)

  3. The National Weather Service produces maps but also a list of rain gauges that are monitored by trained NWS reporters.

Using the USGS stream gaging stations is a bit more complex but rather worth the effort.

There are two figures (graphs) that you will usually see on a USGS streamflow gaging station page. One is the gage height and the second is discharge, a measurement of the volume of water per second that the river is transporting. Gage height is an arbitrary and rather meaningless number but it is easy to measure (more below). Measuring discharge is more complicated and requires more equipment and labor (more below) but the result is more meaningful. However when discharge is measured across a variety of water levels, a ratings curve can be calculated and discharge is estimated from the gage height.

A gage is literally a measuring board for the river's level. Much of the time it is a ruler attached to a bridge or to a post anchored into the river. Waders and boaters of the Wolf River may be familiar with Cap's gage and numbers on it mean something to them. If you understand the context of a particular gage, you may know that 4 feet is baseflow (the "normal" water level), less than 6 feet is wadeable, between 6 and 8 is questionable for wading, and over 10 is crazy whitewater you might not want to miss. Again, these numbers are specific to that that one gage, for other gages - even on the same river, the numbers are different.

Gage height for the Kickapoo River at La Farge for Augst of 2021.

That the river crested at a little bit over 12 feet is not that terribly meaningful to me because I have no reference for that gage height. That is the problem with gage heights, the numbers are only meaningful within context. More over, the numbers are only meaningful for each individual gage and are not transferable across sites. That is, 12 on the La Farge gage is totally independent from the Ontario site upstream where the same flood peaked at 18 feet.

Why stream gages are useful is that the water level can be converted to discharge - the volume of water that passes a cross-section per second - and gage height is cheap and easy to measure. Historically, stream gages were measured visually - that is, someone measured where the water level is on a physical stream gage. Later, gage height was measured by a float in a stilling well. Today, it is done electronically by measuring the difference between atmospheric pressure and water pressure (pressure transducer), gas-purge or "bubbler" systems, or by measuring the height by radar. In each case, the devices measure the depth or elevation of the water. For more about how gage height is measured, see the USGS website on Streamgaging Basics.

A ratings curve relates stage (height) to discharge (volume per second).

We have established that we have a variety of ways to measure stream height - but that it is not a terribly meaningful measurement because it is not universal. However it can be converted to discharge which is the best measurement of a stream's size through a ratings curve. Discharge is measured periodically to develop the curve but because extreme measures - floods and drought - are relatively rare, USGS personnel are often in the field measuring stream discharge during floods. Note how in the hydrograph below has a couple of times where the discharge was measured and not just estimated from the ratings curve.

Discharge of the Kickapoo River at La Farge during the August 2021 flood event.

Maybe you remember a bit of of calculus - discharge is basically integrating under the curve. The stream is divided into a number of cells along its width - the more cells, the more accurate the discharge calculation will be - and the depth of the stream (feet or meters) is measured as is the average current velocity (feet or meters per second). By multiplying these three things (width, depth, and average current velocity) together, the discharge for each cell is obtained in cubit feet or meters per second (cfs or cms). Then summing the discharge across each of the cells, the total stream discharge is obtained. For more, visit the USGS page on how streamflow is measured.

Discharge is the most effective measure of a stream's size. To put things in perspective, the Amazon River, the World's largest river, has a discharge of 7,400,000 cubic feet per second (cfs; 209,000 cubic meters per second), the Mississippi River's average discharge is 593,000 cfs (16,800 cubic meters per second), the Wisconsin River's average discharge is 12,000 cfs (340 cms) at mouth, the Wolf River at New London averages 1830 cfs (51.8 cms), and the Wolf River at Langlade - probably the state's largest trout stream by discharge - averages 438 cfs (12.4 cms). For a bit more perspective, the largest spring in Wisconsin is in Door County and has a discharge of a bit over 18 cfs (0.5 cms; for more on Wisconsin springs). Relatively few trout streams in Wisconsin are gauged but they tend to range from a just a couple of cfs to the Wolf River. Black Earth Creek near Black Earth which is a very large Driftless spring creek has an average annual discharge of about 31 cfs (0.88 cms). Most Wisconsin trout streams are probably between 5 and 25 cfs (0.14 to 0.71 cms).

August 2021 Flood

To illustrate the relationship between the stream gage height and stream discharge, I will present two figures to describe what happens during a flood event. Beginning on Saturday, August 7th, rainfall soaked part of the Coulee Region, particularly hard hit was La Crosse which set a record for 24 hour rainfall that stood since 1884.

In the image above from the August 2021 flood, the baseflow had been about 3 feet as a gage height which is equivalent to about 150 cfs as seen in the discharge figure below. As the storm hit, the rising limb of the flood shows a sharp increase to a gage height of nearly 13 feet. The falling limb - the curve after the flood reaches its peak is more gentle. That is, streams are often flashy - they rise more quickly than they fall. You can also see a couple of other small rainfall events that bumped the river up on August 11th and 12th. Then the stream returned to a higher baseflow - a bit over 4 feet (about 400 cfs in the figure below).

I included this figure mostly to demonstrate that stream height (gage height) and discharge are not related linearly. Notice how much more slowly the stream discharge decreased compared to the gage height. Another thing to note is how the X axis is on a log scale in this figure but not the first one (the one for stream height). For more about reading hydrographs, visit the Engineering Hydrology blog post on the subject.

This flood peaked on the Kickapoo River at La Farge at about 3,000 cfs (85 cms). Upstream at Ontario, the flood peak was 2,300 cfs (65 cms) and it occurred a little earlier. Downstream at the Steuben gage, the peak flood occurred a couple of days later (August 12th) at about 3,800 cfs (108 cms). The 2018 flood - the largest in recorded history on the Kickapoo at La Farge - peaked at about 17,000 cfs (481 cms). At Steuben, the discharge topped 20,000 cfs (566 cms). It was a whole different beast!

The Flood of 2018

Twenty or more years from now people will be talking about the flood of 2018 or at least I hope so because if not, that means a larger and more devastating flood came along. The Kickapoo River is no stranger to floods. In general, the Driftless Area is flood prone. Our steep sided valleys (Coulees) and lack of wetlands due to topography and cultural sedimentation means that rainwater is quickly delivered to steams. Coon Creek, the first watershed project in the county, was lead by Aldo Leopold and farmers in the watershed in response to flood events and the loss of soil to those floods. Contour farming, grassed waterways, and other innovations came out of these efforts.

Peak flood stream flows at La Farge on the Kickapoo River. 2018 is the largest flood on record.

One thing to understand about floods is that they occur in different ways for different streams. Summer storms are often very localized. While the Driftless is not what one might call mountainous, I have been camped out and seen a storm track one valley and miss the one I was camped in. And, of course, I have been in the valley that got "hammered" while the valley next to it received a glancing blow. Such is the way localized storms occur. These floods tend to be occur and dissipate relatively quickly.

2018 flood hydrograph
Flood hydrograph from the 2018 flood - note the large peak and the secondary peak after additional rains fell on saturated ground.

Other floods tend to be more regional - that is a storm front affects a large region. For many of use, the best example of this is the "Great Flood of 1993" that occurred over a huge expanse of the central United States. This flood ranged from the Dakotas east to Wisconsin. 2008 was a similar regional event. These floods tend to be much longer-lived.

Floods and Fishing

Since I have not written much about fishing in this post and this is, after all, a fishing blog, I feel compelled to write a bit about fishing. First, most flood events are small. The stream comes up a bit, the stream becomes a little turbid, and some sediments (silt, sand, and rock) and organic debris - along with food - is washed into streams. Some of the best and most memorable days of fishing I have ever had are right before it rains. Fish seem to know. At the beginning of the rising limb, worms and maybe terrestrials are washed into the stream, nymphs may get dislodged, and fish often feed heavily. Depending upon the magnitude of the flood, trout tend to find a place to "hunker down" around the peak of the flood. Depending upon the duration of the flood's peak, trout are often quite hungry and active as the stream is receding (the falling limb) and returning to a higher baseflow. Often when the turbidity of the stream is at a point where you can see at least a foot into the water, the fishing can be really good. The stained water give you a bit more cover and the fish are hungry after not eating for bit. I have had the best luck on dark streamers, big - often gaudy - dry flies, and if you really want to "slum it", a San Juan worm can be absolutely killer.

Upper Timber Coulee post flood
This would be tough to fish - but tomorrow would probably be a really good day to fish this stream as it clears.

The second bit of fishing information I think is important to convey is that floods rarely ruin streams and do not "kill all the fish". Before the internet warned of the dire consequences of flood events, it was the bait shop and word of mouth that spread tales of streams' demise. Truth is, trout and streams are much more resilient than we tend to think. Yes, there is density-independent mortality associated with floods, particularly of smaller fishes but fewer trout are killed in floods than you might think. And floods do some good - they rearrange streams, often creating new pools, scouring out existing pools, and maybe most importantly, cleaning spawning riffles of fine sediments. Often floods are followed by strong spawning years. Having realized I have not written as much about this as I had thought - expect an examination of the effects of floods in an upcoming post.

Lastly, one of the most effective ways of using hydrographs is to look at current stage compared to historic means for that date. The image above, a hydrograph from the Kickapoo River at La Farge in June of this year. Note that the line of triangles are the mean daily discharge which in the case of the La Farge gage go back to 1939. Since precipitation has increased, our "new" average is probably a bit higher than the average calculated from 82 years of data. But in any case, you can see that during the hot and dry stretch that began the month - La Crosse set several daily high temperature records - the river discharge was increasing until the 17th and 18th of the month delivered rain. In general, the current discharge compared to the historic average gives you a good idea how high or low the water, particularly if you keep in mind that precipitation has increased over time in our area. For more on that and data to support that statement, read the post I wrote on climate change and trout fishing.

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In our country a stream thermometer should be standard equipment by the end of May every year. I have found some of the biggest fish in pools where the water temp below the pool is one or two degrees lower than the temp above, telling of a spring inflow.

Most of the streams I fish are small and do not have a gage, and the gages on the main stem downstream are not the best for predicting fishing success in the upper reaches. When I was a child I rode my bike to the fishing. I had one stream that ran more than 12 miles between bridges. The best fishing was a long walk from the road on any one…

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