Understanding the River Gauges

Understanding the River Gauges

People have always asked me how I know if a river is fishable – which basically means that it is clear and low enough to reasonably expect to catch fish, and is safe enough for watercraft navigation. All of this can generally be determined by the river gauges even if you have never been to or seen the river! All you have to do is simply learn how to read the gauge and understand a little bit about what lakes and dams are built for and have to do with the water that comes in from upstream. If there is no dam, it is even easier to read the gauge and figure out what is going to happen from a rain event.

Two types of dams that affect river levels

There are two kinds of dams, dams that make large impoundments and dams that simply “run the river” and make very small “wide spots in the river” type impoundments.

“Run of the River” dams

With dams that create these wide spots in the river, they just open the gates to pass basically the same amount of water that is coming into the small impoundment, out of the area. So, we call that a “run of the river” dam which takes advantage of rainfall to generate power, but basically just “runs the river”. Because it is so small above the dam they cannot hold much water when it rains. If they tried it would come flooding over the top and put a lot of stress on the structure and make a dangerous situation below it. Many times during floods water does end up actually running over the top of these dams but if you have all the turbines running then what else can you do? Nothing. Occasionally in certain times of the year they will hold as much as they can (again, can’t hold much since the impoundment is small) and then release it all at once to make power for air conditioning, etc. This mainly happens in the summer because in the winter they usually have enough water to make power whenever. There is less evaporation in the winter due to shorter days, longer nights and cooler weather so this usually keeps our rivers at a normal level or above. Not to mention the increase in water use in the dryer months, which also plays a big part in why the rivers are lower during these times.

USGS river gauge

So, knowing this you can use the graphs on the USGS site to predict, very accurately mind you, what will happen. The first graph is an example of a river gauge right below a dam that creates a very small impoundment (run of the river). First off I prefer to use gauge height when looking at a river gauge to determine if it is floatable and fishable. People that only go on CFS graph (cupic feet per second) can be mislead because 1500 CFS on a big river is no big deal but on a small creek it is a raging flood. However, the CFS chart is vital for something else I’ll get to in the next section. First of all you should be made aware that you can change the range of your graph from 1 to 31 days to help you either see the big picture or the smaller picture of what is happening at your particular gauge. I have switched this first graph to 14 days so you can see where this river was before some substantial rain fell in its watershed. The main bulk of water started to accrue on the 26th of May and because it rained several times in a three or four day period, including well upstream in the headwaters, it took the water until about June 1st to be back at a decent level for floating. You can clearly see how the water is passed through now and they can’t spike it up (release a bunch at once) because they don’t have the water upstream to do so. If they did then it would be briefly because there just isn’t any wall of water coming down and you know this by simply looking at upstream gauges to see that they have leveled off. Also, I used this particular gauge on purpose because it had very squiggly lines on it. In my opinion this is due to the reading being taken in an area with severe turbulence (waves). Any gauge that is in the water at a spot that has potential for high waves has a greater likelyhood of having a very squiggly readout, especially below dams where the water boils up and out of the turbines. You’ll notice that the squigglies are only jumping up and down about half a foot on average and maybe some bigger waves register about one foot. Just don’t let it throw you off or freak you out when reading a gauge. You’ll know when they really drop it or raise it because it will move up or down significantly.

USGS river gauge

Ok, so now you are probably asking, “well how did you know it was floatable on June 1st?” This is where the CFS graph earns its money. This chart has a “median” flow level icon. It is the little triangle you see on the graph and that shows you about where it normally is this time of year based on the data that has been recorded since they put the gauge in. Look at the bottom left and you will see how many years of data it is using to make its median mark. This gauge is using 10 years to determine its median and that really isn’t a whole lot compared to some USGS gauges out there that have 90 years of data but it is still plenty good enough. Once the river level gets back near that median, then it is safely floatable 90% of the time. I mean, its not going to get much better than where it normally is, right? If you can’t float it when it is normal then when can you? The only way it gets any easier to float is if there is a drought, but sometimes on rivers with rock a drought makes things miserable because you’re just dragging down shoals and rapids because there isn’t enough water to float over them. My preference is to be on a river when it is at its median level or slightly below. I don’t mind slightly higher either, but when it is higher you can be sure of a couple things – 1. swifter water and 2. stained water. The closer the river gets to the median the clearer it becomes and when it dips down below the median it gets even clearer.

Now that you know this info about the median level you can go back up to graph one and see where the level was on June 1st and then make that your “fishable gauge height mark” for judging this river for floating just below where this gauge is located. Having done so I would say this river is fine anywhere from 26.5ft and below. If it is over 27 I would say it is too high and therefore too muddy to fish successfully. You could still just paddle it though but it would be a quicker ride. 26.5 is your high mark so beware that when the river is this level it is likely to be muddy, especially if the river already has a natural tinge to it or if it drains a heavily urban or construction area.

Mega dams creating large reservoirs

When looking at a graph that is of a river below a major dam creating a big reservoir it changes things up a bit. These dams try to keep the level consistent for property owners on the lake but at the same time also have big swings in river level below the dam because they also generate power there. When a dam generates power that requires a lot of force and power by a large volume of water in order to turn turbines, which in essence is the energy that creates power that lights our homes and the electricity to run the computer screen your staring at right now.

It is important to know if the lake is currently at its desired elevation above sea level. Knowing this can help you know what will happen to the river below the dam. Most lakes have a website courtesy of the power company or whoever controls the dam. Info on lake level can be found there. If they are below normal level then it is likely they will hold more water that is coming into the lake so that they can get back to normal level. A good example of this is the Chattahoochee River below Lake Lanier in the Atlanta area. Due to drought the lake is well below normal level and has been for years. Because of this they do not release any more water than they have to and this creates a pleasant river float below the dam, even after heavy rains. Of course that is trout water for many many miles, but you still get the idea of how knowing if a lake is at, above or below normal level can help you very accurately predict what they will do. Looking at the gauge in a large 31 day range can also help you see trends and learn from them.

If the lake is above full pool you can expect them to be passing a lot of water through the dam, making life tough and potentially dangerous for us river bassers. Since large reservoirs have a duty to homeowners and recreational users to keep the lake at a consistent level, they try to do so when possible. If a ton of rain dumps into the watershed well upstream of the lake then you can expect the dam to keep doing what they have been doing to keep the lake at the consistent level until the water begins to reach the head of the lake – unless it is a MAJOR wall of water that could possibly flood over the dam which could bust it. In that case they would have to do the greater good, which draw the lake down as much as they can before the water gets there to save the dam and try and take some sting out of the imminent flood that is on its way. Aside from that rare situation dams usually do the best they can to keep the lake at a “summer pool” level and a “winter pool.” Many dams have a phone number you can call and talk with someone or listen to a recording about their water release schedule for the day. Your local dam may very well be one of these so try finding out who owns it and visit their website to find a phone number. One last random tip about dams is that you can usually find clearer water below a dam that releases from the bottom. This clear water can be the key to finding some fishable water when other rivers are still murky because the muddy water upstream on a lake takes a long time to get down to the dam, and by the time it does the silt has usually settled creating clear water.

USGS river gauge

The right hand side of this next graph is an example of what a dam on a reservoir normally looks like when they are raising the level just to generate power. The left hand side of the graph you can see they really increased the release level but they were doing this to mainly pass a lot of water through the dam to make sure the lake didn’t get over their desired level. Of course, power is made when this happens just like those smaller dams that use that wall of water to make power. Clearly a significant rain event hit their upstream watershed that day or even several days before depending on how far up it rained in the watershed. Either way they started to pass the water through on the 26th of May because that is when the level rose. Once the water passed through they went back to their traditional schedule of holding water for a little while, then letting it out all at once for a few hours to generate power. And, if for example, there was no more rain in the coming weeks they will eventually cut back on generating power or at least generate for a shorter time because you can’t pass water through the dam that you don’t have or else the lake will go dry and we’ve already covered why they can’t do that due to the agreements with home owners, cities, counties etc.

River with no dams controlling water flow upstream

Free flowing rivers are rivers that have no dams on them and are at the mercy of God’s rainfall on the earth. Until the point where a lake has been created on a river we consider USGS river gaugeanything above this first lake free flowing. Few rivers today are completely free flowing. Smaller creeks will rise and fall faster, and larger rivers will rise slower but will also stay high and muddy longer. Quick downpours will usually make everything rise way faster than steady rainfall because the ground does not have time to soak up some of the brunt of the deluge. The next graph is of a free flowing river. You can see how it rises and especially drops at a slower rate. This chart is for 30 days so it may not be as easy to tell but if you look a the dates at the bottom you can see that it takes a while for the river level to drop.

It will just take time to learn how to use these charts but over time you will get the hang of it. When rain hits look at several of them including tributary gauges on the river system to see how much higher above median they got. That will help you see if a lot of water is coming into the river – aka, it will continue to spike – or if not as much is coming into the watershed. Soon you will be an expert and begin to be able to almost read the minds of the folks who control the dams, because afterall they are using the exact same upstream gauges and techniques we are to determine what to do at the dam!

4 Responses to “Understanding the River Gauges”

  1. PawPaw says:

    Super article BD!!!! I have never used the CFS chart but now that I have read your article, I will definately use it too. Most of my chart reading know how is just by using them during high water times and low water times. Writing down what is a good level and remembering where you wrote it is always good to have.

  2. Great Article Drew, The first time I looked up the USGS data on local rivers it was like trying to read a chinese dictionary! I’ve been on them regularly here lately hoping for a break from the rain.

  3. Shoal Tide says:

    Very informative BD
    thanks

  4. waderjon says:

    Excellent article Drew..I have struggled with the charts.This has cleared some things up for me.
    Thank you

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