Streamflow- Why is it Important?

If you’ve been fly fishing for any length of time you’ve probably seen or heard of the term CFS. Well for those of you who aren’t familiar, CFS is an acronym for streamflow and it stands for Cubic Feet per Second. It’s the scientific measurement of the volume of water passing any given point in a river in one second. In essence CFS tells the fly fisherman how high or low a river is and how fast or slow it is moving.

So why is streamflow important to the fly fisherman? Well, if understood correctly, this little data point can tell you when, where, and even how to fish a river. In some cases it may even help you determine whether or not you want to fish a river at all. Yes, streamflow is a big deal and learning how to read it effectively is a skill every fly fisherman should have.

How to Read the Numbers
One of your most essential tools to understanding streamflow is the universal streamflow graph. There are several online resources out there that offer these readouts, but USGS is the most popular (see below). Here are a few things to keep in mind when looking at one of these flow graphs.

First you will want to identify the daily streamflow. You can find this by cross referencing the numbers running up the left side of the graph (flow rates) with the dates running across the bottom (calendar days). This will give you the current streamflow for that day. Next you will want to identify the historic average of the streamflow. This will give you the average flow of the river over a certain number of years and is represented by small yellow triangles across the top of the graph. The historic average is especially relevant if you are unfamiliar with a river and have no baseline to compare your daily flow to. The below image is a snapshot from USGS showing the daily flows and historic average for the Colorado River near Kremmling. You can see the daily flow for October 15th is 639 CFS and the historic average is around 760 CFS. This information tells us that the river is a bit below normal for this time of year. If you have never fished this section of the Colorado River and have no idea if this is an acceptable flow then the discrepancy between the daily and historic average flow could be a sign telling you to do some more research before heading out to the river. In general though, what you are looking for when comparing the daily and historic average flow is anything that looks out of the ordinary for that time of year.

Another indicator you will want to keep an eye on when reading a streamflow graph is any sudden or drastic changes in streamflow. If you see a significant fluctuation (either up or down) the night before a fishing trip you may want to reconsider. As a rule, major changes in river flow have a negative short-term impact on the fish because it disrupts the trout’s norm and can make them edgy and off balance. Usually 1-2 days is enough time for the fish to adjust and get back to their normal feeding habits. Think of it this way. If you were to wake up one morning and all the furniture in your house was rearranged you would probably be thrown off a bit in the beginning, but after a few days you would adjust. Changing a trout’s habitat is no different.

Now this isn’t to say there aren’t times when a flow change won’t instigate feeding. In the case of a really low flow where trout are struggling because of warm water temperatures and lack of food and protection, a bump can actually turn the fish on. An increase in river flow is beneficial here because it helps decrease water temps and creates additional food and shelter for the fish. That being said, under normal river conditions, a major fluctuation in streamflow (especially a drop) can be a red flag and should be taken into consideration when weighing out the short-term fishability of a river.

Note: I highly recommend journaling your river outings, including the CFS data. This will give you a resource to refer to in the future so you have a personal data point for what high and low really is. In the above case if you had fished this section of the Colorado River at or near 639 CFS and documented it, you would know if it would be worth taking a trip back out to fish it at this flow.

Not all Flows are Created Equal
Another thing to consider when looking at streamflow is that a prime flow for one river might not be prime for another. What I mean by this is that streamflow is relative to the size of its container. A larger river will need more water to fill its banks than a smaller river, so on average it will have a higher CFS. For example a prime flow for a large river like the Colorado is around 1000 CFS (middle section), where a prime flow for a smaller river like the Big Thompson is around 100 CFS. That being said a large river like the Colorado will have more flex in terms of flow fluctuation since there is more room for CFS variance. Think about it. If the Colorado River were to drop 100 CFS from it’s prime flow that would have little to no impact. If the Big Thompson were to drop 100 CFS from it’s prime flow it would be zero. There is a big difference here.

How Stream Flows Impact Trout
When I was fishing the Conejos River this past summer one of the things that really struck me was how different a river can fish based on it’s water level. The average flow on the Conejos for that time of year was about 60 CFS and it was down to a mere 12 CFS. Wow, what a difference 40 plus CFS can make on a small river. The holes that normally held fish were non-existent. The riffles were gone. The pockets were gone. Pretty much all the prime runs were gone. The trout’s entire habitat had been turned upside down. All that was left were the deeper holes and of course that is where the fish were holding.

Now this is an extreme example, but my point is that streamflow dictates where the trout are in the river both horizontally throughout and vertically within. Even in less extreme cases trout will move depending on where they feel they have the best chance to thrive and survive. When flows decrease and the water column flattens, trout will find deeper holes and more secluded areas to hold. When flows increase and water velocity rises the trout will find the outer edges, inside bends, and softer water as places to escape the faster current. Either scenario trout are looking for two basic things – food and shelter, and they will go wherever they must to find it.

As an angler if you can use streamflow as a tool for approaching the river over time you will find yourself having more success. Just as the trout need to adapt to the streamflow, a good fly fisherman will understand this need to adjust and flex as well.

Additional Resources