Reading USGS Streamflows can be one of the most helpful resources for the angler here in Montana.
Weather conditions can change river conditions quite drastically, even in a short period of time. Many of these Streamflow sites are updated every hour, giving you up-to date reports on the river without even seeing it.
If you already know how to read them, maybe you’ll learn a few new tricks. If not, read through this tutorial, and by the end you will be much better at determining the best place to fish.
Here is the link to the USGS Streamflow Station List for Montana. Bookmark this!
If you’re confused on any of these terms, USGS has put together a nice glossary of terms here: USGS Water Term Dictionary.
First off, we’ll start with the basics…
WHAT IS A CFS?!
Cubic Feet per Second, CFS as we refer to it, is a measurement of water volume. I’m sure you’ve heard it before, probably on my fishing reports… “The Gallatin is flowing at 2,000 CFS, dropped down 300 over night..” And you probably said to yourself, “Why the heck does that matter for fishing?” Well it does. It matters a lot. I’ll go into how rising and dropping flows effect fishing, but for now, lets focus on what CFS means.
Like I said before, CFS is a measurement of volume. It is a numerical measurement for the amount of water flowing past a certain point every second. If you’re into math, one CFS equals 7.4805 Gallons of water flowing by a certain point per second. On all of the major rivers here in Montana as well as many smaller streams, and across the US for that matter, there is at least one station set up for measurement of CFS. When these stations take a reading, every hour, and update to the USGS site, we can see how the river is changing, and what direction it is trending.
MAIN LIST OF STATIONS
Below is what the main page looks like. You will see that the stations are broken up by river basin. Here is a general key that will help you figure out which basin the river you are looking for is located in…
West of the Continental Divide = Upper Columbia River Basin
If the river/stream eventually flows into the Missouri River, but is south of Great Falls, MT = Upper Missouri River Basin
If the river/stream eventually flows into the Lower Missouri River, East of Great Falls, MT = Lower Missouri River Basin
If the river/stream eventually flows into the Yellowstone River = Yellowstone River Basin
As you’ll notice, when you visit the main page listing all of the Montana Streamflow sites, you will see a couple of different columns with data.
Here is a breakdown of what those columns mean.
Station Number: The number given to the designated station.
Station Name: The name of the location, usually including name of body of water, and closest town
Long Term Median Flow (Date): The historic median (middle statistic) flow for the site on today’s date. Some sites contain over 100 years of data.
Discharge: Current flow rate, in Cubic Feet/Second measured at the Station.
Gage Height: Another measurement taken at the station, showing current height of the water flowing through the river/stream channel.
Temperature: The temperature, in celsius, of the water measured at the station. Not every station has a thermometer. Some temperatures are measured seasonally.
Date/Time: Date and time of last measured station reading.
SPECIFIC STATION PAGES
When you click on a specific station, links are the station number, you are directed to the station’s page. There you will find several different options.
Towards the top, you will see this box… the HISTORIC DATA RETRIEVER THING.
This magical box allows you to look back into the historic readings from this station. I use this feature a lot when we are coming out of runoff and I’m thinking of fishing a location that I am not very familiar with. One situation I have found for this is when I can remember a time when I fished a specific river and had excellent fishing. I want to go back again, but I’m not sure if it is fishable yet. (There are a lot of other variables that determine whether or not a river is fishable, but this can help). With the historic graphs, I can enter in the dates when I previously fished, and pull up the graph of the flows during that time period and find out what the CFS was when the fishing was so great. There are several other great uses for this, but that is just one that I have found.
Scroll down a little further and you will see the first measurement graph. It is different on each station.
The three possibilities are:
Discharge, Cubic Feet per Second:
This graph shows the trend of the river discharge over the past eight days. This is one of the best indicators of fishing conditions for the angler to look at.
Gage Height, Feet:
This graph follows basically the same trend as the Discharge graph. It is simply another measurement to look at while assessing the river conditions. It is also used in determining Flood Stage during runoff.
Temperature, Water, Degrees Celsius:
This graph is a good indicator when looking at hatch possibilities. Certain bugs hatch at different water temps, and fish feed more or less at certain water temps. A great chart to look at before heading out.
This is a great feature of the USGS website. For every USGS Station, you can sign up for WaterAlert, a real time alert system designed to send you a notification when water levels exceed user-defined thresholds. You can simply go to the site, below, and set the parameters you would like to be notified about and how frequently you would like notifications.
Notice the WaterAlert link at the bottom right of the graph.
This is great for runoff times, when you’re just waiting for the river to get low enough to fish. Take the Yellowstone River for example….we typically don’t fish the Yellowstone until it gets down to below 10,000 CFS after runoff. You can set this to immediately notify you when the Yellowstone first drops down below 10k.
It is also nice when there are changing conditions. Fish typically don’t eat quite as much when the river is rising hard. When there is a spike in the Gallatin River due to last nights rain storms, you can have WaterAlert notify you, and then you can re-assess your after-work fishing plans.
TIPS TO REMEMBER WHEN CHECKING FLOWS
Here are a few tips to think about when you’re looking at the Streamflows.
A couple of major Rivers have Streamflow stations on their tributaries. You should always check those tributaries in addition to the main river stations. They can notify you of changing water conditions that could be effecting the main river.
When the Lamar River spikes, look for the Yellowstone River to have some mud:
Yellowstone National Park’s Lamar River is one of the main feeder streams of the Yellowstone River. When ever there are big rain storms moving through in the summer, be sure to check the Lamar Streamflow. If there is a spike in the Discharge graph, it is highly likely that there will be a mud plug moving through the Yellowstone River in the next day or two.
The Shields River, east of Livingston, MT, is another feeder of the Yellowstone. Although more stable than the Lamar, when the Shields spikes, you can expect a mud plug moving into the lower Yellowstone shortly.
Pulsing Flows out of Dams:
Another thing to look out for, this time effecting the tailwater rivers in our area. During runoff, it is common to see the flows going up and down quite a bit out of the dams. I’m a little unclear on why they (whoever they are) do this, but what I do know is that it can really slow down fishing.
Fish like consistency. They like cool, clean water, and they like it to stay the same. Pulsing flows is when the people running the dams release a bunch of water, then slow it down again, then repeat…fish don’t like that.
Rising Flows Usually Means Slow Fishing
For whatever reason, it seems like fishing is never really good when the river is rising hard. There are always exceptions to the rule…I caught my biggest Brown Trout ever when the Madison was rising several hundred CFS as I was standing in the river….but IN GENERAL, fishing seems to be slow when the rivers are rising.
Dropping Flows Usually Means GOOD Fishing
For whatever reason, it seems like fishing can be very very good when the river is dropping hard. Of course, there have been times when the river drops several hundred CFS and I don’t even see a fish, but it seems like IN GENERAL, those fish are moving around, feeding, and are a lot more aggressive when the flows are dropping. Streamer fisherman love fishing the drop!
ALWAYS CHECK THE FLOWS BEFORE YOU HEAD OUT
I’ve gotten in the habit of always checking the flows before I head out for the day. It is Montana, and you never really know what can happen during the night.
I hope this helped clear up some of the CFS nonsense you read about it our fishing reports. There is a lot of CFS talk during the Spring and early Summer when we are coming out of runoff. It is important though. Even though we wish it were otherwise, it’s just not possible to get out to every river and check the conditions…these Streamflow reports let us make educated guesses as to what might be going on with out actually being there.
If you have any questions on Streamflows, we’d be happy to answer them for you. We’re all pretty good at deciphering what might be happening and whether or not it would be worth driving out to the river to fish or not (For the record, it’s ALWAYS worth fishing somewhere..)
Not only are we you’re local USGS Streamflow experts, we also have two great fly shops full of great gear, flies, and we know a thing or two about fly fishing as well. Stop by and say hi someday!
We’re moving into full on Summer-mode here in SW Montana and I know I’m excited! It’s going to be a good one!