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Know the Flow? Or just go?

Do you check the flow, the “cfs” level, of your destination river before you head out?  Or does it matter to you – you just monitor the river when you arrive and adjust as necessary?  Knowing the “cubic foot per second” (cfs) level can be informative – but there is more to flow than meets the eye.


Let’s start with some definitions. “Cubic Feet per Second” (cfs) is the rate at which the river is flowing past a fixed point on the river.  There is a gauge situated on the river which measures this flow – and through a computer system, this level gets updated and reported every 15 or 30 minutes.  You can see the Division of Water Resources’ site which reports these flow levels by clicking here


So how much water is in a cubic foot?  Picture a basketball – or the box that a basketball comes in.  Or one of those old-fashioned milk crates that you probably used as furniture in your dorm room.  Now let’s say the flow rate is about 55 cfs – about what it has been in Elevenmile Canyon up until a week ago.  Now imagine 55 of those basketball boxes stretched out across a specific point on the river, maybe Messenger Gulch.  You can picture yourself wading across that stretch in that amount of water – carefully watching for rocks and maybe some deeper pools carved out. The picture on the left is Messenger at about 55 cfs.




Yesterday the flow was at 142.  So let’s imagine almost three times as many of those basketball boxes stacked three-high and laid across the exact same point.  Now picture yourself wading across.  It’s a trickier picture to negotiate! The picture on the right is Messenger at about 142 cfs.  Notice how the water lines on the larger rocks  in the left hand picture are submerged as are the smaller rocks in the river.  


All of this would be easy to envision with a relatively flat stretch of water maybe laid out across a river 20 yards wide or so.  But let’s pinch the canyon in a little bit.  Same amount of water all of a sudden is deeper and faster because it is squeezing through a narrower space.  Now let’s change the gradient from 1% to 3%.  The water is cascading through a narrower shoot with more “pitch” to it.  So cfs, width of the river, and grade of the stretch of the water all come together.  


So does it matter if I check the flow before I go out to the river – or do I just adjust when I get there?  If the river is an unfamiliar spot to me, I cannot really visualize what that flow level may look like before I see how wide the river is and at what gradient it is.  So the flow level alone may not be very informative.


But if I am familiar with the river, then knowing the flow level can be very informative.  Greg Blessing, one of the guides at Angler’s Covey, says that when he guides with some newcomers to the sport, knowing the flow level ahead of time helps him plan for where they’ll wade – if at all – and where he will search out the fish for a successful day on the river.  “I like higher flows a little bit more because the fish aren’t as easily spooked.  By knowing the flow, I also know how much weight to use (to get the wet fly down faster).  And lower flows let me know that fish will be congregating in deeper pools … and not be spread out so much on the river.” 


On the Midcurrent website’s, they write that “[w]hile a large river like the Yellowstone might average 10,000 cfs, a mountain stream might average only 10 cfs. Trout feed in currents that are running at 2-6 cfs — usually in the transition areas between faster and slower water, like the tails of pools and the transitional edges between deep and shallow water.” 


So some people don’t check the cfs at all and head on out, knowing that they will make the decisions of where and how to fish when they arrive.  Others like to know the flow before they go. 



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