Determining Optimal River Flow
Bill Leuchten

Determining the optimal flow for a particular river is a complicated business. Some high flows bode well for some rivers and not for others. The same holds true for low flow. There is not much written on the subject because there are no clear cut facts and a lot of "it depends" answers, so who would want to write on a subject like that? Here is my stab.

High flow for many rivers raises the velocity of the current, thus forcing fish to congregate behind all the likely structure like rocks, and off to the sides of the river. An angler can work the banks and structure with good success because the fish are seeking these likely places. The Arkansas and the Roaring Fork rivers, for example, fish extremely well when the flow is high, around 1000cfs. The fish are excited by the increased food supply provided by all the extra water and they are forced into likely areas, mostly the riverbanks. But when the flow drops, thus decreasing the water's velocity, it opens up more riverbed for the fish to occupy. This can make the fishing more difficult. You may need to work a larger piece of the river to locate fish, for they may not be grouped up for easy pickings. To make matters worse, the fish will be tucking in behind rocks and in the indentations of riverbed, forcing anglers to cover large areas of boulder fields. This situation can also make good fly presentations difficult due to the currents created by structure no longer submerged. We all know the challenges of getting good drifts through difficult holes like these. And of course you can never forget that you will be spooking more fish because of the shallower water.

Then there is the other side of this. Low flow on some rivers can force fish into the obvious pockets making them easily accessible to anglers. The "Dream Stream" has fished exceptionally well with low flow. (It also fishes great with a high flow so no real conclusions here). But we've all heard the "the fish have nowhere to go" theory when the flow has forced the fish into the well-known holes.

And to throw another variable into the mix, some rivers experience their high flow in an off-color torrent that is described as being "blown out". This is an example of why an angler can't just abide by an overall policy of "go when the flow is high".

So what can we take from this? Well, every river has its own character; the structure in the river, the indentations of the riverbed, the kind of undercut banks, etc. And the flow will always determine the behavior of the fish. High flow can excite them and put them in likely locations, or it can force them into unfishable areas. Low flow can congregate them and allow you to locate them, or make your approach and drift next to impossible.

I'll just conclude like everyone else. It just depends!

I hope this helps!