An Alternative to “Water Visibility”

Guest Blogger: Mike Cline, Bozeman, Montana

Joe Dellaria’s recent treatise on “Water Visibility” prompted a great deal of thinking on my part.

Having read and contemplated the well-articulated posts, I think there was a key element of why different water conditions resulted in catching fish in different parts of the river that was overlooked. First, generalizations based on a single river are challenging because the impact of the different variables that contribute to good or bad fishing from one stream to another can’t really be compared or evaluated. There is no doubt that water clarity plays a role in success or failure on the stream, but I believe it is an enabling factor to the most important variable—temperature. So, here’s what I think all the variables are in order of priority and how they relate:

Water Temperature: I think this is the driving variable in fish behavior. Trout obviously have a typical temperature range tolerance but are most active when the temps are in the 50-75% regions of that range. At the extreme ends of the range, trout are sluggish and do not feed nearly as much as when the water temperature is in its ideal range. The temperature regime in any given stream is dictated by three variables—bottom color, water sources and water depth. Look at any USGS stream temp data and it follows the pattern of typical day/nighttime air temps—coolest in the morning, warmest in evening. Gases (air) and liquids (water) have remarkably similar behavior when it comes to temperature. The air over a stream does not dictate water temperature as the sun doesn’t heat the air. The sun heats the bottom of the stream which then radiates into the water. A dark bottom will warm much faster than a light bottom. The deeper the bottom, the slower the warming. Water clarity can significantly influence the rate that the stream warms or cools. A stream with a lot of shallow riffle water in comparison to its deep pool water will warm faster. Just as cold or warm air masses move over land changing the ambient temperature, the behavior of water sources has the same effect. High altitude tributaries, heavy snow packs, tailwaters and springs contribute a cooling effect. The lack of those will result in higher water temps as flows thin. Just as very shady waters tend to remain cooler because of the lack of warming from the sun, higher gradient water tend to give off heat and remain cooler because of higher surface areas. I show several examples below of how several large western rivers behave temperature wise as flows change.

Availability of food: Water temperature has a big impact on when hatches occur and the activity of non-insect forage. Fish that might lie motionless at the bottom of a pool early in the day waiting for whatever might come their way will move into the shallow riffle to grab nymphs that are now active because stream temps have warmed. Since all the different aquatic insects typically available in trout streams have different seasonal and temperature requirements, it is water temperature, not clarity that drives their activity. In high water conditions, trout indeed (some trout) will move to the margins because flows are more comfortable, but also because there is the opportunity for more forage being swept along the edges. Fewer fish are typically caught during high water in the deepest areas of the stream not because the trout aren’t there, but because they are much harder to reach effectively with a fly.

A recent experience on the Upper Ruby river near its headwaters demonstrated how the availability of food dictated where the trout were. Near its headwaters, the Ruby is small, not unlike many Eastern streams. However, it is a low gradient meadow stream with little shade along its banks. Just coming out of runoff in late June, the river was slightly off color and the sun was at its highest during the annual solar cycle. The receding, but slightly off-color water had warmed sufficiently to bring out a few May flies and caddis. But you couldn’t budge the trout to the top as they were taking advantage of a far more abundant supply of food. Much of the bank along the meandering section of river I was fishing was mud and grass. During runoff which was heavier than normal in 2018, very large chunks of bank broke off and were dissolving as runoff subsided. The primary food source for the trout and whitefish was earthworms. Even though I was catching fish regularly with small may fly nymphs in the deeper pools, most of those fish were regurgitating globs of earthworms when I landed them. They didn’t need to leave the deep pools to feed, even though there was food available elsewhere and water temperatures were optimal.

Weather: The state of the weather and how it is trending at any given time play a key role. There’s ample evidence that storm fronts impact fishing. As barometric pressure falls, skies cloud up and wind directions change, fishing can improve dramatically. By contrast, as pressure stabilizes, and skies clear after fronts pass, fishing success can suffer. Trout fishing in rivers can be remarkably better on cloudy days than on clear days. This is especially true in brown trout waters with great clarity.

Water Clarity: As said above, the clarity of the water does influence the rate at which a stream warms or cools. It also certainly makes flies easier to see (at least from a human perspective). But, I can cite numerous examples where poor water clarity had absolutely no impact. Here are two. In July, our big rivers experience a hatch coined “Midnight Stones”. These are 1” black stone flies that emerge on the darkest nights. Fish gorge on the nymphs as they crawl out of the river to emerge and the adults as they lay eggs in the river. This is in complete darkness. In daylight, the only evidence of the hatch is the nymphal shucks on the rocks along the edge of the stream. Anglers who fish in the dark do quite well on stone fly imitations. During June and early July when the Salmon Flies hatch, the big rivers are typically still in runoff and anywhere in color from chocolate milk to battleship gray. But that doesn’t keep trout from feeding on adults floating in the river. I have taken trout on dries and nymphs in water the color of chocolate milk shakes—zero clarity.

Species and Size of Trout: This is also a big factor in where you find fish in a stream. In streams with multiple species like we have here in SW Montana you can discern preferences easily. Browns (especially the larger ones) prefer lowlight conditions (either shady side, deep undercuts or depth). Rainbows (even large ones) tolerate much faster and shallower water. Cutthroats prefer the deeper, eddy like pool water. Whitefish (although technically not a trout, but still a salmonid) hang in fast riffles or tail out water almost 100% of time, regardless of conditions.

Fish Density: The “fish per mile” of any given stream will also drive where and when you find fish. Simply put, the more fish, the more different places you will find them. Streams with low fish densities will find most of the fish in optimal locations for the conditions and availability of food. As densities increase, fish will move (generally by size and species) into less than optimal locations as long as they lie within their tolerances. Some estimates claim the Madison River boasts some 3000 catchable trout per mile. With an average width of 270 feet, each of those trout would have approximately 46 square feet of living space. (About the size of a typical walk-in bedroom closet.) Not difficult to find fish under those conditions. By contrast, the Jefferson River near Twin Bridges is a bit less than half the size of the Madison but estimates of “trout per mile” are around 300. Much more difficult to find fish on the Jefferson than the Madison merely based on fish density.

Here are three graphic relationships between stream flow (think depth and clarity) and water temperature. Graphs depicted represent temperature and flow from June 1 – August 15, 2018. Unfortunately, the graphs don’t tell you the weather conditions, so weather influence can’t be accounted for.

Madison River: This is essentially a tailwater with a minimum flow regime after runoff. After runoff the river is very clear and generally not impacted by rainfall. The bottom is a medium color.

Big Hole River: This is a free flowing, undammed stream whose sources do not emanate from high altitude or heavy snow packs. The bottom is very dark, and the water color is clear but a highly stained tea color.

Yellowstone River: This is a free-flowing undammed stream significantly influenced by high altitude water sources and heavy snow packs. The bottom is a medium color. High elevation thunderstorms can muddy tributaries which will eventually affect clarity to some extent in the mainstem.

In each of these cases, when you have spikes in stream flow (think clarity and depth changes) you have effects showing in the temperature regime. Those effects being either a drop-in stream temperature or a decrease in the amplitude of daily temperature changes. Even weather can be somewhat deduced by the amplitude of daily temperature changes. Heavily overcast skies and storm fronts may well be the cause of less daily temperature change even when flows are stable.

Again, generalizations based on a single river are challenging because the impact of the different variables that contribute to good or bad fishing from one stream to another can’t really be compared or evaluated. Is water clarity an important variable in finding and catching fish? Indeed, it is, but to some extent we probably tend to apply too much anthropomorphism to water clarity. Its what the fish can see and feel that is important, not us humans. Temperature on the other hand is a given behavioral driver for all ectotherms.

One thought on “An Alternative to “Water Visibility”

  1. Michael Vorhis

    Very detailed treatment Mike, thanks. You paint a very intertwined picture, and I suspect there are many more inputs yet, into the equation, before it approaches a complete story.

    I suspect that once the number of variables exceeds about two, the “equation” becomes too complicated to use on-stream; there are too many conditions to assess and react to, and then there’s the guesswork of figuring out which condition trumps which, during a given season in a given place on a given day. I believe Joe’s singular discussion of one aspect, that being water visibility (which is really already multiple aspects including light source, light angle, water clarity, water depth and water surface choppiness), was intended to provide a theory simple enough to be actionable on a given day, once the angler is there on the water.

    Similarly my little discussion about light “igniting” insect activity was intended to be simple enough to be easily actionable–I know it has helped me on that one stream.

    That’s not to say that either of those theories is anything like complete.

    Your longer list of many interwoven factors gets us closer to a possibly more complete picture (although again only as complete as we humans can hope to be). That’s the upside; the downside is that a far more complex picture elicits a more complex series of confusions on what to do about it…and many of the resulting decisions are likely to conflict with each other.

    In short, I for one am a limited creature with a limited intellect. I can draw conclusions and make decisions the night before about a variety of different factors, but once I’m on the water I’m likely to be able to juggle only a factor or two in the ol’ noggin. If I try to juggle dozens of factors in my head, I come back around to a shrug and a guess–I come full circle and start to think like I have no theories at all.

    I find your statistical approach very interesting; baseball has gone that way too, in recent years. I do look at a water depth chart, and I keep clarity and bottom color in the back of my head, from memory, for the waters I fish…and I guess I have a very rough idea of fish population densities…but it’s all from guesswork, and kept in memories. The weather I take as it comes, and I do my best to react accordingly–same as I do with water temperature and speed. I have no control over such things so I just get there and then see.

    I do like both approaches–the “bigger picture/prior planning” approach, especially good if you have a lot of different rivers to choose from like you do in Montana…and the “keep a useful rule of thumb in mind” approach, which works for me when I have only one choice of stream.

    Whoever said fly fishing was simple never tried it. : ) Thanks again.

    – Mike

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