Taking the temp of the water in a mountain stream

City Rat

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Ok still wearing my newbie cap I thought that I'd throw this question out there. I have been told in both fly fishing classes as well as read in several books that trout really become active in mountain streams where I will be fishing once the temperature of the water hits 40 degrees for 4 consecutive days. So trying not to over think this but since temps fluctuate by a wide margin in the spring, sometimes 20+ degree swings in a single day, does it matter if the water temp hits 40 degrees for only about an hour a day , i.e. that's the high temp for the day and all of the rest of the day its in the 20's and 30's or is this a thing like it needs to be 40 or above consistently for 4 days? Again I hope that I'm not over thinking this. Any insight here would be great. Thanks.
 

plecain

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Interesting question.
I've only really thought about what the temperature is 'now'. Fish take on the temperature of the surrounding water very quickly.
If it's a situation where the fish move a good distance based on the water temp, then I'd guess the temperature over time would matter more.
 

triggw

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Trout become active when their food--the bugs--becomes active. That's not when the water hits any particular temperature, rather it's when a certain (for each species of insect) number of degree days have accumulated. So it's actually a somewhat broader measure than just hitting 40 degrees for 4 consecutive days. It depends on what the preceding days have been like.


On the streams you fish, hitting 40 degrees for 4 consecutive days may be a reasonable way to guess that things are heating up, but I've seen fish rising heavily in water that was frozen at the edges. So it really depends on the stream and the bugs. You just have to get out there and keep checking:)
 

cooutlaw

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Here's two quick reads that may help expand understanding albeit requiring piecing together semi opposing takes:

Trout and Water Temperature: How Hot is Too Hot? | Hatch Magazine - Fly Fishing, etc.

Trout & Temperature – Tenkara Talk


As other's have stated, the ambient and water temp fluctuations may elude to increased or decreased fish and insect activity, but is not the end all, be all, to base your fishing parameters around.

Further, water temp has an effect on oxygen content as well, and as ethical sportsmen, we have a responsibility to not overstress trout when their health and potential propagation is at risk, I think many overlook this aspect, of not fishing when trout are already struggling to make it through warmer water conditions during peak ambient temperature periods.
 

City Rat

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Here's two quick reads that may help expand understanding albeit requiring piecing together semi opposing takes:

Trout and Water Temperature: How Hot is Too Hot? | Hatch Magazine - Fly Fishing, etc.

Trout & Temperature – Tenkara Talk


As other's have stated, the ambient and water temp fluctuations may elude to increased or decreased fish and insect activity, but is not the end all, be all, to base your fishing parameters around.

Further, water temp has an effect on oxygen content as well, and as ethical sportsmen, we have a responsibility to not overstress trout when their health and potential propagation is at risk, I think many overlook this aspect, of not fishing when trout are already struggling to make it through warmer water conditions during peak ambient temperature periods.[/QUOTE]

Cooutlaw,
Very interesting, Thanks for the links. Now help me understand what you mean by the italicized bit above.
 

silver creek

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Ok still wearing my newbie cap I thought that I'd throw this question out there. I have been told in both fly fishing classes as well as read in several books that trout really become active in mountain streams where I will be fishing once the temperature of the water hits 40 degrees for 4 consecutive days. So trying not to over think this but since temps fluctuate by a wide margin in the spring, sometimes 20+ degree swings in a single day, does it matter if the water temp hits 40 degrees for only about an hour a day , i.e. that's the high temp for the day and all of the rest of the day its in the 20's and 30's or is this a thing like it needs to be 40 or above consistently for 4 days? Again I hope that I'm not over thinking this. Any insight here would be great. Thanks.
I'm a bit confused by your post. As a newbie, I think you might have some temps mixed up.

First of all, how can the temperature of water in LIQUID form in the 20's since water in a stream is fresh water freezes at 32 degrees or below? Salt water or water under high pressure can get below 32 degrees and still be a liquid but this is not in the situation you mention.

Secondly, trout are lethargic at low temps and at 40 degrees, I would think they would be very lethargic. I don't understand how books could say that trout become "active" when temps hit 40 degrees.

The only situations where I have seen trout to be "active" is when the water temps are CONSTANTLY low. For example in bottom draw tailwaters like the San Juan where the water coming out Navaho Dam varies from 39 - 46 degrees. On the San Juan the trout just below the dam have adapted to these low temps and will feed in 40 degree water.

https://www.sciencebase.gov/catalog/item/4f4e488ce4b07f02db51cbfb

Are you sure the temperature was not 50 degrees instead of 40 degrees for these mountain streams. The 50 degree temp would conform to what is usually taught. See:



http://flyfishing-and-flytying.co.uk/articles/view/degrees_of_success/

Using Water Temperature in Locating Fish | Encyclopedia.com

Optimum Water Temperatures for Fly Fishing - dummies

Why Water Temperature Plays a Role in Your Fishing Success

http://flyfishing-and-flytying.co.uk/articles/view/degrees_of_success/

I am willing to learn when I am wrong so if you have references to which books say trout “really become active” at 40 degrees in these mountain stream, I would appreciate it.
 

City Rat

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Super interesting, Yes I am sure that it was 40 degrees for 4 consecutive days , I sat in the class last Saturday and we were all told this very very clearly. Now Once I get home I will find the reference in the book and let you know. I will note the class is very localized to fly fishing for wild trout in the local mountain streams near here so there is a chance that this is a local species adaptation.
 

cooutlaw

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Here's two quick reads that may help expand understanding albeit requiring piecing together semi opposing takes:

Trout and Water Temperature: How Hot is Too Hot? | Hatch Magazine - Fly Fishing, etc.

Trout & Temperature – Tenkara Talk


As other's have stated, the ambient and water temp fluctuations may elude to increased or decreased fish and insect activity, but is not the end all, be all, to base your fishing parameters around.

Further, water temp has an effect on oxygen content as well, and as ethical sportsmen, we have a responsibility to not overstress trout when their health and potential propagation is at risk, I think many overlook this aspect, of not fishing when trout are already struggling to make it through warmer water conditions during peak ambient temperature periods.[/QUOTE]

Cooutlaw,
Very interesting, Thanks for the links. Now help me understand what you mean by the italicized bit above.
Well, if you read the first link, you will note that trout suffer to maintain sufficient oxygen once temperatures rise above their natural tolerance, this affects not only their ability to grow, gather food, maintain health in general, which could ultimately lead to death and not being able to spawn or perpetuate the species. Anglers that fish trout in extremely warm water at the peak of summer temps are putting them in greater jeopardy of perishing after they extend the energy and oxygen during a catch, which ultimately lowers their survival rate after release. Hence the reason trout in lakes are at deeper (cooler) depths during the height of heated months. If the water temp is at or nearing their peak tolerance level, the ethical thing to do is call it a day and go home...let them have a better shot at living for another day.
 

rangerrich99

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I've never heard the "40 degrees/4 days" thing, so I can't really comment on that. Other than to say that ambient air temp and water temp are two completely different things. Air temp may fluctuate by 20 or more degrees during the day, but water takes a lot longer to change temp significantly. So while the low air temp might be 35 and the high ends up being 75 over the course of a hypothetical day, the water temp won't change much over the course of that same day, so it was 45 at dawn, it's likely to be within a few degrees of that by nightfall.

What I have heard and have found to be fairly accurate, is that several days of similar weather (pretty sure I read three days) will lend itself to better fishing. In other words, when I look at weather, I want to see at least three days of roughly the same temps, weather conditions, etc. before the day I think I'm going fishing. Most of the time the fishing seems more predictable on that fourth day.

For instance, about three years ago I was fishing in WY (I know, there's no fish there), and every day was the same. Sunny and relatively warm in the morning, light breezes. decent but not strong feeding activity.

About 3pm everyday there was a thunderstorm that rolled through, dropping temps by as much as 15 degrees, high winds, and of course heavy cloud cover. After 3 days the fish seemed to know the storm was coming. They began to feed more heavily as the clouds rolled in and shut off about an hour after. Feeding again picked up about two hours or so after the daily storm rolled off to the north. Air temps were extremely consistent except for one day when the storms rolled in really early and stayed all day. Fishing was sporadic all day and into the following morning until after noon.
 

silver creek

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I've never heard the "40 degrees/4 days" thing, so I can't really comment on that. Other than to say that ambient air temp and water temp are two completely different things. Air temp may fluctuate by 20 or more degrees during the day, but water takes a lot longer to change temp significantly. So while the low air temp might be 35 and the high ends up being 75 over the course of a hypothetical day, the water temp won't change much over the course of that same day, so it was 45 at dawn, it's likely to be within a few degrees of that by nightfall.

What I have heard and have found to be fairly accurate, is that several days of similar weather (pretty sure I read three days) will lend itself to better fishing. In other words, when I look at weather, I want to see at least three days of roughly the same temps, weather conditions, etc. before the day I think I'm going fishing. Most of the time the fishing seems more predictable on that fourth day.
Rangerrich,

Cityrat's OP below is that water temp is 40 degrees for 4 days. No where in his post does he state air temp and that is one of the parts I found confusing in his post. The only temp he specifically mentions is WATER TEMP. Although he says the temps vary by 20 degrees, at no time is it stated that this is air temp rather than water temp. The assumption grammatically must be that all temps are water temps.


Ok still wearing my newbie cap I thought that I'd throw this question out there. I have been told in both fly fishing classes as well as read in several books that trout really become active in mountain streams where I will be fishing once the temperature of the water hits 40 degrees for 4 consecutive days. So trying not to over think this but since temps fluctuate by a wide margin in the spring, sometimes 20+ degree swings in a single day, does it matter if the water temp hits 40 degrees for only about an hour a day , i.e. that's the high temp for the day and all of the rest of the day its in the 20's and 30's or is this a thing like it needs to be 40 or above consistently for 4 days? Again I hope that I'm not over thinking this. Any insight here would be great. Thanks.
 

rangerrich99

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I saw that SC. However, he subsequently says that water temps can fluctuate by as much as 20+ degrees in a single day, which seems unlikely. I was attempting to get him to be more precise, if possible.

Regardless, stable water temps/weather conditions seem to contribute to more consistent fish behavior, IME. The "40/40" thing I can't speak to.
 

City Rat

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I've never heard the "40 degrees/4 days" thing, so I can't really comment on that. Other than to say that ambient air temp and water temp are two completely different things. Air temp may fluctuate by 20 or more degrees during the day, but water takes a lot longer to change temp significantly. So while the low air temp might be 35 and the high ends up being 75 over the course of a hypothetical day, the water temp won't change much over the course of that same day, so it was 45 at dawn, it's likely to be within a few degrees of that by nightfall.

What I have heard and have found to be fairly accurate, is that several days of similar weather (pretty sure I read three days) will lend itself to better fishing. In other words, when I look at weather, I want to see at least three days of roughly the same temps, weather conditions, etc. before the day I think I'm going fishing. Most of the time the fishing seems more predictable on that fourth day.

For instance, about three years ago I was fishing in WY (I know, there's no fish there), and every day was the same. Sunny and relatively warm in the morning, light breezes. decent but not strong feeding activity.

About 3pm everyday there was a thunderstorm that rolled through, dropping temps by as much as 15 degrees, high winds, and of course heavy cloud cover. After 3 days the fish seemed to know the storm was coming. They began to feed more heavily as the clouds rolled in and shut off about an hour after. Feeding again picked up about two hours or so after the daily storm rolled off to the north. Air temps were extremely consistent except for one day when the storms rolled in really early and stayed all day. Fishing was sporadic all day and into the following morning until after noon.
Honestly I think that the 40 degrees 4 day thing maybe the local shorthand for what you are describing, i.e., 4 days of consistent weather and water temps. Thanks for the explanation.
 

Ard

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If I may ask, what class was this?

And...………

Having a stream thermometer isn't a bad thing, I have one around here somewhere that I bought over 40 years ago. I did use it a few times but essentially (in my case) taking water temps is more of a curiosity thing, one of those FYI type things to know.

The fact that you are interested in such things speaks well for your future as a fisherman. It is curiosity and the want to be able to make relationships in your mind with memories that helps one learn their way in the outdoors. If you do decide to carry a thermometer and do take water temps then by all means keep notes because such things aren't of much value unless you compile records enough to draw some conclusions from what you found. When I did those things I kept note of location - air temp - water temp - general weather conditions and insects encountered. The number of fish caught or at least moved to the fly is also good to record. By doing that you can develop your own book to go by within a few short years.
 

Lonnie Utah

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When most of us talk about the water temperature of a stream, lake, river, or other body of water we think of it as a single uniform number. But nothing could be farther from the truth. This is a pretty complicated topic due to the physical characteristic of water. Most bodies of water have a range of micro-climates that are influenced by any number of variable such as depth, current, velocity, inputs of other water bodies (both surface and subsurface), etc. As most of you know, the density of water changes with it's temperature. It reaches it's maximum density at 4 Deg C or just over 39 deg F. Additionally, unlike most substances, the total amount of dissolved oxygen in water is inversely proportional to water temperature. That is, cold water can hold more dissolved oxygen than warm water.

Fish are poikilotherms (organism that cannot regulate its body temperature) so they have to depend on the environment to put themselves in a range of water temperatures that maximize their metabolism. For most commonly fished for salmonids, this is in the mid to high 50's. (I THINK I remember reading a paper in grad school that found that Rainbow trout reach their maximum metabolic rate at around 55 deg F, but that was 25 years ago, so don't quote me on that one but it should be in the ball park.) Regardless of the actual number, below those temps, their basal metabolism rate slows and they do not require as much food. This means they are less inclined to eat our flies. Above those temps, the systems in their bodies (respiration and heart rate for example) are in overdrive and they become stressed. So during the colder seasons (late fall, winter, late winter and early spring), fish are always seeking the warmest water they can find. That's why you commonly find wintertime fish in the deepest slowest pools, because the warmest water (around 40 deg C) is denser and sinks to the bottom of the pools. Conversely in the warmer months, they are looking for cooler and more oxygenated water. That could be a deep pool, or pocket water with lots of oxygenation or any other combination where the fish find "optimal living conditions" (This could be the point where I break out the explanation of an ecological niche as a n-dimensional hypervolume, but I think ya'll would kill me. lol.)

So I think the answer to the question is, at warmer temps, the fishes metabolism is simply starting to ramp up from what it was over the winter. This means they have to eat more to keep up with their increased metabolism. Weather it's 2 day, or 3 or 4 or 10, that jumps starts them to eat more, it's hard to say. The point is, the fish are ALWAYS trying to find the spot in the stream where they are the most comfortable, and can maximize their food intake with a minimum of physical effort.

That being said, a thermometer is one of the most useful tools a fisherman can carry with them. Joe Humphreys in his book "Trout Tactics" goes into great detail on how and when to find fish using a thermometer. Many of the systems he speaks of are the small mountain streams your are asking about. While it is a bit dates, it's an excellent read for a beginning fisherman. Edit: But as Ard says above, after many years of fishing and learning your local area, you really don't NEED it any more. You learn the patterns and habits of your local fisheries and it becomes more of a guideline than a rule...
 

silver creek

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I also think a thermometer is a good thing especially for a novice fly fisher.

To be truthful, with over 30 years of fly fishing, I still carry the thermometer but rarely use it. The reality for me is that after taking stream temps and then correlating it with what I feel through my waders and by dipping my hand in the water, I can now get very close to the actual temperature without using the thermometer.

To do this, all you have to do is to be in touch with your surroundings. I have discovered where springs enter the stream I fish, just by noticing the water got a bit colder.

I would be interested if anyone else has found this to be the case.
 

Lonnie Utah

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To do this, all you have to do is to be in touch with your surroundings. I have discovered where springs enter the stream I fish, just by noticing the water got a bit colder.

I would be interested if anyone else has found this to be the case.
Yes.

When I lived back east, I could tell you where the groundwater inputs were on the streams I fished regularly. Of course, I mostly wet waded back then and I could tell the difference in water temps almost instantly.

One thing I forgot to mention in my post, that I was reminded of as I typed the above, is that early, mid and late season I would change where I fished in the watershed in search of the "magic" water temps. Early season, I would almost always fish down low in the watershed were I was more likely to find warmer water temps. In the heat of the summer, the converse was true. I would find the smallest headwater streams where I could still make a cast. Those often contained the coldest, clearest water as they were mostly spring fed. Again, exact water temps didn't really mater, as you're looking for the optimal water temps that you can find that are close to you.

I'm fortunate now to live close to several large tail-waters that moderate the water temps all year long.
 

City Rat

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Guys,
Thanks very much and sorry for any confusion earlier. As was noted I was always referring to water temps regarding 40 degrees for 4 days, yes I did grab a water thermometer, however my reference to wide temperature swings, which happen to air temps but not necessarily to water temps, may well have confused the thread.
 

City Rat

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Ok still wearing my newbie cap I thought that I'd throw this question out there. I have been told in both fly fishing classes as well as read in several books that trout really become active in mountain streams where I will be fishing once the temperature of the water hits 40 degrees for 4 consecutive days. So trying not to over think this but since temps fluctuate by a wide margin in the spring, sometimes 20+ degree swings in a single day, does it matter if the water temp hits 40 degrees for only about an hour a day , i.e. that's the high temp for the day and all of the rest of the day its in the 20's and 30's or is this a thing like it needs to be 40 or above consistently for 4 days? Again I hope that I'm not over thinking this. Any insight here would be great. Thanks.
Ok, I have to admit to unintentionally leaving a key bit out of the original post, i.e. "...that trout really become active in mountain streams where I will be fishing once the temperature of the water hits 40 degrees for 4 consecutive days..." After digging into my class materials further the 40 degrees for 4 days actually refers to when the mayflies will usually begin hatching here and THEREFORE that is why the trout really become active... Yeah I know a key detail. At any rate I also found a blog post from the guy who wrote the book and gave the class on the same point: Mayfly Nymph Blog by Harry Murray at Murray's Fly Shop
 

irideaduck

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I don't own a thermometer. I was out last spring with one of the members of the US fly fishing team. The first thing that morning we did was measure the water temperature. With the temperature being on the colder side he said the fish are most likely going to be in slower/deeper water and fishing would be slow. We caught a few fish and broke for an early lunch. Going from memory, temperatures by noon has risen some 8 deg F. We were still fishing the slower water. A few hours later the temperature was up some 12-14 deg F (mid to high 50's) from the morning and we were catching the fish in the faster water.

The point of my post was that we should expect to find fish in different areas of the river based upon the different water temperatures.
 
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