Stream temperature calculator

[Scudly]

Jay Ford Thurston's new book says water temp must be at least 40 and rising for trout to be active this time of year.

Quick estimator he provides. Take the current temperature it will be when you fish. Add 48 to that number and divide the total by 2. This should equal what the stream temp is.

40 outside + 48 = 88 divide by 2 = 44

30 outside + 48 = 78 = 39, too cold for fish to be active.

Thurston also says he always carries a thermometer with him. he ties to a string and it dangles in the water while fishing. If the temp drops by even one degree he goes home.

Important factor this time of year. I should NOT have fished yesterday am. Should have fished yesterday afternoon. Only caught two dinky brook trout. Fish were inactive for the most part.

His new book gives the 100 best streams in WI based on priority. His book also gives the top twelve streams for pre-season catch and release where to catch the big boys.

[Jerry]

What about the groundwater temp factor in spring creeks?

[waterbound]

That's exactly what I was thinking Jerry. Some rivers I have fished are downright cold year-round and others warm up more as the day and season progresses. But I think it's just a rule of thumb.

[goblueM]

sorry, but that's total bunk

spring fed streams in the summer, ANY stream right now with varying precip/snowmelt... it just doesn't work out. maybe in a few perfect situations it could work but there are far too many variables and far too much variation in daily temps compared to water temps

it just doesn't add up

YES in colder water you should fish when it's warmer in the day in general, but i think this methodology is extremely flawed

[Watershed]

The more I fish, the more I find "rules of thumb" set by fishermen to be false. Active fish can be found throughout the year. Pay attention to the fish and make the adjustment on the water. If I allowed myself to fish only when conditions were perfect, I would probably never get out to fish.

[Watershed]

Could not agree more gobluem.

[itchmesir]

where does this random number of 48 come from?

[grate8]

yea the temp droping by one degree means you have to be smarter, not go home. ha

[Lucky Dogg]

48 it the temp of underground caves. it's also the temp spring water comes out of the ground at.

[goblueM]

absolutely not. caves do not always have constant temperatures, nor are they all at 48 degrees.

also, springs do not have 48 degree water constantly. i can think of about 3 exceptions to that right off the top of my head, not counting ocean vents, which can be extremely hot spring water

spring water temperature relies on the depth and size of the water source, (since rocks generally get warmer as depth increases) and various other heat imputs, such as other ground water, surface water, etc

It may be that in certain small local areas in MN/WI, the spring water is 48 degrees when it exits the ground. But you definitely can't apply that to the entire region and certainly not on a larger geographic scale

[Lucky Dogg]

the cave system that starts at the mouth of the root river is ALWAYS 48. just like the spring that feeds into the trout hatchery. call the dnr and let them know that you know more than they do.

[Scudly]

I have to agree with Lucky Dogg. Spring fed trout streams are always a constant temperature. Thurston explains "cold water springs , wiht a year round temperature of 48 degrees, emerge in the region after passing through limestone bedrock. The guy always carries a thermometer with him and he believes his formula estimator works pretty good. I tend to agree. However I believe it is probably more accurate in the pre-season than in the the heat of the summer.

Water has its greatest density at 39 degrees. Water in our lakes turns over when water on the top drops to 39 degrees. The heavier water settles to the bottom. Trout seldom hit when water temperature is below 40 degrees. They lie dormant in cold water.

Furthermore Thurston describes three key temperatures:

40 degree rise which we have discussed. You want to be out on the water pre-season when it is 40 and rising. Following this formula, don't bother fishing when it is below 34 degrees outside (or falling).

45 degree rise. Water will rise to 44 degres and no higher until the ince and snow has melted along the stream bank. The air temperature can rise but the stream will not because there are still a number of ice shelves dripping to keep the water cool. 45 degrees tells the trout spring is here, the show is ALL gone!

49 degree rise. Water comes out of the ground at 48, plus or minus two degrees. 48 degrees is normal for trout. Once trout stream temp rises to 49 and higher this is yet another trigger for feeding.

Take it or leave it. Makes sense to me when we are talking about limestone spring fed creeks.

[Watershed]

I will have to leave it. To follow such a strict list of rules only limits what you can accomplish on the water. Over the past several years, the winter season has easily become my favorite time to trout fish our driftless area streams. Many of my fish in the winter season have come on single digit air temp days with water temps just barely topping 33 degrees. For someone to tell me that fish do not or will not eat in this cold of a water temp have just not put the time in to figure them out. A fishes feeding trigger is not only dependent on water temps but also fly or bait presentation.

I do think that water temp can be used as a guide for what techniques to use and what areas of a stream should be fished.

[goblueM]

Scudly, your quote "spring fed streams are always at a constant temperature" contradicts the formula. IF they are at a constant temperature how does the formula work by taking the air temperature? It's completely fallacious.

I think a blanket formula is not very useful. The high specific heat of water makes it that way. Essentially this formula is adding on 5 degrees of water temperature for every 10 degrees that it rises in air temperature. Horribly horribly wrong. It underestimates the stream temp in winter, overestimates it in summer, and is innacurate at best during the spring and fall.

Think about it... if it is 16 degrees or less, according to the formula the water should be frozen. But the streams still run in winter and they are above freezing

Conversely, if it is 95 outside, the water would be 71.5. But this just doesn't happen if you are fishing in a spring fed stream.

Any way you spin it, the formula just doesn't hold water.

Spring fed water might be at 48 degrees in several locations. I'm not disputing that. I'm just saying that applying a blanket statement to a broad geographic reason just doesn't work. For example, spring water in the Ozarks in Mizzou is considerably warmer than 48 degrees. Hot springs in Wyoming are near boiling. It's geology - rocks get warmer the farther below ground you go. If water that has slowly trickled down (warming as it goes), then finds a fissure and travels up to the surface faster than it seeped down, then it comes out warmer.

[Scudly]

Quote:

Conversely, if it is 95 outside, the water would be 71.5. But this just doesn't happen if you are fishing in a spring fed stream.

I have seen the streams get up to dang near 70 when it is 95 out. Rush River being one example.

Personally I will never trout fish again in outside air temps below 25. It is unpleasant, the fish generally inactive, and I'd rather save my vacation time fishing when it is nice out.

To each their own. Anyway I don't follow Thurston's words of wisdom to a T. It is just something I have read and I'm not telling you it is the truth, nor do I firmly agree with him at this point because I have not studied water temp myself. I just fish when it feels right to me.

[goblueM]

fair enough! tight lines