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Water temperature: diurnal range

Anthony Pearson

Anthony Pearson

Senior Member
I don't carry a thermometer but was wondering if anyone out there has mapped the temperature of a small river to see how much it drops on a clear night. I was on the Nidd yesterday and the bites stopped coming not too long after darkness. It was a cold night (by comparison with the daytime temperature) and I suspect that this is what put the fish off there feed.
 
I compared on the Nidd a couple of years ago and it didn't change but I left midnight or so. I'm sure you've noticed that on slightly chilly nights the water suddenly feels relatively warm to what it did in daylight.

On a much grander scale we get the Liverpool Bay sea temperature twice a day at work, it usually reaches it's annual peak in September and minimum around March. Huge time lapse between the warmest and coldest months for air temperature.
 
Not a small river Anthony but here is a link to the EA website that monitors the Hampshire Avon river temperature constantly and shows the overnight temperature drops. Also available on an excel spreadsheet if you click on the link which takes a reading every 15 minutes.
Updated daily at 9am.

http://www.knappmill.co.uk/id2.html


Regards John
 
Thanks for that John. There are some interesting drops that stand out. It would be interesting to hear from those who fish this river to see if it affects their catch-rate.
 
Surely it depends how deep the water is ? I would imagine that a 3 ft deep swim would cool quicker than a 6ft deep swim once the sun had gone off it after a warm summers day . Wind plays a factor as well . Lots of variables
 
I agree Mike, so a river with riffles and pools should generally lose more heat than one with a steady flow and regular depth etc. I don't know what part the thermocline plays in all of this either. We are however led to believe that a rise in temperature can induce feeding (especially in the colder months) but my query was whether a greater than normal drop in the daily water temperature had an adverse affect?
 
Broadly speaking the fishes behaviour / activity as a cold blooded creature will be affected by sudden abnormal temp drop or even a sudden abnormal rise in temperature but otters swimming by , state of the moon , predators in the swim , wind , dissolved oxygen level [ doesn't that drop at night , could be wrong ] will also have an effect . Anyway barbel go to bed after 1am . There was a thread on here some time ago about barbel feeding tailing off during the small hours
 
Thermoclines (or the effects of a thermocline) don't exist in rivers Anthony, not even in the very deep rivers abroad. They exist in the oceans and lakes only.

Mike, disolved oxgen levels in water depend on a number of factors, but the day/night differential you are talking about is that caused by plant photosynthesis. During the sunlight hours oxygen is produced by plants as a byproduct of converting CO2, sunlite etc. into glucose. During the dark hours, the process kind of reverses itself and oxygen is used by plants, and CO2 produced. Roughly speaking. Hence the differential.

Chears, Dave.
 
David, it was working until then! The wrong phrase but are you saying that the temperature of water in a particular river is the same throughout, no matter what the depth?
 
Iam sure Dave can explain but I think he is talking about the thermocline which doesn't apply in rivers , however I don't think he is saying it's the same temp whatever the depth in a river . If you have a copy of Dick Walkers still water angling there is a diagram in there that explains the principle of the thermocline .
 
i would have thought the tempreture at different depths in a river are pretty much the same as the water is moving and mixing all the time unlike stillwater
 
Once you start reading into this, you come across interesting information:

It can be assumed that the water flow in natural rivers of small depth is under isothermal regime (with equal temperature) because of the intensive turbulent mixing of water (Penchev, 1972). The only exception is the narrow strips along the shores, where water is warmer. For this reason it has been accepted that the measurement points should be at a distance of minimum 1-2 m from the shores and at a depth of 30-50 cm from the water surface.

VICAIRE - Module 2 - Chapter 5

Next question: do the fish move close to the river banks to continue feeding when the temperature is dropping in the main flow?
 
I catch most of my barbel within a rod's length of the bank after dark - might be something in it.
 
Sorry Anthony, been busy with the ABF charity thing. If you google it, you will find that the thermocline is actually a thermal barrier that forms between the different layers in fluids, including air. In oceans or lakes, if forms a barrier between the rapidly changing warmer, upper layers and the cooler, more stable water below . As such, it doesn't occur in rivers.

Cheers
 
My mistake David as I indicated earlier. I wasn't sure if there was any 'shearing' in river flow that left the deeper water undisturbed (apparently not).
 
Interesting about the warmer water at the sides though, isn't it Anthony? Definitely food for thought there, come the winter.

Talking of which...have you shrunk my coat yet?

Cheers Dave.
 
For the price I paid it was a bargain. It's almost coat length so will cover that small-of-the -back cold spot that usually happens when the shorter ones ride up. It'll never shrink down to a medium though! ;)
 
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