Salmon behaviour in relation to water temperature and river level

Do salmon react the most to water temperature or river level!

For us salmon fishers this is an important question in terms of choice of equipment and fishing strategy! My theories for this article are obviously not scientifically proven, but based on my own observations from many rivers in the Trøndelag region on which I have spent thousands of hours throughout more than 30 seasons along the rivers.

There are many different aspects of salmon fishing that I find fascinating, and which is the reason as to why I keep going full steam ahead with salmon fishing, just like I did 30 years ago. It is very stimulating to continuously learn more about salmon behaviour through your own observations, this combined with casting practise and the development of fishing equipment means that we always develop as salmon fishers. Fly fishing for salmon is a very exciting hobby, and I am fascinated by all the things we still don’t know about the life of salmon, this allows for your own studies which in time might turn into fishing theories. The reason as to why I am so interested in this question of salmon behaviour and water temperature and river levels is that I always want to grow and evolve as a salmon fisherman, the goal is not to fill the freezer with salmon, but to gain as many experiences as possible that will make me a better salmon fisherman through increasing my fishing experience. The total experience is what matters; from observing river conditions and choosing a fishing strategy and equipment, to actually fooling the salmon into taking your fly. The more difficult the conditions, the bigger the challenge, and the bigger the joy when the take finally happens!


Make time for observations.

The big question:
What affect salmon behaviour the most, the river level or the water temperature? Many fishermen will reply as follows: 1) river level, and 2) water temperature, and to a certain extent I agree with this. However, is it possible that what will affect salmon will vary from one time to another? I see this as a very interesting question as I have experienced lots of different combinations in terms water temperature in relation to river levels.

When the fishing season starts the river is often high and cold, for the fly fisher this means sinking lines and large tube flies, as the fly line will have to penetrate the surface current so that the fly will get down to where the salmon are. Today this is an obvious fishing strategy for most fishermen during such conditions. But what if the river is low and cold?

June: To be fair, it has to be said that in June the river will most likely not become low enough to be compared to flowing at summer river flows, but it can get pretty low for being June if the weather is cold and the snow melting stops. When the river is lower the surface current will not be as swift, creating a more even water temperature from the surface to the river bottom (see the article “Fishing with a fly rod during spring flood” in the menu “Fishing”). When the river is lower we do not need the heaviest sinking lines as the surface flow is slower now, but in terms of the water temperature the salmon will still lie deep and move in a concentrated fashion upriver until it meets some sort of an obstacle where it will rest. In the places where the salmon stops I will use heavier sinking lines than I would when fishing for running salmon. I am also mindful of the fly choice in terms of if the fish is resting or swimming. In other words, I will still use large tube flies, as large as the ones I would use on a much higher June river. If the water level is lower the salmon will swim a bit faster upriver, so it is wise to increase the length of time where the fly and the salmon meet, this is best done by fishing a large tube fly, remember to tie flies using lots of flash for this. Fly fishing is always a big challenge in June regardless of river level, just don’t be fooled by relatively low river conditions compared to “normal” June water levels in terms of fly choice.

July: Can of course be variable in terms of both these factors too, but I experience that the water temperature is more stable compared to the air temperature. If we have heavy rain decreasing the air temperature and increasing the river level (creating a summer flood), the water temperature will still remain relatively high as the rainwater creates higher temperatures than snow melt. The summer is generally more stable during July, thus this is often the month with the most stable water temperature.



Yes, the fishing strategy was spot on once again!!

August: Several times during the autumn month of August I have experienced that the river can get both low and cold, this is the result of a lack of rain in the last part of July, so if the weather becomes cold without rain we will suddenly have low and cold river conditions. I have measured temperatures as low as 10 °C at river levels defined as low. This is actually very cold for this river level. Should we then choose fishing strategy based on river level or the water temperature?

In periods of summer heat the river flowing at the same level be as much as 8-10 °C warmer. Then the insect life will also be much more prolific, and the average water temperature will be more similar during day and night, this makes for better fishing conditions than when the river is low and cold. When the river is low and cold the insect life will be limited, in addition to the water temperature varying a lot between day and night. This can also be taken advantage of in general you know which pools are best suited for different light conditions considering that when the river is low, the water is crystal clear and the salmon spooky. This means that it might be wise to fish during the night in a pool rather than fish during the day in the same pool when the water is warmer. The factors that makes the difference are the character of the pool in terms of depth and current speed, and also the pool’s function, i.e. is it obstructing the salmon running further upstream when the river is low, or is it part of a spawning area? All of this will affect the behaviour of the salmon in terms of temperature variations at the same river level. If you fish more shallow and swifter areas these are often the best day fishing spots. The deeper and slower pools are often better to fish throughout dusk and during the night than during the day. This is a fishing strategy I also follow on low and warm rivers.

During August most salmon will seek to its childhood pools (spawning grounds), this combined with the fact that many areas will act as obstacles for salmon runs when the river is low, this will in a low and cold river lead to the salmon lying deep along the river bottom. If you then have chosen a floating line you might have to wait a while between takes. We often see salmon jumping during such conditions, this is why it is so easy to be tempted into fishing a floating line, but the salmon is not resting or taking flies just below the surface. This jumping is nothing else than the salmon marking its territory in terms of its spawning instinct. This has taught me that the fishing strategy must be chosen according to the water temperature in this situation. This means that we must fish deeply. And the best thing is if the fly surprises the salmon, this is possible by fishing deep and fast, then the attack will come! Another experience I have is that salmon are not likely to take a fly when it drifts towards it too slowly and too high up in the water.

The salmon must not have too much time in which to consider the fly, realising that it poses no threat and thus choosing not to attack it. I have also noticed that it will take larger flies more easily than smaller flies, this is similar to what happens in June. Thus it is obvious that water temperature affects the salmon regardless of river levels. I have focused on these conditions for several years, but it still feels odd to fish flies with wing length of up to 6-10 cm on low rivers during the day. This should be done using a lighter rod as the river is low and the salmon is very spooky, so the line weight you use will have a lot to say for your success. Otherwise you might scare the salmon instead of irritating it, there is a big difference between a scared and an irritated salmon, one will bite and the other will not! One more tips when it comes to flies for low and cold rivers in August; avoid too much flash in your flies, this will have the opposite effect, now you should not increase the amount of time where the salmon and the fly meet, but actually shorten this moment. A large and camouflaged fly in earthy colours fishing at the correct speed and depth has given me many great fights during these conditions.

Always check the water temperature regardless of the river level!

Jan Erik


jørem vald namsen

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