It's not often that salmon anglers are faced with high, warm water conditions in the latter part of the season.
I have experienced this twice now in Norway, and this situation is most common on rivers with a lake above their system that has a big enough catchment to collect localized rainfall, and or rivers that are fed via glacial melt water.
This scenario can be quite testing on fisherman when you have not experienced these conditions before, as there is a conflict of interest when it comes to techniques to try and catch salmon on a frequent level.
The first factor that will weigh in your favour will be oxygen supply. This will dictate their behaviour in terms of how active they will be when reacting to your fly choices.
Secondly is flow rate, this generally goes hand in hand with oxygen levels. The more water you get coming down the river, the more oxygen the river will generate especially at the heads of pools. This is a “go too” spot as fish will generally gravitate to the areas that have the most oxygen.
As a rule of thumb, rivers that have less flow often have depleted levels as dissolved oxygen levels decrease due to the heat of the water, in essence it dissolves faster and evaporates due to the nature of the gas.
In rivers with high flow rates, it tends to hold oxygen due to the extreme amount of rough water which saturates the water. This is best when the whole river is considered to be full of rough water in the middle of the main current.
What we have found is that when you have heavy flow rates and high temps due to the conditions described above, we almost go back to June fishing tactics when it comes to fly choices, but not the same line choices. We will fish larger flies than normal for late season fishing, and stay away from heavy lines in sensitive areas (slack water) that you know hold fish.
Fish large flies on the surface and keep them moving, be active in your fishing approach.
I found this works best in the heads of pools on the seams and areas that have lots of slack water next to heavy current. It will also work in fast water, but sometimes the fish will come up to the fly but not take it aggressively. So this method is also good for finding fish to target with a different method.
The next option you have is to fish downstream with a floating line and long leader with a much smaller but heavier fly, in my limited experience with these conditions, I have found some of the more passive fish react better to a small fly, but it has to be quite close to the fish to react. The method is to fish from above the fish, drop in a square cast down to 45 degree angle depending on where and how deep the fish is sitting, and simply dump some slack line by mending in the current to get the desired sink rate and simply hold the line when you think the fly in in the zone. Remember that it's easier to generate depth vertically than horizontal. And don’t forget to scale down in the diameter of your leaders as a thinner diameter will have much less resistance in current.
Please remember that this is just some info on one particular river, it may work on others it may not, but try this next time you are on a river in similar conditions. You might get a surprise!
text & photos : Scoty De Bruin