Firstly
The stretch just upstream of the rush you can see at the right is a fairly fast flowing rocky part of river which is home to the stone clinging nymphs of the march brown.
This part above also has a lot of ranunculus weeds which are home to the LDO nymphs. When the flies hatch they take between 30secs and 60 secs approx to struggle free of their shucks and take to the air. This gives them just enough time to go through the rush and many end up cripple or drowned in the lovely deeper run you can see in the picture.
The water in the picture is a nice deep run which has slower water to either side, giving the fish two places to lie (the strip on either side between fast and slow)and a good flow of water
In which they can intercept at speed. The water is not so fast or rough that your flies shoot through with loads of drag, but a short line is still advantageous
.
If you sit and wait, there will be a rise almost as soon as the first fly floats past, and suddenly this wee stretch of water that looked empty will suddenly come alive with fish.
Other that that to me it just looks
right Thats why i took the photo, it seemed like a better idea than trying to describe it
Sandy