When a tree falls into a river, the attached root system creates two prime positions where browns like to sit: on the pillow in front and in the trough behind. No matter where you are in the world, water and gravity act in the same way. A root mass in the river will divert a fast stream of water around the front corner of the roots, creating a scoured-out trough behind the deflection. Similarly, when water runs head-on into a root mass, it “bounces” off the front of the structure before finding an alternate way around. This small window of water at the head of the root ball creates a pillow where fish can surf relatively effortlessly in search of food.
When fishing for browns, I approach from downstream. I find it’s very difficult to approach from upstream on smaller waters without being detected. When I see a classic root ball I stop well below and watch the water for a minute or two. I look for rises or sub-surface disturbances, flashes of sunlight reflecting off the flanks of nymphing fish, or shapes moving on the bottom. Being able to sight a fish before casting dramatically improves the probability of getting a take.
If I don’t see a fish, I’ll make a cast with a long leader to put the fly directly in the seam as close to the back of the deflection point as I can. This puts the fly at the head of the seam, usually without any fly line covering the back end of the trough. If you cast ahead of the deflection point, you run the risk of inducing a downstream take from a fish sitting on the pillow, and you don’t want that.
If the first cast results in nothing, I’ll make one or two more before heading up beside the trough to look at the pillow. To properly see into (and fish) the pillow, you need to be almost perpendicular to the root ball. If you’re fishing from the outside bank, take a few steps back from the edge or you’ll end up towering over the fish as you get around the root mass, in prime position to watch the prize of the day swim away from you, spooked. Once you can see the water above the root mass, repeat the minute or two of observation before casting. If you can’t see a fish, cast 4-5 feet upstream of the root mass with an upstream mend or reach cast. It’s important to keep the belly of your line out of the faster water, which is being pulled down and around the root mass, because this will drag your fly unnaturally past the fish’s nose.