Reading Trout Water // Secondary Seamlines
I've personally found trout to be much easier to catch if I understand where they are holding and why. Many years ago I was guiding for steelhead in cold weather. I was a young guide with more ambition than knowledge, and my work ethic compensated for my sincere lack of knowledge on how to read river water.
I remember rowing my boat with anglers using nymphs in a big pool and we were all getting a bit frustrated trying to make sense of some of the confused currents that were either boiling, mixing, or floating the wrong direction. Were were trying to fish right in the middle of the pool where the water was confused. It didn't know where to be. Should it back eddy upstream, should it swirl, or should it continue downstream in the general direction of travel? The water was either slack or confused. Finally after several very unsatisfying drifts, I pushed the boat down stream and just as the first strike indicator starting drawing forward into an area of stable and natural flow... boom. Steelhead on. It was a dime bright 10 lb. wild female. Heck of a fish. We landed that fish at the tailout near the bottom of the run. I had mentally marked the spot, so we walked back up the shoreline and decided to fish that spot on foot.
The next angler lays out a cast, but the indicator stalled out and the drift wouldn't float any where. it just hovered in the swirls. I suggested a big downstream mend so that it would begin to draw ahead. The indicator found the current we were looking for.... it just barely begin to move with the natural direction of travel. DUNK. Bobber down. Another fish hold at just the point the slack and swirly water returned to its default flow. This was about an 8 lb. male with a beautiful red stripe.
I have never forgotten that day where I learned that secondary seamlines are incredibly important. While I lucked into this while guiding steelhead, river dwelling trout love this water as well. I've caught many Rainbows, Cutts, and Brown Trout by not overlooking a secondary seamline. While fish will certainly hold in this location all year, If find that fall - early spring is when the biggest fish prefer to hold in these easy lies. Water sub 50 degrees has a fairly high level of dissolved oxygen, so the trout don't need to put their heads in the riffle in order to breathe.
Do Your Strategies Change?
Typically you'll be in deeper water. The general fly selection and strategy will not change, but a big factor in play is that the trout can see you and your cast much better in the secondary seam as there won't be riffles, chop, white water, or rocks to mask your delivery. The head of a pool with all the chaos of fast water is easy. Just cast up, your flie(s) land in the riffle and enter the pool at high speed and as they slow down entering the pool, trout will pick them off. In secondary seams in open flat water, the trout have much greater visibility. I find that planning one long cast (reasonable distance) and trying to cover as much water with one clean drift is a big advantage. I'll even break for a few seconds, take a deep breath, and try to keep the water fairly virgin when fishing over these more open swaths of current. Also, look for ways to fish your drifts in a neutral manner that doesn't require mending. Typically this is a 40-50 degree upstream angle out and across (assuming dry flies or dead drifted nymphs).
Next time you walk up on a pool, look for the transition between slack and slow and just as the current begins to move downstream try focusing on that spot. Good water there!