Studies have shown that between 80% - 90% of a trout's diet consists food consumed sub-surface. Translation... nymphing is downright effective and puts fish in the net throughout the year. While fly choice and depth control are certainly important elements to effectively nymphing for trout, I would argue that presentation is paramount and it is my humble opinion that no piece of gear will immediately improve your presentation than a longer rod.
The standard, do-it-all trout rod for many has long been 9 foot 5 weight. Why? It has been a tried and true performer when it comes to all things western trout fishing. It covers many bases in terms of presentation and technique, but could a fly rod offer more performance when nymph fishing?
Enter the 10 foot 4 weight fly rod. The 10 foot 4 weight is the ultimate Western trout nymphing rod. It protects light tippet, gives you invaluable extra reach that facilitates excellent presentation, and has the leverage to effectively nice trout. Don't believe us...
Effective mending is essential to achieving a more natural drift. Ultimately, the angler wants their flies to be moving at the same rate of speed as the water where the fish is holding or feeding and a good mend allows that to happen. The longer 10-foot rod allows for the angler to pick up more line without disturbing the ongoing drift of their flies. This allows for a more effective mend and therefore a more effective "dead drift."
When I'm throwing two or three flies and split shot all under an indicator, I find it's best for my sanity that I reduce the number of times I make a traditional, overhand cast. The longer 10-foot fly rod acts as a longer lever than it's 8.5' or 9' brothers. That longer lever generates more power with less effort and allows for you, the angler, to roll cast with greater ease. This has two benefits: (1) Significantly less effort is required to roll cast your fly line, leader, tippet, and flies to the normal holding water. (2) The longer rod (aka lever) can throw more line than a shorter rod. This will provide
Extra length provides extra leverage, it's as simple as that. Extra leverage allows for you to move the hooked fish out of the strong current with less effort. This translates to a shorter, more efficient fight with fewer opportunities for losing fish. If a fish dives into an undercut bank, the extra length will help you pull that fish out of the danger zone more effectively.
This is a combination of point 1 and 2. With a longer rod, you can cast (roll and overhand) further and mend more effectively. Combine these facts with the longer rods ability to keep line off the water and a 10 foot rod allows you to extend your drifts and increase the length of your presentation. This allows for your bugs to be in the feeding zone for longer and increases the number of opportunities you have to catch fish.