To add to DonO's post, it's not always competition to be first that puts the biggest trout in front of a rock. If you study the hydrodynamics of water flowing around a rock, you'll see that the front of a rock has a cushion with much smoother flow, the "food lines" are compressed into a smaller space, and there's a lovely little dead spot where a floating dun will pause momentarily before being swept around the rock. Behind the rock you get a dead area but the currents on either side are generally full of eddies. This makes food interception more difficult. Another interesting point, to which DonO will attest, is that the size and shape of the obstruction is frequently important. If you stand in Buffalo Run on the Yellowstone, the cutts will line up downstream of your legs, not in front. I believe that to be because your leg is too small (and likely the wrong shape) to produce a viable front cushion. Of course it could be as simple as the fact that when you move you mess up the front cushion but the rear deadspot follows you like a puppy. HOWEVER, when it comes to migrating fish, like salmon, that aren't feeding, you'll often find them at the side or just at the edge of the current behind an obstruction. This is the part of the theory of groynes that are built out into some salmon rivers. Not only do these make for casting platforms, but they hold fish along the newly created downstream current seam.
cheers Paul www.galesendpress.com -- Paul Marriner Outdoor Writing & Photography. Owner: Gale's End Press. Member: OWAA & OWC. Author of: A Compendium of Canadian Fly Patterns (co-author), Stillwater Fly Fishing: Tools & Tactics, How to Choose & Use Fly-tying Thread, Modern Atlantic Salmon Flies, Miramichi River Journal, Ausable River Journal, and Atlantic Salmon.
