One of the biggest myths in fly fishing is that trout don't feed well in high, dirty water. Nothing could be further from the truth. The trout actually gorge themselves quite often during high, dirty water conditions. The fact that you aren't catching them doesn't mean that they aren't feeding. The tactics do change a little however, especially from the way most people believe that they need to present their flies during the dirty water times. Here's a few tips that may help you out:
Lose the Flash
Most people tend to think that they need to throw big, bright, flashy flies when fishing in dirty water so the fish can see their offering. This may be one of the biggest myths out there. The trout sees very well in dirty water...very well. Just because you can't see the fly doesn't mean that they can't. In fact, flashy flies will often turn them away. Think about it like this...nothing in nature produces flash without sunlight reflecting on it. In dirty water there would not be enough light penetration for anything to produce flash, so it is not a natural occurrence and the fish know that something is up. Go to a dark, or at least a darker fly, and see if that produces some better results for you.
Downsize Your Fly
Another myth related to fish not being able to see your fly is to think that it has to be a large fly in order to get noticed. Again, these fish make a living eating. They see your fly very well. The reason they don't eat it actually may be that it is too big. Keep in mind that there is a huge amount of dislodgement occurring during high water. The trout are literally being bombarded with food floating downstream that has been washed out from under the rocks, off the banks or wherever else it usually hides out. In fact, that's the reason that most people find the fishing slower during these times. The trout simply have a huge amount of food available to them and will often key in on the size food that they are actually seeing as a natural occurrence. If your fly is way too big, then guess what, it just doesn't look right to them and they will leave it alone. In fact, try fishing a two fly rig (which you should be doing anyway) with two very different sized nymphs, say a size 10 stonefly versus a size 16 hares ear. Bet you'll be surprised at how much better the smaller fly does in getting their attention.
Beat the Banks
This can be very important. It only makes sense to present your fly to areas that are actually holding fish. During high water, whether it's dirty or not, the fish will often be pushed to the banks in order to find more suitable flows to hold in. Just like any other time, they do not like fighting the current when they don't have to. Many times you walk right past them on your way out to fish the middle of the river. Big mistake! Pay attention to the slower currents that are often found in the more shallow water near the banks. If you are truly in high water conditions you can bet that the fish will not be holding in the middle of the river without having a good current break to hide behind. But overall, you'll find them tucked away safely within a few feet of the bank enjoying the softer water and gorging themselves on the abundance of food that just magically appeared when the water rose.