You can run, but you can't hide -- high water is coming, or it's already here.
Fishing in early summer runoff or storm flood conditions in streams and rivers is a classic challenge for fly fishermen. While swapping out your river gear and heading for the nearest lake to go throw some long casts or chase carp and bass is always a viable option, insightful and experienced river anglers get after fish during these conditions using out of the box thinking that can put you on the board too.
In coming to grips with the question myself, the first thing I had to realize was this: The fish are still there. They don't head downstream, sulk on the bottom, or go on a three week fast -- they have to deal with this, same as you do. In working to understand what was going on, I had to ask myself the same question as always: "What would I do here if I was a trout?"
For developing a high water strategy, I think it is helpful to keep three things in mind when trying to understand trout behavior in these conditions. Nymphing is a core technique to apply to the following, though dry flies -- and, especially -- streamers can be used with excellent results.
#1: Don't fight the current. This applies all year long, not just at flood stage, but is most obvious during runoff. Trout can and will cross heavy current, navigate rapids and even jump small waterfalls when so inclined, but to do it hour after hour, day after day is just not an efficient use of hard won food energy. Even in low water, trout will seek out lies that provide relief from unnecessary effort, like behind boulders and corners. In high water, these lies get buried under massive flows and the fish are forced to find easier water to hold in. Often, this means being pushed right to the banks or into large eddies adjacent to the main current. Many, many times, I have found big trout sidled up with their shoulders literally pressed against one bank, hovering in the hydrostatic drag provided by grass and roots just ahead of them. Bottom line: In most rivers and streams, trout feed and hold in knee deep water, no matter what the total flow is.
#2: Follow the food. Again, fish do this all year long, but in normal conditions, drifting insects are usually spread across the river structure, where fish can fan out to stake their own feeding territories. During runoff, fish are forced to congregate not only due to the conditions mentioned earlier, but also because food hangs up and gets concentrated in lower velocity water. Heavy flows tear a lot of insects out of the bottom strata, including common emergent species like mayflies,caddis and stoneflies, but also aquatic terrestrials like annelids (worms) and other crustaceans such as sowbugs and shrimp. Where fast water meets slow, the resulting interface stalls suspended food in whirling vortices that cause a smorgasbord of food to dance in front of fish who have positioned themselves to take advantage of the situation.
Water clarity during these conditions is the typical objection that most fisherman have to this, and to a certain extent, this is valid -- four inches of visibility is common during these periods. However, fish have different vision than we do, and see most food items with no trouble in murky conditions. Juvenile brown trout have been determined by researchers to be able to see in the ultraviolet spectrum, an adaptation called tetrachomacy. New fly tying materials with UV reflectance can potentially increase prey targeting for fish that are operating in this realm during low visibility. Additionally, in contrast to the many emergent insect species that have adapted muted colors allowing them to survive in clear water, aquatic terrestrials can be brightly colored, with worms coming in gaudy reds and purples, and crustaceans flushing with excitement to pink or orange from their already visible pearly white and greys when dislodged from under rocks and torn out of the banks.
#3: Safety. The third condition that needs to be satisfied is for fish to feel comfortable enough to settle down and eat. Turbid water provides better cover for fish from overhead threats like eagles and osprey, but if the available water is an eddy on a former gravel bar that is only eight inches deep, even four inches of water visibility is not going to give a fish the confidence to let it's guard down if the overhead surface of the water is smooth. Look for fish to be congregated under the rippled surface at the interface of fast and slow water. When fish are tucked up under the bank, don't be surprised if you spook them out with heavy footsteps -- they feel vibrations through the saturated dirt better than you would believe.
Rigging and presentation in high water is not necessarily all that different from that used in "normal" conditions, but rather, adjusted to take objective factors into consideration. Presenting a nymph or dry to a fish in a bank hugging scenario can be an interesting challenge, as the holding lane can be literally six inches wide -- throw too far in either direction and you're either in the grass, or drifting outside the fish's comfort zone in the current. Cast upstream from below the fish, put it right in his face without spooking him, and he'll usually eat the first thing he sees. You don't need to use huge flies, or be rigged to run deep -- in most rivers, I never vary the distance of my indicators, keeping them at about 3 1/2' from the point fly to run knee deep all year long, adjusting only the weight and pattern seasonally. Typically, however will use somewhat heavier tippet material nymphing in these scenarios than during the rest of the year, dropping to 3x or 4x from 5x / 6x to be able to stop larger fish that have jumped into heavy current or to keep them from bulldogging under banks. With lower water clarity, you can get away with this, so.... I do.
Counter intuitively, streamer presentation is also more precise than during the rest of the season. Either upstream or downstream casts can be effective to bank huggers, but in either case, stealth is the order of the day as to be able to put your fly in the strike zone via short, accurate shots with any action from stripping imparted at just the right moment. Fish aren't as likely to chase a streamer from any distance with low visibility, so you need to be right on the money, and the best way to do that is to make short casts. Patterns that displace a lot of water that fish can feel with their secondary senses are always a good call, and four words here are useful: Wooly Buggers. Black ones.
The biggest fish in any river are always going to take over the prime lies -- that is, the ones with the best protections and food. Some of the best places for a big fish to hole up in during runoff are going to be way up in undercut banks below trees with root systems dangling into the water. These situations are almost impossible to present to without taking the chance of instantly losing your rig to a snag. Here is where thinking interpretively can pay big dividends -- unweighted, subsurface streamer patterns intended for bass with weedless hook guards can allow you to present to these situations with impunity. Most of these patterns are intended for stillwater use, and employing a weighted poly leader system with a heavy 0x or 1x tippet can allow you to harass fish that would normally never even get a chance to see your fly.
High water doesn't mean you have to put your gear away for weeks. With a little different approach, you can take fish when everyone else is off the water, and some of them may be the season's best!