Traveling through darkness with hours to go; trying to reach a river just before daybreak. The sky unleashes a torrent of rain that wasn't predicted by any weather outlet. The river rises, and you arrive to stand above a writhing, screaming, out-of-control beast.
Fluctuations in water level can move fish around quite a bit, but if rising or dropping water makes angling tough, it might be because the angler doesn't quite grasp how fish react to those variables. Or it could be the fish just aren't there at all.
Very high and very low water levels create adversities for migrating fish. Whether to migrate to spawn, as salmon and steelhead do, or to utilize seasonal habitat, like systemic trout, bass, or whitefish, extremes in water levels can postpone fish movements. Until the need to spawn or move becomes urgent, fish might stay put.
Understanding how water levels influence fish movement, behavior, and their ultimate location is one of the fundamental pillars for river rats like us to understand in order to catch fish consistently.
High water pushes fish to the banks. The priority, especially for migratory fish, becomes finding the path of least resistance. The main river becomes even more dangerous for fish than humans. We can leave, they can't.
Visibility is reduced as logs and debris come tumbling downriver. In mountain streams, big rocks come rumbling through the pools. High water carries a heavy silt load, which probably irritates gills and eyes. During such times, fish tend to seek areas near the bank where the current is blocked or slowed significantly, for safety and comfort. Silt settles out a little in those areas as well, and it isn't being driven into the fish.
Smaller fish are more affected by high, fast water than large fish. Baitfish and young-of-the-year gamefish can be virtually swept away and killed by major floods — getting crushed or disoriented — making them easy prey. Year classes can be decimated if the timing of the flood is such that newly-hatched fish haven't had time to develop enough savvy to follow the rising edge of the river into the shoreline brush. All of which provides reasons for predators to locate along the banks — that's where the food is. Salmon and steelhead are the exception as they generally do not feed during migrations.
Rivers want to flow in a straight line, bending only when forced. So the highest current speeds will be on the outside of a bend or toward that side of the river where the outside of the last bend was situated. It's easy to distinguish the varying current speeds in straight sections of river by watching foam or debris travel, or by throwing twigs and sticks out and monitoring progress. Results can surprise, because current might be blocked by underwater points, boulders, wing dams, or sand bars. Find the slowest speeds in each segment of the river being fished and focus on the zone where faster water meets the slowest water.
Less experienced anglers tend to focus on the deep water of a pool no matter what the conditions are. But when a river is even slightly high and cloudy, fish like steelhead and salmon feel safe in the water where we normally wade. These fish — and all migratory fish — prefer to travel and hold along the path of least resistance, where current is reduced, if they feel secure. As a general rule, always start fishing where bottom can't be seen, even if it's just a few feet from the bank you're walking on
Fish locate along the edges of eddies and the slow side of current seams between fast and slow water. The most active fish tend to hover right at the upstream tip of an eddy or seam, holding in the slow water and watching things go by in the faster water. Spend more time focused on those areas and less time casting to deep water.
The best presentations in high, cloudy water tend to be bright with contrasting colors. Glow or fluorescent orange contrasted with bright chartreuse, bright green against black or white, silver plate coupled with copper or blue glow — every river has its own substrates creating its own water color that suggests various patterns. Drop it in the water and watch. Whichever pattern remains visible at the deepest point is a good place to start.
When rivers run at median levels, trout and salmon spread out more than in other conditions. They use larger portions of pools, more pocket water, and many spots that are too exposed in low water and uncomfortably fast or dangerous in high water. Where you fish now is determined by proximity to spawning windows and the urgency (or lack thereof) to reach seasonal habitat.
The rule of thumb in all conditions is to make short casts first, so the line isn't dragged across fish that might be closer than expected. In moderate flows, the first cast should be to the inside edge of a pool or run, where the bottom can barely be distinguished. (Each successive cast should be 6 inches longer than the last in all conditions.)
The inside edge of a pool, closest to the inside of the bend, becomes the path of least resistance for migratory fish in moderate flows. Any shallower and they become visible, losing security. Any deeper and they're forced to work against stronger currents. This is not to say fish won't move up through the center of a pool, but the main highway is always the path of least resistance as moderated by the need to hide. Fish tend to know when they can be seen from above and position themselves accordingly.
By all means, cover the entire pool. But angling efforts should focus on pockets and the inside edges of runs and pools from the head to the tail. Make a few extra casts to those areas before working deeper into the heart of a pool. Colors and patterns on days with average flows can range from bright and gaudy to very subtle, depending on water clarity, but they tend to fall somewhere between those extremes.
Low water means reduced current. Rules regarding the path of least resistance only apply at night in low water, if at all. The priority for migratory fish, during the day, becomes security. Hiding big bodies from the view of predators is job one.
The keys to low water become depth, shadows, cover, and broken water. Steelhead, trout, and salmon can feel secure under two feet of ultra-clear water if the surface is rippled or broken overhead. Steelhead often reject wood cover in all other conditions, but can be found lingering in the outer branches of fallen trees in low, clear water. Deep pools and overhangs that produce shade become prime location factors.
Low water can concentrate fish in pools, but pockets in riffles and rapids should never be overlooked. The rule in low, clear water is to fish every spot where bottom can't be seen. Low, clear conditions offer the best opportunities to gain a high vantage over the river and, using polarized sunglasses, spot fish in the pools and deeper runs. This saves a lot of time that might be spent working pools devoid of fish.
When water levels are really low, a rain event might draw migratory fish like steelhead well upriver — but if the need to spawn isn't urgent yet, and water levels begin to drop, that body of fish might begin to slip back downstream by a mile or more per day until water levels stabilize or rise again.
Color and pattern selections in low water should be subtle. Match jig color to baits. Use natural black, olive, or brown flies. Plastics and beads should be translucent, allowing light to pass through.
Of course, water levels are not the only things that position migratory fish in streams. Water temperature and season play a role. Fish tend to hold in slower water when it's cold and faster water when it's warm. And the closer it gets to the end of that day-length window for spawning, the more fish ignore water levels. Obviously, water level is only one variable in a vast pastiche of indicators and clues, but ignoring it would be unthinkable for a true river rat.