Spring fly fishing is like a box of chocolates, you never know what you’re going to get. One day you’re fishing dry flies on a 70-degree day and four days later you may be dredging deep pools while it snows. Spring is a season of change and with that change comes volatile weather and fishing conditions. While air temperatures tend to rise, streaks of winter weather always find their way into the mix. Fluctuations in air temperature and precipitation directly impact the progress of snow melt, run off and river flows, but if you know what to look for, you can anticipate these changes and react accordingly.

In order to anticipate changes in flow, we need to first understand what runoff is and what causes it. To put it simply, runoff is when excess surface water flows or “runs off” the land. When the ground is saturated, excess moisture from rain and snow flows across the land and spills into creeks, streams and rivers. During the spring, this occurs when warm air melts the snowpack that accumulated during the winter. This process starts at lower elevations and gradually works up to higher elevations as we progress through spring.

Over the last two weeks, we observed two different forms of pre-runoff on our freestone rivers. We saw a couple rivers rise at a rather rapid pace before stabilizing or declining. On other rivers, we observed daily fluctuations in flow with spikes during the day and decreases overnight. Despite overnight decreases, flows remained on an upward trend. The primary difference between these two scenarios is overnight temperature. Warm temperatures during the day encourage snowmelt and runoff but if temperatures dramatically drop overnight, snow melt will taper off until temperatures rise again the following day. Keeping a close eye on afternoon and overnight air temperatures will allow you to anticipate changes in flow until runoff season kicks off in late May/early June.

Now let’s use that logic to predict runoff and flow fluctuations on the Middle Colorado River this week. Looking at the Wunderground weather forecast below, we can expect relatively warm afternoon and sub-freezing overnight temperatures with some precipitation sprinkled in the middle. Based on temperature alone, it’s reasonable to expect a modest level of runoff and a gradual rise in flow during the day. However, cold overnight temperatures will pump the breaks on snowmelt, leading to daily fluctuations rather than continuous runoff. Precipitation in the forecast would typically suggest increased runoff, however, minimal accumulation is expected, so we don’t expect it to have a major impact. In the case of significant precipitation, we would expect either bigger daily spikes or sustained runoff over those three days.

weather example

Now that we know how to anticipate runoff, let’s get into the fun stuff and discuss how pre-runoff impacts fishing and the adjustments you can make to be successful.

Flows and water clarity: When flows rise, water clarity and water temperature decreases. Faster water and cooler water temperatures will encourage trout to move into soft water where they are protected from debris and can conserve energy. Rather than targeting riffles and fast runs, focus on the outer seams, banks, pools and slow runs. Decreased water clarity provides trout with a sense of protection from aerial predators, so don’t rule out soft and shallow water.

If overnight temperatures drop and delay runoff, flows will likely drop overnight and hold until runoff begins the next afternoon (assuming warm afternoon temperatures). This will improve the water clarity and push trout back into their typical spring lies during the morning.

Bugs: As flows increase, bugs are kicked up from the river bed and tumble downstream. The presence of bigger meals excite trout and increase feeding. Worms, leeches, stonefly larva and caddis larva are hot menu items and will produce results in the lead position on a nymph rig. Dead drifting and stripping streamers is also highly effective as it imitates a bait fish struggling to keep up with the current. Now, don’t get me wrong, trout are still keyed in on midge and baetis offerings, they’ll just get excited when they see a larger meal floating their way. Regardless of the patterns you use, large and/or flashy patterns will effectively attract attention in off-color water.

Approach: Whether flows are on the rise or a decline, making adjustments to your weight/depth when nymphing is critical. When flows are up, apply extra split shot or use tungsten bead nymphs to get your flies down quickly. When flows drop, less weight is typically needed unless your targeting riffles, pockets and faster runs.