Best Time of Day to Fly Fish for Trout

How to pick the hour from the season and the water temperature, what low light changes, and when to stop.

From the book Fly Fishing for Nerds — $9.74 →

A clock time on its own is a thin plan. Noon in July and noon in March ask for different hours. Check the water temperature and the light before you rig, and write both down.

Use the same loop as the observe page: observe, then hypothesize, then test. Temperature and light are the observation. One sentence about the hour is the hypothesis. A handful of clean drifts is the test.

1. Read the temperature before you trust the hour

Use a stream thermometer when you have one. The water-temperature guide is how to take that reading, and where the bands below come from. A public gauge for that river is the next best figure, if a gauge exists. The river-flow guide is how to read the discharge on that page. On many rivers the water is coldest around dawn and warmest in the afternoon, because water lags the air. A guess from your hand is too coarse for the stop rule further down.

Four bands are enough to choose an hour:

• Cold, about 45°F and below. The feeding window is short, and it usually sits in the warmest part of the day.
• Moderate, about 45–60°F. Trout can feed across more of the day. A hatch, or a change in light, can still move them.
• Warm, about 60–68°F. Look for low light, shade, and any water that is cooler than the main flow.
• About 68°F and above. Stop for the day.

A small creek and a river below a dam can sit in different bands on the same afternoon. Write the temperature down. The band is the guess you are about to test.

2. Apply the season once you have the number

Spring, while the water is still cold. Plan the fishing for late morning into afternoon, after the day has had time to take the chill off. Use first light to walk the bank and see the reach. If insects are on the water at 11, fish the insects. That observation outranks the clock.

Summer, on a river that warms through the afternoon. Fish the morning, and come back for the evening. At midday, look in deeper water and along shade. A bottom-release tailwater or a spring creek can stay in the 50s at noon. The thermometer is the check.

Fall, as nights cool the river. Morning, midday, and evening can each be a fair session while the water stays in the 50s. If a warm afternoon climbs toward 68°F, end the session.

Winter. Fish a short session in the warmest part of the day, often early afternoon. Slower drifts, deeper water. Two hours in that window is a complete test. Starting in the dark, in water near freezing, makes a poor test.

3. What low light changes

Dawn, dusk, a heavy cloud deck, and the shade along a bank make you harder for a trout to see. Caddis are often busier in that light. Shallow edges that look empty at noon, including tailouts, can hold fish then.

Temperature still leads. At 40°F at sunrise, wait for the water to warm, or fish a deep slow lane and keep the session short. At 66°F in bright midafternoon, leave the sunny tailout for evening. Fish the shaded bank now, or come back later.

4. Give trout a break above about 68°F

Warm water holds less oxygen, and a trout you fight has a harder time recovering after you let it go. If the river is already about 68°F when you arrive, choose a cooler creek, a dam release that is still cold, or go home. A few casts in the riffle still count as fishing them. If the morning started cool and the afternoon crosses about 68°F, stop at the crossing. A fish that cannot swim off upright is a late sign that the water is too warm, or the fight was too long. End the day.

Keep the thermometer with the nippers. The number in your hand beats a memory of how hot the air felt in the parking lot.

5. Make the hour one test

Before the first cast, write the sentence. "52°F, overcast, late morning: nymph one run for a handful of clean drifts." Or "41°F at dawn: walk the bank now, and nymph the deep run after lunch, once the water has warmed." If the test comes up empty, change one thing: the lane, the depth, or the plan to return at another hour. Pick the lane with the reading-water guide.

6. Other reasons to quit

Stop when the wade looks wrong, when the river is rising into color you cannot read, or when weather is moving in. Safety and access are the first item on the bank checklist. Stop when you are tired enough to wade badly. Stop when the note is written: time, water temperature, light, lane, result. Another hour of the same empty drift repeats a test you already finished.

Season, water temperature, and where trout sit as the day changes are covered at more length in Fly Fishing for Nerds: A Data-Driven Guide to Finding, Hooking, and Landing Trout. Readers can take 25% off ($9.74) with code NERDS25 at https://leanpub.com/flyfishingfornerdsadata-drivenguidetofindingfishfly-fishing-for-nerds/c/NERDS25.

The Riverbank Checklist is the one-page bank routine, with a line for today's hypothesis and the result.