Fly Fishing After Rain

How rain changes the flow and the color, how a rising river differs from a falling one, and when to stay on the bank.

From the book Fly Fishing for Nerds — $9.74 →

Rain on a freestone does two things you can check before you rig. It adds water, and it adds dirt. Read the gauge for the water. Look at the river for the dirt.

Use the same loop as the observe page: observe, then hypothesize, then test. The gauge and the color are the observation. One sentence about the edge you will fish is the hypothesis. A handful of drifts is the test.

1. What the rain changes

On a freestone, rain that runs off the ground shows up as discharge. The river-flow guide is how to read cubic feet per second, and whether today’s number is ordinary for that gauge.

The color comes with the water. USGS calls turbidity the clarity of the water: how much material in it scatters light. During low flow many rivers run a clear green, often under 10 NTU. In a rainstorm, particles from the surrounding land wash in and the river turns muddy brown. High flow does a second job. Faster, deeper water stirs sediment off the bed and holds it up.

USGS describes the load itself. A storm brings eroded soil and debris. Fast water can pick up larger particles than slow water, which is why a river looks muddier in a storm than it does at low flow. So much can move at once that more than half the sediment a river carries in a year may pass in a single storm. Leave a glass of that water still, and the dirt settles. The river does the same thing wherever the current slows: an eddy, the inside of a bend, the soft side of a seam.

Two rivers in the same rain do not have to match. A steep freestone colors up and speeds up. A spring creek, fed from the ground, often rises less and stains less. A tailwater follows the dam release, so rain on the parking lot may not be the water at your feet. If the reservoir above the dam is taking floodwater, muddy water can still come through later. Read the gauge on the reach you will fish.

Temperature can move in the same storm. Take it anyway. The water-temperature guide is what the number means, including when to quit near 68°F.

2. Rising water and falling water

Rising means more volume is passing the gauge. Unless the channel got much wider, the main lane got faster. A trout that was holding in a moderate current may not hold there. The slower water is along the bank, in an eddy, on the inside of a bend, or on the soft side of a seam. That is the hypothesis. Test that edge for a handful of drifts before you change flies.

Falling is the other half of the same storm. Speed drops, and sediment starts to settle in the slower water, the way it settles in the glass. The river often clears at the edges before it clears in the tongue. The lie that looked right at the peak can be too shallow by the time you wade in. Walk the reach again with the reading-water guide and mark what is actually there today.

The crest is usually the worst hour to fish. Speed and color are both at their peak, and from the bank you cannot tell yet whether the river is done coming up.

3. Clarity is what you can see

Stained, off-color, and blown out are angler words. USGS does not publish a fishing status by those names. Turbidity in NTU is the meter reading, and most of us do not have the meter. The bank check is whether you can see enough to know if a drift worked.

If you cannot see the seam, you cannot tell a clean drift from a dragging one. Another fly does not fix that. When the bubbles or the texture line come back, the seam guide is which side to fish.

4. How long until it fishes again

There is no hour count that fits every river. A small steep creek after a short shower can be fishable the same afternoon. A larger river, or one carrying a lot of clay and silt, can stay brown for days after a soaking rain. Fast water carries the bigger particles. When the river slows, those drop out first, and the fine material is what you still see. Anglers often wait a day or two. That is a fair guess for a modest rain on a small freestone, and it is only a guess.

Three checks, in order:

1. The rain has stopped on the drainage that feeds your reach, not only over the parking lot.
2. The graph has peaked and is falling. A river that is still rising is not past the rain yet.
3. You can see enough to judge a drift, using the boot check above.

The upper river often clears before the water downstream, because the lower river is still taking dirty water from every tributary. A gauge below a large tributary measures a different mix than the bridge above it. The river-flow guide is how to pick the gauge on your reach.

Write down the discharge or gage height you already know you can wade on this river. PingWhen (paid, from $9 a month) can email you when a USGS gauge crosses a flow or gage-height level you set.

When the three checks pass, fish the soft edge. When they do not, going home is a finished session. Write the CFS, whether it was rising or falling, and what you could see. Next time this river comes up, that note is a better wait than a number of hours you remembered.

5. What to fish when the water is high or off color

Fish where a trout can hold. That is the soft edge, not the middle of the fast lane. In stained water a small pale nymph at the end of a long drift is easy for the trout to miss. Start with one larger, darker fly close to the bank: a stonefly nymph, a worm, or a woolly bugger worked along the edge. Rain washes worms and terrestrials in and knocks nymphs loose, so a bigger fly is the first thing to test.

Make a handful of clean drifts, or a handful of casts along the bank, before you change flies. The lane and the depth matter more than the next pattern in the box. If you cannot see the take, the indicator guide is the rig. The no-hatch guide is the same discipline on a clear day: one fly, then a change of depth or lane.

Heavier tippet is reasonable. Sticks are coming down, and a trout that cannot see the leader well is not a reason to fish 6X. The tippet guide is what the X number measures. Step up one size from what you used in clear water last week, and stop there. A woolly bugger in the starter sizes sits on 3X.

If the boot check fails, another fly will not save the session. Come back when you can see the edge.

6. Safety

The National Weather Service puts a number on moving water: six inches of it can knock you off your feet. Do not walk into floodwater. A knee-deep crossing that was easy last week can be that hazard after one night of rain. If the access road is under water, turn around.

• If the gauge is rising steeply, stay out of the channel. The river can come up while you are in it. Fish from a bank you can leave, or do not fish.
• Stained water hides the bottom, the next drop-off, and the wood. Wade less than you would in clear water. Probe with a staff or a stick.
• In a canyon, rain you cannot see can arrive as a rise. If the river starts coming up, or you hear water upstream that was not there, get to high ground.

Saturated banks slump. An undercut that held last month may not hold today. Footing and flow are the first item on the bank checklist.

Rain, flow, and where trout hold when the river is up get more room 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 routine for the bank once you are there: safety first, then one hypothesis and a short test.