Freestone · Chalk · Île-de-France / Seine-et-Marne and Marne (above La Ferté-Gaucher)

Grand Morin (Upper)

The upper Grand Morin is the obvious 1ère catégorie candidate for a Paris-based fly angler — classified as first-category water upstream of the Moulin de Montblin at La Ferté-Gaucher.

Likely too hot — check on arrivalThe water is probably above the brown trout safety line — carry a thermometer.

Water estimated at about 23°C from air temperature — there is no water-temp gauge here, so treat the figure as a guide. If the estimate is right, this is above 22°C and you should not be targeting brown trout. But shade, cold tributaries, and recent rain can hold a river two or three degrees below the air-driven figure. Carry a thermometer. Check it on arrival. If the river is genuinely at 22°C or above, fish another species or come back another day. If it is cooler than the estimate: dawn and dusk only, barbless hooks, land it fast, wet hands, no air shots. Stop the moment a fish is slow to recover.

DepthShaded riffles, deep pools, spring inflows, cold tributary mouths. That is where the fish will be.
TimingDawn and dusk only. Nothing in the middle of the day.

Poorlive now

Not the day for it

River temperature is the limiting factor today, not the flow. This is not the day to push it. Check the thermal advice above before you head out.

90% confidence

Notify when prime

What moved it

LevelDry recentlyNo gauge reading
Water temp22.6°C
ClarityClear

Today’s fly

CDC Emerger16-18

Dead-drift

Rules at a glance

Permit required

Live now

Conditions on the water

Estimate

Trends shown where the gauge supports them

Level

Dry recently

No gauge reading

Water temp

22.6°C

coldwarm

Clarity

Clear

Weather

33°C

Light cloud

Wind

NE 15 km/h

calmblown out

Pressure

1013 hPa

lowhigh

Rain · recent

0.0 mm

dryspate risk

Rain · ahead

0.0 mm

dryspate risk

No gauge on this water — readings are regional estimates from weather, not a live gauge.

Water temperature for brown trout

Warm — slow

23°C est.ideal 10–16°C

0°14°28°

How to fish it · for brown trout

When

From the 2nd Saturday of March opening into early September. Best window April–June with small olives and grannom; shoulder through summer.

Where

Upper reaches around La Ferté-Gaucher and upstream towards Esternay. Pool tails, faster glides, and the seams behind overhanging willow on the smaller upper beats.

Method

Upstream dry to risers; dry-dropper through the riffles. Walk-and-spot.

Kit

9 ft #4 (or 8 ft #3 on tighter beats), floating line, 12 ft leader to 3–4 lb fluoro.

Why this works

⛔ Water at 22.6°C. This is the line where catch-and-release kills brown trout. Do not fish for them today.

Hatch timeline · todayPeak at 12pm

Hatch predictions

Today's headline hatch shown — see all 12 active hatches hour by hour with Pro.

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Evidence

Survey-backed · regionalModerate confidence

Backed by regional invertebrate surveys; no sampling on this exact reach yet.

Through the year

0–3 scale · July highlighted

Trout season

Grannom

Evening Sedge

Large Dark Olive

Iron Blue

Numbers are intensity 0 (none) to 3 (peak) — a guide, not a guarantee.

Ranked for today

Brown Trout fly box

Permits & access

Permit required — see local rules.

1ère catégorie above Moulin de Montblin (La Ferté-Gaucher)
AAPPMA permit + carte de pêche required
Check the Seine-et-Marne federation's annual notice for category boundaries, catch limits and method restrictions.

What's coming

Plan ahead

5-day outlook

Water here around 23°C. Nothing cooler within range — carry a thermometer.

About this water

The upper Grand Morin is the obvious 1ère catégorie candidate for a Paris-based fly angler — classified as first-category water upstream of the Moulin de Montblin at La Ferté-Gaucher. Above that boundary the river fishes as a small chalk-leaning stream through Esternay and the eastern Seine-et-Marne / Marne countryside; below it slides into the more mixed, coarse-fish water that defines most of the regional rivers. Modest stream trout in modest numbers under modest pressure. Travel with images of the Andelle or the Risle in your head and you will be disappointed; travel with low expectations and a willingness to enjoy small fish in a small river, and the upper Grand Morin is a sound way to spend a morning.

Chalk

The full read · show the working · for brown trout · confidence 90%

How the 6 is built — score × weight = contribution

Temperature3 × 28%0.8

Flow60 × 22%13.2

Clarity95 × 18%17.1

Feeding Time60 × 13%7.8

Pressure80 × 7%5.6

Insect activity60 × 12%7.2

Limiting factor: Water temperature (22.6°C) is outside Brown Trout's preferred range (10–16°C)= 52

Can you trust it?

Water temperatureair-to-water estimateestimated

Level / flowon-river gaugeobserved

Prey activitysurvey-backed invertebrate datasurvey

What would change the calculation

Cooler water — back toward the 10–16 °C ideal band — would lift the score and ease welfare.

The dawn and dusk windows score higher than the midday lull.

Directions

Apple Maps Google Maps

Seasons & zones

Trout2nd Saturday of March → 3rd Sunday of September

Related guides

Booking & contacts