Karst · Limestone · Jura / Doubs

Cusancin

A small clear river flowing over a low weir, lined with autumn trees, in the Cusancin valley, France.
Contributor photo

cusancin

michèle gallecier - CC BY-SA 2.0

The Cusancin is a short, secretive limestone tributary of the Doubs that rises at two resurgence springs at Cusance and runs for maybe fifteen kilometres before joining the Doubs at Baume-les-Dames.

Poor · Brown Trout
CDC Emerger · 16-20
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
What moved it
  • LevelDry recentlyNo gauge reading
  • Water temp22.2°C
  • ClarityClear
Today’s fly
CDC Emerger
CDC Emerger16-20
Dead-drift
Conditions on the water
Estimate
Level
Dry recently
No gauge reading
Water temp22.2°C
ClarityClear
Weather28°C
WindSW 9 km/h
Pressure1016 hPa
Rain · recent0.4 mm
Rain · ahead16.3 mm

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

Water temperature for brown trout
Warm — slow
22°C est.ideal 1016°C
0°14°28°
How to fish it · for brown trout
When
Through the open trout season. The Cusancin is small intimate karst water — read each lie carefully. Spring olives, summer sedges, autumn small flies.
Where
Pool tails, weeded glides, the cushion behind chalk boulders. The Cusancin is short and small-scale — short casts, careful approach.
Method
Upstream dry to spotted rising fish; long fine leader. The water reads clear — careful approach matters more than fly choice. Move slowly upstream, cast accurately, accept the result.
Kit
9 ft #4 — French karst-stream default. Floating line, 14 ft leader to 4 lb fluoro. Polarising glasses non-negotiable.
Why this works
⛔ Water at 22.2°C. This is the line where catch-and-release kills brown trout. Do not fish for them today.
Hatch timeline · todayQuiet day

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Sub-surface · on the nymphunder the surface

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Beyond the hatch · worth watchingambient prey

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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
J
F
M
A
M
J
J
A
S
O
N
D
Trout seasonSeason
1
2
2
2
2
2
1
Grayling seasonSeason
1
2
2
2
1
Brown SedgeHatch
2
3
2
Freshwater ShrimpHatch
2
3
3
3
3
3
3
3
3
3
3
2
Blue Winged OliveHatch
2
3
3
3
3
2
Mayfly (Green Drake)Hatch
2
2

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

Ranked for today
Brown Trout fly box
Gammarus Scud (Olive)
Gammarus Scud (Olive)
12-16 · Gammarus Scud
Dead-drift, sub-surface
Permits & access
Permit required — see local rules.
  • AAPPMA permits required
  • Some beats private; fly-only and no-kill parcours on parts of the river
  • Low limits and short rods recommended.
What's coming
Plan ahead
5-day outlook
Water here around 22°C. Nothing cooler within range — carry a thermometer.
Gallery · 2
  1. A small clear river flowing over a low weir, lined with autumn trees, in the Cusancin valley, France.
    cusancin
  2. Terrain map of the venue
    Terrain map
About this water

The Cusancin is a short, secretive limestone tributary of the Doubs that rises at two resurgence springs at Cusance and runs for maybe fifteen kilometres before joining the Doubs at Baume-les-Dames. It has no business holding the quality of fish it holds. The water emerges from the Jura plateau cold and clear and stays that way through the worst of August, which is exactly when other Franche-Comté rivers are gasping. Wild brown trout and grayling are both present and both take fly life seriously. This is a proper spring creek — the kind of water where you can stand above a glide, watch three fish working different lanes, and take twenty minutes to pick the one you want to try first. The river is narrow enough that a wrong cast puts everything down, and the fish are educated enough that you won't get many second chances. Fish it when you're feeling patient.

  • Limestone
Water quality (WFD)
  • EcologyModerate
  • ChemicalUnknown
What this classification means

WFD ecological status Good → Moderate corrected to EEA WISE 2022 (3rd RBMP) classification for waterbody FRDR11271 (was a stale earlier-cycle value). Source: EEA WISE SWB_SurfaceWaterBody (discodata.eea.europa.eu), verified 2026-06-18.

WFD classification · FRDR11271

The full read · show the working · for brown trout · confidence 90%
How the 10 is built — score × weight = contribution
Temperature5 × 28%1.4
Flow60 × 22%13.2
Clarity95 × 18%17.1
Feeding Time75 × 13%9.8
Pressure80 × 7%5.6
Insect activity47 × 12%5.6
Limiting factor: Water temperature (22.2°C) is outside Brown Trout's preferred range (10–16°C)= 53
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.
Directions
Seasons & zones
  • Trout2nd Saturday of March → 3rd Sunday of September
  • Grayling3rd Saturday of May → 3rd Sunday of September

    France national rule: grayling opens 3rd Saturday of May in all waters. In 1st category (salmonid) rivers, grayling closes with the trout season on the 3rd Sunday of September. Departmental regulations may impose additional restrictions. Always check the relevant département's 2026 arrêté préfectoral.

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