Freestone · Mixed · Wallonia / Luxembourg Province

Semois

Semois — File:Frahan JPG01.jpg

One of Wallonia's defining southern rivers, winding through wooded valleys and long glides with a mixed trout-and-grayling identity depending on section.

Poor · Brown Trout
Pheasant Tail Nymph · 14-18
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
  • Level0.01 mLast reading 13h ago
  • Water temp23.2°C
  • ClarityClear
Today's fly
Pheasant Tail Nymph
Pheasant Tail Nymph14-18
Upstream
Conditions on the water
Live gauge
Level
Steady
0.01 m
Last reading 13h ago
Water temp23.2°C
ClarityClear
Weather32°C
WindSE 8 km/h
Pressure1021 hPa
Rain · recent0.0 mm
Rain · ahead0.9 mm

The nearest gauge doesn't record temperatures, so we've estimated it — defer to the river keeper's advice.

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
Nymphing can work through most of the day.
Where
Cover mixed depths.
Method
Start with tight-line nymphs and adjust if fish rise or drift higher.
Kit
9 ft #4 rod, floating line, 12 ft tapered leader to 4–5 lb fluoro tippet.
Why this works
Water at 23.2°C. This is the line where catch-and-release kills brown trout. Do not fish for them today.
Sub-surface · on the nymphunder the surface

Hatch predictions

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Beyond the hatch · worth watchingambient prey

Hatch predictions

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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 · August highlighted
J
F
M
A
M
J
J
A
S
O
N
D
Trout seasonSeason
1
2
2
2
2
2
1
Grayling seasonSeason
2
1
1
2
2
2
2
2
2
GrannomHatch
2
2
Evening SedgeHatch
2
3
3
3
2
Large Dark OliveHatch
1
2
2
1
Iron BlueHatch
1
2
2
1

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

Ranked for today
Brown Trout fly box

A proper Ardennes river-trout pack for Wallonia, centred on clear to lightly coloured rivers with trout and some classic fly-fishing culture.

SB
Small Black Beetle
14-18 · Pattern
Upstream dry
Permits & access
Permit required — see local rules.
  • Walloon permit required, but some stretches may sit under additional society, forest-zone or local rights structures depending on section.
What's coming
Plan ahead
5-day outlook
Cooler water nearby · 1
Probably above the Brown Trout safety line — these are likely cooler, but carry a thermometer.
Pisciculture de Vendresse
35 km
~20°C
Rainbow trout
Gallery · 2
  1. Semois — File:Frahan JPG01.jpg
    File:Frahan JPG01.jpg
  2. Terrain map of the venue
    Terrain map
About this water

One of Wallonia's defining southern rivers, winding through wooded valleys and long glides with a mixed trout-and-grayling identity depending on section. More culturally important than flashy, and best treated as a core Belgian river rather than an afterthought behind the better-known East Belgium names.

  • Mixed
Water quality (WFD)
  • Ecology: Moderate
  • Chemical: Failing to achieve good
What this classification means

WFD re-coded 2026-06-18: prior id BERW06_SM01R was a wrong-basin code (no row in EEA WISE). Re-mapped to BERW17_SC28R 'Semois III' near the venue coordinate (49.85,5.08, Bouillon area). Status: EEA WISE 2022 (3rd RBMP). Confidence: medium — VERIFY reach against a map (Semois has 4 reaches I–IV). Source: discodata.eea.europa.eu.

WFD classification · BERW17_SC28R

The full read · show the working · for Brown Trout · confidence 90%
How the 2 is built — score × weight = contribution
Temperature1 × 28%0.3
Flow80 × 22%17.6
Clarity95 × 18%17.1
Feeding Time55 × 13%7.2
Pressure80 × 7%5.6
Insect activity48 × 12%5.8
Limiting factor: Water temperature (23.2°C) is outside Brown Trout's preferred range (10–16°C)= 54
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 MapsGoogle Maps
Seasons & zones
  • Trout 3rd Saturday of March → 30 September
  • Grayling 1 June → 28 February
Related guides
Grayling Playbookgrayling tacticsEuro Nymphingsubsurface methods
Booking & contacts