Stillwater · Granite · North Wales / Gwynedd, GB-WLS

Llyn Trawsfynydd Reservoir

Llyn Trawsfynydd from the shore, open water filling the foreground with moorland hills beyond.

Llyn Trawsfynydd sits high in Snowdonia — a dramatic, wind-blown upland water surrounded by moorland and crag.

Good · Rainbow
Booby · 10-12
Goodlive now
Reasonable autumn fishing likely at Llyn Trawsfynydd Reservoir
Useful ripple, fishable wave. Fishable ripple — drift the productive shore.
65% confidence

Llyn Trawsfynydd Reservoir is often best in spring through autumn.

What moved it
  • WindSE 34 km/hFresh breeze
Today's fly
Booby
Booby10-12
Washing-line, sub-surface
Conditions on the water
Estimate
Wind
Fresh breeze
SE 34 km/h
N
SE
from the south-east
Wave60 cm wave
Water temp17.7°C
Air temp18°C
CloudOvercast
Pressure1008 hPa
Rain · recent0.0 mm

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

Head to
Wind driftSE wind
South-eastern end

A southeasterly has held for 12 hours, running colder than the water (14°C air against 18°C water). The end it blows into is taking that chilled surface water, so try the sheltered south-eastern end instead — off the back of the wind, where the undertow returns food along the bottom. That chill sits in the top of the water, though, and the bed is slower to feel it — on a bottom bait it is the undertow, not the temperature, that puts you at this end. Treat it as a tiebreaker between good swims rather than an override — features and angling pressure usually count for more.

How to fish it · for Rainbow
When
Conditions are not ideal but fishable at Llyn Trawsfynydd Reservoir.
Where
Start with Cormorant (10-12) — on the bob / top dropper. Rainbows respond to more active retrieves — try short strips between pauses. If that does not produce, switch depth or speed before changing the pattern entirely.
The plan
Plan A

Start with Cormorant (10-12) — on the bob / top dropper. Rainbows respond to more active retrieves — try short strips between pauses. If that does not produce, switch depth or speed before changing the pattern entirely.

Plan B

If the main plan stalls, drop to a size 14–16 midge — switch to a washing-line or suspender rig and change drift line before you change food group.

Watch for

Keep an eye on changing conditions — wind shifts and cloud breaks can trigger short feeding spells.

Bank

Bank-only venue. Fish from accessible margins, points, weed edges, and inflows.

Sub-surface · on the nymphunder the surface

Hatch predictions

Supporters see the full sub-surface read.

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Beyond the hatch · worth watchingambient prey(1)

Hatch predictions

Top prey shown — see all 5 as a supporter.

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Through the year
0–3 scale · September highlighted
J
F
M
A
M
J
J
A
S
O
N
D
Trout seasonSeason
1
2
2
2
2
2
2
1
Reservoir BuzzerHatch
2
3
3
3
3
3
2
Daphnia SwarmHatch
2
3
3
3
2
Lake OliveHatch
2
3
3
3
3
3
2
Silverhorns & LonghornsHatch
1
2
2
2
1

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

Ranked for today
Rainbow fly box

Boat-friendly, competition-influenced, often wind-driven, with strong top-of-water periods but also classic deep searching and washing-line phases.

What's coming
Plan ahead
5-day outlook
Condition match
45%
Cloud50%
Wind30%
Temp60%

Conditions are away from Llyn Trawsfynydd Reservoir's sweet spot — it usually fishes best in ripple wind with cloud skies.

Other water nearby · 5
Afon Prysor
1.0 km
Brown trout
Llyn Eiddew Bach
5.0 km
Brown trout
Llyn Eiddew Mawr
6.0 km
Brown trout
Llyn Tecwyn Uchaf
6.0 km
Brown trout · Rainbow trout
Llyn Dubach y Bont
7.0 km
Brown trout
Gallery · 5
  1. Llyn Trawsfynydd from the shore, open water filling the foreground with moorland hills beyond.
    Llyn Trawsfynydd
  2. Llyn Trawsfynydd, Gwynedd — a still lake view with open water and the surrounding upland landscape.
    Llyn Trawsfynydd
  3. Llyn Trawsfynydd lake shore with open water and moorland backdrop under an overcast Welsh sky.
    Llyn Trawsfynydd
  4. The southern end of Llyn Trawsfynydd, open water, a grassy shoreline and the Rhinogydd hills beyond.
    The south end of Llyn Trawsfynydd
  5. Terrain map of the venue
    Terrain map
About this water

Llyn Trawsfynydd sits high in Snowdonia — a dramatic, wind-blown upland water surrounded by moorland and crag. It's fished from the bank only (no boats), and the isolation and altitude give it a genuine mountain-water atmosphere. Wild and stocked trout inhabit the water; the scenery is spellbinding. Spring and early summer (April–May) see the best hatches — small olives and early buzzers. The granite moorland setting means soft, slightly acidic water and fast clearing after rain, so lure fishing can be briefly productive after spates. This is a place where you go for the surroundings as much as the sport — the kind of water that teaches you patience and presence.

  • Reservoir
  • Granite
Water quality (WFD)
  • Ecology: Moderate
  • Chemical: Unknown
What this classification means

NRW WFD Cycle 3 (lake) — Llyn Trawsfynydd: Moderate ecological status. Re-tagged 2026-06-18 from a Dwyryd river reach to the reservoir's own lake waterbody. Source: NRW DataMapWales WFD Cycle 3 classification layer.

NRW (Wales) · GB31034870

Why this score
  • Wind conditions (windy) are not ideal for this water.
  • Buzzer is in its seasonal window, boosting the chance of targeted feeding.
  • Reservoirs of this kind fish well at this time of year.
Directions
Apple MapsGoogle Maps
Seasons & zones
  • Trout 15 March → 5 October
Related guides
Big Stillwater Playbookreservoir tacticsReservoir Entomologymatching the hatchThermal Layers & Troutreading depth and temperature
Casting instructors near here
Booking & contacts
Boards & regulators