Spate · Slate · North West Wales / Gwynedd / Eryri

Glaslyn

A rocky river gorge with large boulders in low, clear water, flanked by wooded hillsides, on the Afon Glaslyn near Aberglaslyn.
Contributor photo

IMGP8547

Matt Buck - CC BY-SA 2.0

Eryri water of the proper romantic kind — out of Llyn Dinas under Yr Wyddfa, through the Aberglaslyn pass, and down to Tremadog Bay at Porthmadog.

Poor · Brown Trout
CDC Emerger · 16-18
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.
80% confidence
What moved it
  • Level0.63 mLast reading 17h ago
  • Water temp21.4°C
  • ClarityClear
Today’s fly
CDC Emerger
CDC Emerger16-18
Upstream dry, in the film
Conditions on the water
Live gauge
Level
Steady
0.63 m
Last reading 17h ago
Water temp21.4°C
ClarityClear
Weather23°C
WindSW 9 km/h
Pressure1021 hPa
Rain · recent0.0 mm
Rain · ahead0.0 mm

Live readings — water temperature is an estimate where the gauge does not record it.

Water temperature for brown trout
Warm — slow
21°C est.ideal 10–16°C
0°14°28°
How to fish it · for brown trout
When
Through the open trout season. Spring olives later than lowland rivers — the Snowdonia water warms slowly. Summer sedges and beetle falls in the warmer months.
Where
Upper Glaslyn around Beddgelert and the Llyn Dinas inflow. Pocket water and small pools — small-stream scale.
Method
Upstream dry to risers; dry-dropper through the riffles. North Country spider on the swing in cooler water. Spate-driven on falling water — pick the window after rain rather than fishing a flat low river.
Kit
9 ft #4 — Welsh freestone default. Floating line, 9 to 12 ft leader to 4 to 5 lb fluoro. Studded boots — slate slabs run slick.
Why this works
⚠️ Water at 21.4°C — above the brown trout caution line (18°C). Fish dawn or dusk only. Land it fast, wet hands, no air shots. Otherwise very poor — water clarity is in the right range.
Hatch timeline · todayQuiet day

Hatch predictions

Today's headline hatch shown — see all 4 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
J
F
M
A
M
J
J
A
S
O
N
D
Salmon runRun
1
2
2
2
2
2
1
Sea trout runRun
1
2
2
2
2
2
2
1
Trout seasonSeason
1
2
2
2
2
2
1
Daddy Long LegsHatch
2
3
2
March BrownHatch
2
3
2
GrannomHatch
2
2
Large StoneflyHatch
2
3
2

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

Ranked for today
Brown Trout fly box
BB
Black Beetle
14-18 · Pattern
Upstream dry, on top
What's coming
Plan ahead
5-day outlook
Cooler water nearby · 3
Water here around 21°C — probably warm enough to stress brown trout. These are likely cooler.
Afon Prysor
14 km
~17°C
Brown trout
Artro
17 km
~17°C
Brown trout · Salmon · Sea trout
Dwyfor / Dwyfach
14 km
~18°C
Brown trout · Salmon · Sea trout
Gallery · 2
  1. A rocky river gorge with large boulders in low, clear water, flanked by wooded hillsides, on the Afon Glaslyn near Aberglaslyn.
    IMGP8547
  2. Terrain map of the venue
    Terrain map
About this water

Eryri water of the proper romantic kind — out of Llyn Dinas under Yr Wyddfa, through the Aberglaslyn pass, and down to Tremadog Bay at Porthmadog. The Glaslyn is one of the recognised North Wales sewin rivers and a true spate stream — response time around three hours, levels swinging hard with the Snowdonia weather. NRW data is sobering: salmon at risk with a downward trend. Treat the river accordingly — the fish need every consideration. Slate and schist bedrock underlie this river. Atlantic salmon are under serious conservation pressure — Natural Resources Wales mandates catch-and-release for salmon on all rivers, so all fish must be returned.

  • Slate
Water quality (WFD)
  • Ecology: Moderate
  • Chemical: Unknown

NRW (Wales) · UKGB110065053960

The full read · show the working · for brown trout · confidence 80%
How the 19 is built — score × weight = contribution
Temperature11 × 28%3.1
Flow80 × 22%17.6
Clarity95 × 18%17.1
Feeding Time60 × 13%7.8
Pressure80 × 7%5.6
Insect activity56 × 12%6.7
Limiting factor: Water temperature (21.4°C) is outside Brown Trout's preferred range (10–16°C)= 58
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
  • Salmon1 April → 17 October
  • Sea trout3 March → 17 October
  • Trout3 March → 30 September

Sea trout: Variable seasonal (2026) — Recognised North Wales sewin river but condition-sensitive rather than guaranteed. NRW 2024 classification: sea trout at risk. Acid flushes, mining legacy and slate spoil shape the water; fast 3-hour spate response makes timing critical.

Salmon: Negligible (2026) — NRW 2024 Glaslyn catchment summary classifies salmon as at risk with a downward trend; sea trout also at risk. Acid-vulnerable soils, mineral workings, slate and quarry legacy, conifer forestry are the documented pressures. All-Wales catch-and-release applies under the 2020–2030 byelaws.

Related guides
Yorkshire & SpidersNorth Country methodsEuro Nymphingsubsurface methods
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