Rivers: Afon Nant Peris
FSC Rhyd-y-creuau Field Centre
The Afon Nant Peris is a small mountain stream running down the Llanberis valley situated
on the North-Eastern flank of Snowdon. It has its source at Pen-y-Pass (SH 647 557) at a
height above sea level (OD) of 359m. The Pen-y-pass is also the site of a youth hostel and
the start of two main footpaths up Snowdon, the Miner’s and PYG tracks. The river flows
for approximately 6km in a North-Westerly direction before it reaches Llyn Peris (SH 599
588) at a height above sea level of 105m.
A 3D perspective of the Afon Nant Peris and the Llanberis valley with Snowdon nearby
Once the river reaches Llyn
Peris it is diverted into an
artificial underground
channel bypassing Llyn Peris
and draining into Llyn
Padarn further North-West
near the village of Llanberis.
The river then continues
naturally as the Afon Seiont
meeting the sea eventually
at Caernarfon (GR 476 627).
The river system from
Pen-y-pass to Caernarfon is
approximately 28km in
length.
The Afon Seiont meeting the sea at Caernarfon
Rivers: Afon Nant Peris
FSC Rhyd-y-creuau Field Centre
Caernarfon
Marchlyn Mawr
Llyn Padarn
Afon Seiont
Afon Nant Peris
Llyn Peris
Source
Snowdon
OS map extract showing the Afon Nant Peris and it’s continuation channel, the Afon Seiont
Location of fieldwork
OS map extract showing the Afon Nant Peris and fieldwork location
Data collection normally
takes place on the first
2.5km of the Afon Nant
Peris until the river passes
under the A4086.
This is predominantly due
to health and safety
considerations, with the
river commonly
exceeding knee height
after this point.
Groups are often dropped
off at the Pen-y-pass car
park and picked up
downstream at the
Cromlech boulders, huge
masses of volcanic
breccia (volcanic rock
composed of angular
volcanic fragments
cemented together).
Rivers: Afon Nant Peris
FSC Rhyd-y-creuau Field Centre
The Afon Nant Peris flows down an unusual
valley. During the Devensian glacial event
(ending approximately 10,000 years ago) large
quantities of ice accumulated in the mountain
cwms and valleys, flowing downhill under the
influence of gravity. These glaciers carved out
large glacial troughs with steep sides and flat
bottoms. The Peris flows down such a valley
and is therefore termed a ‘mis-fit stream’.
The valley flattens out in places and energy is
directed laterally causing a series of meanders
to form.
The Afon Nant Peris showing a flat bottomed U-shaped
valley with meanders.
The Afon Nant Peris is a relatively small
mountain stream that responds very quickly
to rainfall events, with many tributaries and
storm channels flowing straight down the
steep valley sides. A high annual
precipitation rate and thin, saturated soils
also contribute to the channel filling up very
quickly. These factors combined result in a
short ‘lag’ time and high peak discharge.
Fortunately the normal levels of the river
allow students to safely enter the river and
gather data.
Tributaries bursting their banks on the Afon Nant Peris
Even though the glacier that filled this valley
has gone the catchment area has still been
subject to cold temperatures due to its high
mountainous environment. Freeze-thaw
weathering has been, and still is, a major
factor influencing the areas present
appearance.
The upper catchment area is characterised by
steep slopes covered by loose scree and
boulders. At times these large accumulations
of material have developed immediately
adjacent to the Afon Nant Peris.
The Afon Nant Peris with bankfull discharge during heavy rain
Rivers: Afon Nant Peris
FSC Rhyd-y-creuau Field Centre
These large angular rocks have been
displaced by the action of freeze-thaw
and have tumbled down from the cliff
faces above over the last 10,000 years.
The scree slopes are a constant input of
bedload into the river and can affect
channel characteristics e.g. reducing
velocity and efficiency, modifying
channel dimensions and increasing
bedload size and angularity.
Scree and boulder fields in the upper catchment of the Afon Nant Peris
The valley exhibits a diverse geology including sandstones, Rhyolitic tuffs, pillow lavas,
Breccias and welded ash tuffs (Pitts Head Tuff). Differences in rock type play a big role in
erosion rates and river dynamics in the catchment. The valley itself is aligned with an
ancient fault system along which a large caldera collapsed in the area causing violent
volcanic eruptions. Surrounded by the light grey slabs of Rhyolitic tuff at the source, you are
really looking at ancient volcanic ash from this major eruptive event that settled under a
shallow sea 430 million years ago.
Bedded
Pyroclastic
Formation
Sandstones,
siltstones and
mudstones
Sandstones,
siltstones and
mudstones
Intrusive Igneous
Simplified geology of the catchment area. Map taken from the British Geological Survey
Lower Rhyolitic Tuff
Rivers: Afon Nant Peris
FSC Rhyd-y-creuau Field Centre
The Afon Nant Peris differs slightly from a theoretical long profile showing several
irregularities and small ‘knick points’ along its course.
Combinations of factors have influenced these irregularities including sea-level changes &
the recent action of ice eroding distinct steps in the valley profile.
Long profile of the Afon Nant Peris
Energy production
When the river reaches Llyn Peris lower in the valley it is diverted underground into an
artificial channel. This channel bypasses Llyn Peris and drains into Llyn Padarn further
downstream. This is due to Llyn Peris being a storage reservoir as part of a large pumpstorage HEP scheme.
Water is released from a higher lake called Marchlyn Mawr on the slopes of Elidir Fawr and
races down giant pipelines. Electricity is generated from large turbines turned by the power
of the descending water. Water that accumulates at the bottom storage reservoir (Llyn
Peris) is then pumped back up into the higher storage reservoir overnight when energy
demand is lowest.
During the evenings the water levels of Llyn Peris can be extremely high and there is a
danger of the dam being overtopped. This is why the Afon Nant Peris is diverted past Llyn
Peris otherwise serious flooding could be possible. When the river leaves Llyn Padarn it
becomes the Afon Seiont, flowing for a further 14km before meeting the Menai Straits at
Caernarfon.
Marchlyn Mawr reservoir. This forms the ‘top lake’ in the pump storage hydro-electric scheme
Rivers:
Rivers: Afon
Afon Nant
Nant Peris
Peris
FSC
FSC Rhyd-y-creuau
Rhyd-y-creuau Field
Field Centre
Centre
Flood Prevention along the Nant Peris
Flood management of the Afon Nant
Peris centres around the village of
Nant Peris, a small settlement at the
end of the Llanberis valley.
Gabions
Gabions (wire cages containing rocks),
levees (artificially raised banks) and
stone walling of the river bank can be
observed immediately upstream of the
village.
Levees
Basic flood defences immediately upstream of Nant Peris village
After the river passes the village of Nant Peris it is
diverted by a dam into an artificial underground
channel, bypassing Llyn Peris for 2km eventually
draining into Llyn Padarn.
The channel diversion has been designed as a self
regulating flood protection scheme. In heavy rain
when river and lake levels rise, some of the water can
spill over the artificial channel into Llyn Peris utilizing
it as extra catchment storage. Given particularly high
river levels, flood gates are opened to allow more
water into Llyn Peris.
Flood gates open - 2009
Flood gates open during high river levels in 2009
Flood gates open during high river levels in 2009