Historic Landscape Characterisation

Mynydd Hiraethog

Lakes, Reservoirs and Pools

Streams, rivers, natural lakes and reservoirs form a distinctive and important element in the landscape of Mynydd Hiraethog. The natural lakes are Llyn-y-foel-freche, Llyn Bran, Llyn Alwen and Llyn Aled which are between about 2ha and 45ha in extent and together total about 86ha. Of these, however, both Llyn Aled and Llyn Bran have been enlarged by the construction of dams in the earlier 20th century. A further small natural lake known as Llyn Dau-ychain, about the size of Llyn-y-foel-freche, lay in the area now occupied by the Alwen Reservoir. Llyn Alwen, the largest of the undeveloped natural lakes is an important element in the designation of the Mynydd Hiraethog SSSI. A number of smaller pools are also to be seen on Mynydd Hiraethog, notably on the western side of the moor on Fawnog-fawr and Swch Maes Gwyn, many of which occur in areas of peat bog and probably derive from the peat cutting which continued to be carried out into the 1950s and in some instances more recently.

The major reservoirs on Mynydd Hiraethog are the Alwen Reservoir (150ha), Aled Isaf (28ha) and Llyn Brenig (354ha), with a total surface area of about 530ha, submerging earlier prehistoric sites and largely abandoned fields and farms of medieval and post-medieval origin in the case of the Alwen and Brenig reservoirs. The earliest the Alwen Reservoir, built between 1911-16, in the relatively narrow valley of the Alwen to the west of Pentre Llyn Cymmer. The tall curved dam of rock-faced concrete blocks with its Italianate valve house lies in a woodland setting to the east of the dam, the reservoir snaking its way into the heart of the moorland further west. Below the dam are modern water treatment works as well an older works, barrack housing and a terrace of workers' houses contemporary with the construction of the dam. The reservoir was first built to supply the town of Birkenhead but today it acts as a regulating reservoir supplying north-east Wales, water supplies being taken from the river it feeds rather than the reservoir itself. The smaller reservoir at Aled and Aled Isaf are slightly later in date, having been completed in 1934 and 1938 respectively. Aled Isaf, again has a tall curving concrete dam, with a simpler concrete valve house, which carries the modern road. The smaller reservoirs at Llyn Aled and Llyn Bran were created by extend pre-existing natural lakes by means of much lower embankments of earth and stone which also act as road causeways, with a stone valve house in the case of Llyn Aled and a sluice in the case of Llyn Bran. Llyn Aled and Aled Isaf are linked, and had originally been intended to supply water to Rhyl and Prestatyn on the North Wales coast, though due to lack of funding the aquaduct was never built and today the reservoirs are used to control the flow of water into the Afon Aled. Llyn Brenig, the largest of the reservoirs on Mynydd Hiraethog, built between 1973-76, with its massive earth dam across the broader valley of the river Brenig, reflects a more recent generation of design and engineering involving large-scale earthmoving rather than the use of concrete, The reservoir provides water for homes and industry in north-east Wales as well as helping to regulate the seasonal flow of the Dee and supplying the Llangollen Canal. The contrast between the design and scale of the Alwen Reservoir and Llyn Brenig provides an interesting illustration of the changes in concept and design of reservoirs and their impact upon the landscape during the course of the 20th century, influenced by the technology available to the construction industry, which in turn influenced the kind of valley chosen for reservoir construction and the kind of landscape created as a result, which have become an important focus of recreational activities, considered below.