The Dyfi Catchment is located in west Wales, in Gwynedd. The town of Machynlleth lies in the centre of the catchment.
Situation, relief and drainage
The Dyfi Catchment and Woodland Research Platform is situated in the Dyfi catchment, with an altitudinal range from 905 m at Aran Fawddwy in south Snowdonia to sea level at the Estuary at Aberdovey. On the eastern slopes of Aran Fawddwy is the small lake named Creiglyn Dyfi, the source of the River Dyfi. The Catchment area extends into the estuary in order to encourage research that encompasses both terrestrial and marine science, and the links between them. The Catchment drainage is dominated by the Dyfi, but there are many tributaries. The Dyfi floodplain is approximately one third km wide at the junction with the Afan Cerist 40 km upstream from Aberdovey and widens considerably as the estuary is approached. Above the floodplain, the relief is characterised by steep hills and mountains between a fluvially incised landscape.
Western Wales is amongst the sunniest and mildest parts of Britain, but the coastal regions are very exposed to westerly winds. Rainfall in the Dyfi catchment is comparatively high, from c. 1000 mm at the coast to over 2000 mm at the fringes of the catchment. The coast benefits from an early rise in spring temperatures, but summer temperatures are comparatively low. Accumulated temperate decreases with altitude and distance from the coast.
Geology and soils
The geological succession in the Dyfi Catchment spans the mid-Ordovician (Caradoc Series) to mid-Silurian (Wenlock Series) Periods. The oldest Ordovician rocks occur in the northern part of the area where the Aran Fawddwy Formation comprises 350-400 m of acid ash-flow tuffs. In the south, late Ordovician (Ashgill Series) argillaceous sedimentary rocks crop out in the core of the Machynlleth Inlier and in periclinal folds along the western margin of the Plynlimon Inlier. The Ordovician sequence is conformably overlain by Silurian sedimentary rocks dominated by a thick succession of turbiditic mudstones.
The main soils of the Dyfi valley are podzolic soils, ground water gleys and peat soils. Podzolic soils have a peaty surface layer and humus and/or iron enriched subsoils. Ground water gleys occur along the River Dyfi and are permeable, seasonally waterlogged and affected by the groundwater table. Peat soils are classified as having more than 40 cm of organic material that has a well drained surface layer. Soils of the alluvial flats and terraces are gleyed and peaty. Saltmarsh soils, located along the estuary, are naturally wet, have a loamy texture and are lime-rich, but saline. Loamy and clayey floodplain soils and soils of the coastal flats have a natural high groundwater with moderate fertility.
Woodland and forest
Native woodland which developed over much of the area in postglacial times was greatly reduced from Neolithic times onward. Remnants of native woodland survive in the valleys and on the lower hillsides. There are 1,325 hectares of Ancient Semi Natural Woodland and 1,359 hectares of Plantations on Ancient Woodland Sites (PAWS), giving a total land cover of 3.5% for ancient woodland in the catchment.
Large areas were acquired by the Forestry Commission in the 1930s and many upland sheep walks planted with conifer species. During the 1960s to the 1980s, private conifer forests were also established. Currently 14 % of the catchment is conifer woodland.