Afon Dyfi ger Mallwyd

Site of Special Scientific Interest · 52.6994, -3.6887

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Also within

Gwynedd CountyPowys County Eryri / Snowdonia National Park

Habitats

Fast flowing river and stream

Geology & landscape

Geological Period: Quaternary

Landscape processes: Fluvial, Glacial

Description

Description Afon Dyfi ger Mallwyd is of special scientific interest for the wide range of fluvial and glacial/fluvioglacial landforms (e.g. terraces, morainic mounds, kettle basins and an alluvial fan). Analysis of these landforms and of the sediments exposed in river bank sections has facilitated the development of an integrated model of the evolution of this fluvial system since Late Devensian times. The site incorporates a stretch of the floor of the middle Dyfi Valley extending downstream from the road bridge at Dinas Mawddwy for a distance of approximately 2.4 km. This reach of the Afon Dyfi is important for a sequence of river terraces. They are unusually well-preserved due to the local confinement and low lateral mobility of the river. Three main terraces provide an important record of Holocene river changes and good bank sections reveal the composition of the terraces and their relationships to the fluvioglacial deposits which underlie them. The terraces are derived from reworked glacial deposits and comprise mainly medium to coarse gravels typical of many rivers in mid-Wales. Remarks: The site lies within Snowdonia National Park.

Plant communities

A9 — Fast flowing river and stream, Marginal and inundation, Pond ditch and canal, Sluggish river and stream

Potamogeton natans community

Schoenoplectus lacustris, Juncus bulbosus, Sparganium angustifolium, Sparganium erectum, Sparganium minimum, Alisma plantago-aquatica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Elodea canadensis, Lemna minor, Lemna trisulca, Nymphaea alba, Callitriche platycarpa, Callitriche stagnalis, Littorella uniflora, Potamogeton natans, Ranunculus trichophyllus, Glyceria fluitans, Chara subspinosa

A14 — Fast flowing river and stream

Myriophyllum alterniflorum community

Isolepis fluitans, Juncus bulbosus, Fontinalis antipyretica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Lemna minor, Callitriche stagnalis, Littorella uniflora, Potamogeton filiformis, Potamogeton gramineus, Potamogeton natans, Ranunculus peltatus, Chara spp., Nitella hyalina

A17 — Chalk river and stream, Fast flowing river and stream

Ranunculus penicillatus ssp. pseudofluitans community

Berula erecta, Nasturtium officinale, Elodea canadensis, Lemna minor, Callitriche stagnalis, Veronica beccabunga, Polygonum amphibium, Polygonum hydropiper, Potamogeton crispus, Potamogeton pectinatus, Ranunculus penicillatus ssp. pseudofluitans

A18 — Fast flowing river and stream

Ranunculus fluitans community

Fontinalis antipyretica, Fontinalis squamosa var. curnowii, Myriophyllum alterniflorum, Myriophyllum spicatum, Elodea canadensis, Mentha aquatica, Lemna gibba, Lemna minor, Potamogeton perfoliatus, Ranunculus fluitans, Glyceria fluitans, Phalaris arundinacea