River Mellte

Afon Mellte or the River Mellte is a river in Wales. It is formed by the confluence of the Afon Llia and the Afon Dringarth. It then flows south through the village of Ystradfellte to Pontneddfechan where it joins with the Nedd Fechan to become the River Neath (Welsh: Afon Nedd). The river derives its name from Mellt - the Welsh word for 'lightning' - after its tendency to rise and fall rapidly in response to heavy rainfall.

The entire river runs underground for 1/2 km at Porth yr Ogof as it crosses the Carboniferous Limestone outcrop. It resurges at Pwll Glas. There are fifteen known entrances to the cave system. It is a complex maze of passageways, large and small, mostly developed along one or two major bedding planes. In extreme flood conditions the entire cave can flood to the roof. It is used heavily by school and adventure groups though sections of the cave can be hazardous for the unwary. There have been a number of fatalities over the years.

Downstream of Porth-yr-Ogof and within 'Waterfalls Country', the river flows over several falls, the most significant of which are Sgwd Clun-gwyn, Sgwd Isaf Clun-gwyn and Sgwd y Pannwr - meaning 'white meadow fall', 'lower white meadow fall' and 'fall of the woollen washer (or fuller)'.

The larger part of the river and its valley is protected both as a site of special scientific interest (Dyffrynoedd Nedd a Mellte a Moel Penderyn SSSI) and as a special area of conservation (Coedydd Nedd a Mellte SAC) in respect of the bryophyte flora which the deep and wooded nature of the valleys help to support.

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Wikipedia