The river, which from its source to perhaps as far as Deri, is more of a brook and is shallow, reasonably swift-flowing and fast-rising. It rises on the moorland just north of the village near Rhas Las pond and its flow is supplemented by springs that emerge from the many coal tips which surround the village to the west, north and east. These coal tips act as reservoirs which absorb the rainfall and releases it over a prolonged period.
Over recent years, following the removal and landscaping of these tips, it is noticeable that the “dry weather flow” of the brook is less than it was when the tips were their original size.
Prior to, and following the closure of the local collieries, the river ran clear, however, this was not always so since, during the 1960’s, 70’s and 80’s a company called Ryan established a washery at the site of the tips between Fochriw and South Tunnel, where the residual coal which was left in the tips was recovered and the slurry produced by this, and the mechanical recovery processes, caused heavy pollution. However, now that this activity has ceased, the brook once again runs clear and has become a nursery stream for trout, whereas during the 1950’s only fish, locally known as barabits, were in evidence.
As an aside, the coal content of the waste tips was as high as 40% and the earlier the age of the tip the higher was its coal content. The reason for this is that during the early mining era little use was found for small coal since the saleable produce was lump coal. Thus, this unwanted product was disposed of with the slag, rock and other detritus that formed the slag heaps.
The rocks of the region were formed between 280 and 240 million years ago. This remote age is known as the Carboniferous period and it began with a deep trough occupied by a clear sea. Here the sediments deposited were the accumulation of corals, shells and fine mud, largely built of the remains of such marine life. The resultant rock, Carboniferous Limestone, was extensively quarried for use in iron-smelting and for the production of lime, stone-dust and railway ballast. However, this quarrying is now mainly for ballast and gravel for use in the construction industry.