The Gwenllech also known as "White River" or "The River of White Stones", is in Kaedwen and flows through Kaer Morhen valley, past the keep itself, in the foothills of the Blue Mountains. Farther west it joins the Buina.
Its name means White Stones in Elder Speech.
“
The Gwenllech, the River of White Stones, flowed down the heart of the valley, foam seething between the boulders and logs washed along by the current. Here, in its upper reaches, the Gwenllech was no more than a wide but shallow stream. Up here it could be crossed without any difficulty. Lower down, in Kaedwen, in its middle reaches, the river was an insurmountable obstacle, rushing and breaking against the beds of its deep chasms.
”
— pg(s). 46, Blood of Elves (UK edition)
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The ford which Geralt must cross to get from the camp to Kaer Morhen keep is on the Gwenllech.