Complex sentence: The river used to be sourced from the many rivers and streams running off the mountains surrounding the Vyrnwy valley, however since the Lake Vyrnwy dam was built in the 1880's the river has flown directly form the base of the dam.
Split into simpler sentences: The river used to be sourced from the many rivers and streams running off the mountains surrounding the Vyrnwy valley. However, since the Lake Vyrnwy dam was built in the 1880's, the river has flowed directly from the base of the dam.