Complex sentence: The River Tawe which was already being polluted by the coal mining industry suffered badly but even worse affected was the stream that meandered through the lower half of the valley, the Nant y Fendrod.
Split into simpler sentences: The River Tawe was already being polluted by the coal mining industry and suffered badly. Even worse affected was the stream that meandered through the lower half of the valley, the Nant y Fendrod.