Complex sentence: The source of the river is an area called Will Narr at Hordern Stoops, along Spitler's Edge-the Chorley/Bolton boundary-on the West Pennine Moors and feeds the Yarrow reservoir, which in turn feeds the 3 reservoirs at Rivington and Anglezarke.
Split into simpler sentences: The source of the river is an area called Will Narr at Hordern Stoops, along Spitler's Edge-the Chorley/Bolton boundary-on the West Pennine Moors. The river feeds the Yarrow reservoir, which in turn feeds the 3 reservoirs at Rivington and Anglezarke.