Complex sentence: The River Brue originates in hills, to the south west of the catchment area, close to the border with Dorset which are also the sources of the River Wylye and the Dorset Stour which flow south to the English Channel.
Split into simpler sentences: The River Brue originates in hills to the southwest of the catchment area, close to the border with Dorset. The same hills are the locale of the sources of the River Wylye and the Dorset Stour which flow south to the English Channel.