Complex sentence: The pump operated until 1951, by which time the local drainage system had been linked into King's Sedgemoor Drain, which discharged further down the River Parrett; the water levels dropped and the pump was unable to draw the water from the rhyne.
Split into simpler sentences: The pump operated until 1951, by which time the local drainage system had been linked into King's Sedgemoor Drain which discharged further down the Parrett. As a result the water levels dropped and the pump was unable to draw the water from the rhyne.