Complex sentence: The lake provides hydro-electric power via Manapouri Power Station, in the West arm, which discharges water through a 10-km tailrace tunnel to Doubtful Sound and the sea.
Split into simpler sentences: The lake provides hydro-electric power via the Manapouri Power Station sited in the lake's West arm. The station discharges water 10km away through a tailrace tunnel to Doubtful Sound and the sea.