The Clutha River is New Zealand’s largest river, both in terms of discharge and catchment size. It has a large seasonal flow variability with large spring and summer flows and very low flows in winter. The river is important for its contribution to energy production from two large hydro dams, irrigation for agriculture and horticulture, and tourist activities. The Clutha River flows are modified by discretionary releases of water from the only storage lake in the catchment, at Hawea. The mountainous South Island topography to the west means that a large part of the catchment is in a rain shadow and is regularly subject to long periods without any significant precipitation, similar to many other rain shadow catchments globally.