The Rangitata Diversion Race was a dream in the minds of the pioneering farmers in Mid Canterbury when the 750,000 acre plain was first farmed in the mid-19th Century. Regarded as the largest area of nearly flat land in New Zealand, all development was closely linked to water problems – water races to supply stock on light soils and water courses to drain heavy swamp lands.
Despite high expectations of development, it took the massive unemployment of the 1930s depression to provide the catalyst for the work to start in what would become the RDR. The Minister of Public Works, Bob Semple the ebullient and colourful minister in the first Labour government wrote in a foreword to a government booklet ”Water put to work”; “We as a nation cannot afford the continued idleness of such extensive resources not only for our own good, but for the benefit of the world at large”. Work began on the scheme on April 2nd 1937 and was completed in November 1944. Race water first generated electric power at Highbank power station on June 8th 1945. Expected to cost £1.5 million pounds, the scheme costs grew to £2 million pounds.
Since 1945 there have been a number of amendments to refine the efficiency of the RDR. Montalto, the second hydroelectric power station was built in 1981 and started producing electricity a year later. The Sandtrap was designed and built at a similar time to remove much of the suspended sediment from the water by reducing the velocity of the flow. The sediment causes wear and tear on the hydro electric turbines and can cause gastric problems for sheep and lambs when washed out onto paddocks. Conversely the sediment is a good bonding agent that can help to plug microscopic holes in the race lining.
The RDR Resource Consents were renewed by Environment Canterbury in 2008. Part of the consent conditions required the RDR to install fish diversions to reduce the number of native and exotic fish from becoming entrained in the Race. 2Kms downstream from the Rangitata River intake a behaviourial screen called a Bioacoutic Fish Fence that uses bubbles and underwater sounds to divert fish to a channel returning them to the river, was installed in 2007. At the South Ashburton River, a physical rock gallery was installed in 2008 together with a bypass to return fish to the river.