In 1967, the construction of the Snowy Mountains Scheme (Scheme) resulted in significant alteration of the hydrology of the Snowy River, with 99% of the mean annual natural flow diverted to the Murray-Darling Basin. The post Scheme hydrology resulted in no pre-regulation hydrological metrics being achieved in the reaches below Jindabyne, NSW, and a significant decline in the condition of the river. In 2002, the Australian, Victorian and NSW Governments agreed to release up to 21% of the mean annual flow (i.e. 212 GL/year) in a staged release program to improve river condition. The key challenge was how to restore the hydrology of the montane river, knowing none of the pre-scheme hydrological metrics could be achieved.
Since 2002, four methods have been used to deliver environmental water in the Snowy River. These methods included: (i) a tributary release (i.e. 2002-2006), (ii) a default settings based on monthly average targets (i.e. 2006-2009), (ii) a building blocks approach based on perceived ecological water requirements (i.e. 2009-2013), and (iv) a surrogate unregulated montane river approach (i.e. 2013-16). A complimentary strategy was also adopted in the fourth stage to increase basal resources (i.e. Dissolved Organic Carbon (DOC), dissolved silica and macro nutrients) and manage a-seasonal warm water via tributary autumn release.
An evaluation of the hydrological changes associated with the four environmental water strategies implemented between 2002-03 and 2015-16 is presented. The tributary release provided a minor hydrological signal reflective of a small unregulated montane river, the default monthly settings provided a muted seasonal signal and provided limited flow variability. The building blocks method provided higher flow events while using over 50% of the annual volume in a single event, and thus limited daily flow variability. The flow scaling method reflected the hydrology of a montane river while still achieving high instantaneous discharge rates.