07 Nov Andrew Greenfield
Program Manager – TLM Hattah For the last 8 years I have been working as a floodplain ecologist at the two Living Murray Icon Sites managed by the Mallee Catchment Management Authority. This includes the Lindsay-Wallpolla Islands Icon Site and the Hattah Lakes Icon Site. The focus of my work has been to restore floodplain communities through the reinstatement of more natural water regimes to a floodplain subjected to 95 years of highly regulated water.
Restoring a Disconnected Floodplain in a Highly Regulated River System – The Hattah Lakes, Australia
Throughout the world waterways are regulated to improve the reliability of water supply for human needs and agriculture and hydroelectric power generation. Regulation of rivers for extractive purposes alters the natural flow characteristics of the river system through tempering the seasonal flood peaks, potentially changing timing of flood peaks and decreasing flow volume in the river. The cumulative effect is the decrease in out-of-channel flows and the loss of floodplain inundation.
The Hattah Lakes in northwest Victoria, Australia forms part of the floodplain in the Lower Murray River and is subject to relatively low annual rainfall (270 mm p.a.). The relatively low rainfall in this area means the regular flood events across the floodplain are critical to the ecology of the area. River regulation in the River Murray has resulted in a decrease in regular mid-sized flood events across the Hattah Lakes floodplain. These mid-size events filled wetlands, provided connectivity between the river and the floodplain and provided periodic water to forest and woodland communities on the higher terraces of the floodplain.
Restoring floodplain inundation requires innovative thinking and engineering. The development and construction of water delivery infrastructure at the Hattah Lakes, has allowed water for the environment to be delivered across the floodplain at the Hattah Lakes. Over the last 5 years, water delivered to the Hattah Lakes floodplain has provided filling and drawdown events in the lakes and inundation across the River Red Gum woodland and Black Box woodland communities.
The delivery of water to the floodplain has played a crucial role in the restoration of plant and animal communities. Where water has been delivered tree canopy health has improved, waterbird breeding events have been stimulated and rare and listed species have been reinstated.
Restoring Vegetation on the Hattah Lakes Floodplain by Delivering Water for the Environment
Decreased overbank flows to the floodplain of the Lower River Murray has negatively impacted vegetation communities in this reach. The extended period of minimal floodplain inundation has left the River Red Gum and Black Box woodlands of the Hattah Lakes floodplain showing signs of severe water stress. In addition, understory communities inundated frequently under natural flow conditions have shifted to a community dominated by dry and drought tolerant species. In order to restore the hydrological character of the Hattah Lakes floodplain to pre-regulated conditions, water delivery infrastructure was built at key points on the floodplain to be able to mimic flood events across the floodplain.
Delivering environmental water to the Hattah Lakes floodplain has resulted in recovery of vegetation communities. Most noticeable is the improvement in canopy condition of trees on the floodplain. In addition to the improvement of tree health, the understorey vegetation has seen a shift from the dry and drought tolerant functional groups to the terrestrial damp functional group on the lower terraces of the floodplain and aquatic species returning to wetlands.
At the species level, over 30 rare and threatened species are being regularly sampled since watering actions have commenced. In 2017/18 a healthy population of Slender Spurge (Sauropus trachyspermus) was sampled. The slender spurge is a plant listed as very rare in Victoria and is known only from the River Murray floodplain around Mildura and to the Hattah Lakes (about 100 km of floodplain in total). This plant typically occurs following flood events and has not been recorded since 1982. The second unique plant to be recorded is Pluchea rubelliflora. This is the first record of this plant in Victoria and represents a substantial range extension for this species.