Discussion Paper - Part A - Victorian Environmental Assessment ...

The North East catchment region includes the Ovens,King, Kiewa, Mitta Mitta and Upper Murray rivers andcovers approximately 1.9 million hectares. Although thisis only 2 percent of the Murray-Darling Basin, the regioncontributes 38 percent of the total water to the system(NECMA 2004). All these rivers enter the River Murrayupstream of Yarrawonga.As its name suggests, the Goulburn Broken catchmentregion includes the Goulburn and Broken Rivers. TheGoulburn River is Victoria’s largest river basin covering anarea of 1.6 million hectares (7.1 percent of the State).The Goulburn River is approximately 570 km long andhas a mean annual water discharge of 3040 GL. TheBroken River (a tributary of the Goulburn) basin covers772,386 ha (3.4 percent of Victoria), including theBroken Creek catchment that includes the decommissionedLake Mokoan. Over half of the annualstream flow occurs between July and September, with amean annual flow of 325 GL (GBCMA 2003).The Goulburn and Broken River basins together coverapproximately 2.4 million hectares (10.5 percent ofVictoria and 2 percent of the Murray-Darling Basin).Combined, the river basins generate around 11 percentof the basin’s water resources and flow into the RiverMurray upstream of Echuca. These rivers provide largevolumes of water to the River Murray during floods,thereby avoiding the physical constraint imposed by theBarmah Choke—see below.Combined, the North East and Goulburn Brokencatchment regions contribute 49-50 percent of theMurray Daring Basin’s overall water resources.The North Central catchment region contains theLoddon, Campaspe and part of the Avoca River basins.The Campaspe River with an approximate annual flowvolume of 315 GL and the Loddon River with 263 GLflow directly into the River Murray. In contrast, the AvocaRiver discharges into the Avoca marshes and Lake Boganear Swan Hill (NCCMA 2003).The Mallee catchment region contains three main basins,the Mallee Basin covers 2,802,688 ha, the Avoca Basincovers 1,235,246 ha and the Wimmera River Basincovers 2,401,130 ha (Mallee Catchment ManagementAuthority 2003). Rainfall is relatively variable in thiscatchment region. Compared with the Goulburn Brokenand North East catchment regions, the Mallee catchmentcontributes little if any run-off into the River Murraysystem, reflecting its semi-arid climate.Flood RegimesUnder natural (pre-European river regulation) floodconditions, around half the surface water run-off fromthe catchments of the Murray-Darling Basin reached thesea. River flows varied considerably across the system involume and between years. Before river regulationaround 11,300 GL of water flowed down the RiverMurray to its mouth in South Australia during a typicalyear and varied from approximately 2500 GL during adry year to 40,000 GL in a very wet year. This variation isa key feature of the flooding and flow regime of theRiver Murray.Under current water regulation conditions (see chapter15), flow is reduced to about 3000 GL in a typical year,or 27 percent of flow under natural conditions (Murray-Darling Basin Commission 2002). The extraction of thisamount of water from the River Murray system hasadversely affected many species in the study area, mostsignificantly in wetlands and river red gum forests. It hasalso significantly affected water quality and salinity (seechapters 5 and 15). However, the extracted watersupports important agriculture around the study area(see chapters 8, 13 and 15).For flooding to occur across the River Murrayfloodplains, river flow must breach the channels andflow over the river bank onto the surroundinglandscapes. Flooding enables some tributaries such asthe Lachlan which normally terminates in marshes, toreach the Murray River. Remaining surface water infloodplains and wetlands eventually permeates throughthe soil to recharge ground water systems. Prior to riverregulation the natural flood regime of the River Murraycould be divided into three types, large-scale floods,mid-range floods and low flow conditions (Gippel &Blackham 2002).Large-scale floods generally occurred once every 20 to100 years. These floods extended from the River Murraychannel and out over the vast floodplains (see Map 4.5for distribution of a 1 in 100 year flood). These floodswere crucial for connecting wetlands, swamps andmarshes along with the broader floodplain to the mainriver systems and for transferring energy and nutrientsbetween the river and floodplains. They were also crucialfor flushing the entire river system. Large-scale floodslasted for 2 to 6 months, depending on rainfall andsnow in the upper catchment. Large floods on theMurray usually coincided with large-scale floods onVictorian rivers such as the Ovens, Broken, Goulburn,Kiewa and King.Discussion Paper41