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

Large flood events occurred in 1917, 1956 and 1973.According to Gippel and Blackham. (2002), peak flowsrecur every 20 years or more and their frequency hasremained relatively unchanged with river regulation.Major floods are usually preceded by heavy rain that fillsstorages and reduces demand for water diversion. Withthe huge volume of run-off, dam storage capacity isreached, forcing large volumes of water to be releasedout of the storages (Gippel & Blackham 2002). However,such floods may become less likely in the future, due toclimate change reducing rainfall and run-off.Mid-range floods occurred approximately once every2 to 10 years and were usually of shorter duration thanthe large floods (one to six months). Monthly flows atthe SA border exceeded 1000 GL and occurred in 45percent of months (Gippel & Blackham 2002). Suchflooding regimes are crucial for the sustainability of thevarious ecosystems found within the study area.Today, with regulation these floods are only expected tooccur in 15 percent of months, with mid-range floodslasting only one to three months. Studies by Thoms etal. (2000, cited in MDBC 2005b) indicate that thesetypes of floods occur much less frequently now thanthey did prior to river regulation. Floods that onceoccurred every second year now occur every 6 to 8years, while floods that once occurred every 10 yearsnow occur every 25 to 30 years. This change in flowregime through river regulation reduces the frequency offlooding of wetlands, riparian vegetation andfloodplains, with dramatic consequences for species andecosystems that may be able to survive some reducedflooding but not to this extent (see chapters 5 and 14).The third type of flow regime is low flows, whichincludes periods of no flow and even—in extremedroughts—drying of much of the river bed so that theRiver was reduced to a chain of often saline ponds.Today, with river regulation and the demand for areliable water supply by downstream water users suchevents are highly unlikely to occur in the future. Now,there is nearly always some flow in the River other thanat the most downstream reaches in SA where regulationis thought to have increased the frequency and degreeof low flows.The flow regime prior to regulation was alsocharacterised by seasonality of flooding. For example,flooding usually occurred from July (winter) toNovember (late spring). Reduced flows occurred duringthe drier seasons from December (summer) to May (lateautumn). The seasonality of natural flooding alsoinfluenced water temperatures. Cold water flowedduring late winter–late spring particularly with themelting of snow. Warmer water flowed in summer–lateautumn when there was less water and higher landsurface temperatures.The river red gum forests and associated ecosystemshave adapted to these conditions of seasonal flowvariability, temperature variations, drought, and wet anddry cycles to form the rich biodiversity of the river redgum forests we see today. Figure 4.2 illustrates therelationship between the river’s flow regimes and itsanabranches, billabongs, lakes and ecologicalcommunities—particularly the lateral flow of waterunder flood conditions across the broader floodplain.Today, the natural variation of the River betweenfloods and low flows has largely been eliminated,resulting in the degradation of large areas of river redgum forests and their associated ecosystems.Environmental flows (see chapter 15) are designed toaddress these ecological concerns.Figure 4.2 Relationship between flow regimes of the river and ecological communities.(a) Low flow(b) High flow: benches covered and billabongsjoined to river(c) Over-bank flow: terrestrial floodplains,temporary wetlands inundated(d) Flow recede: wetlands recharged; organicmaterial and billabong organisms wash to riverSource: MDBC (2002)44 River Red Gum Forests Investigation 2006