Linking the geomorphology, vegetation and soils on the Murray River floodplain for salinity management
Wong, VNL, Clarke, J & Pain, C 2008, 'Linking the geomorphology, vegetation and soils on the Murray River floodplain for salinity management', paper presented to the Australian and New Zealand Geomorphology Group 13th Conference, Queenstown Tas., 10 - 15 February.
The Murray River floodplain supports a number of competing land uses, including significant areas of national parks and state forests alongside both irrigated and dryland agriculture, horticulture and croplands. As part of a multi-disciplinary study, associations between geomorphology, soils, regolith and vegetation were
integrated to answer land management questions related to salt mobilisation, irrigation practices and the overall influence of salt on floodplain health.
The Murray River floodplain downstream of Wakool Junction consists of three discrete floodplain meander belts (Fm1, Fm2 and Fm3 units) and an alluvial terrace inset into the Murray River Gorge. The modern floodplain (Fm1) is characterised by neutral to slightly acidic sandy soils of low salinity. Open forests of River Red Gums (Eucalyptus camadulensis) with minimal understorey dominate this unit. The older
floodplain units (Fm2 and Fm3) generally support low open woodlands of salt-tolerant Black Box (E. largiflorens), with occasional River Red Gums occurring in drainage depressions. Isolated trees of Mallee eucalypts (Eucalyptus spp.) and Black Box with a Saltbush (Atriplex spp.) shrubland are typically found on the Fm3 unit while the vegetation on the terrace unit is predominantly Saltbush (Atriplex sp.) shrubland.
The Fm2, Fm3 and terrace units are characterised by heavy-textured soils which increase in pH with increasing distance from the main river channel. These clay soils are sodic, which seal on wetting and are higher in salinity than those found on the Fm1 unit, suggesting that limited flushing of salts occurs. Fining of sediments also occurs with increasing distance from the main channel, which limits recharge on the older floodplain units. The terraces are capped by relict sand dunes, resulting in higher rates of localised recharge, forming perched watertables above the floodplain clays. Due to river regulation, a large number of channels are now abandoned and lined by cracking clays which also seal on wetting, and are therefore unlikely to be
areas of high recharge. Active channels and those which receive environmental water allocations sustain communities of River Red Gums, or Black Box along the smaller and drier channels. The area outside of the Murray River Gorge, classified as the uplands unit, is characterised by well-drained loamy sands and sandy loams, with moderate amounts of carbonate in the intermediate dunes. This unit has largely been cleared for agriculture or horticulture, with significant areas subject to irrigation and are therefore potentially areas of high localised recharge. The integration of the geomorphology, vegetation and soils on the Murray River floodplain has been a key component in the estimation of recharge rates for salinity and land management.