FLOODWATER RECHARGE PROCESSES IN THE CHOWILLA ANABRANCH SYSTEM, SOUTH-AUSTRALIA

We report the results of a study into the interaction between floodwat ers and an unconfined alluvial aquifer in the Chowilla anabranch syste m of the River Murray during a large flood. Data on watertable elevati on, groundwater chemistry and unsaturated zone salt storage were colle cted before, during and after a flood in 1990 which inundated all but the very highest points of the floodplain. These data, combined with a nalytical modelling of the watertable behaviour throughout the flood, led us to conclude that diffuse vertical recharge of floodwater to the unsaturated zone is of little importance. As a consequence, only limi ted teaching of salt from this zone to the groundwater occurs. This ap pears to be due to a reduction in the infiltration capacity of the sod ic clay surface soils of the floodplain which disperse and swell when wetted with the low salinity floodwater. This suggests that the unsatu rated zone is not the major source of salt which enters the River Murr ay following floods. However, from previous studies it is clear that t he floodplain is an important source of saline groundwater which is ad ded to the river following floods. An alternative hypothesis to explai n the observed salt accessions is that areas of the floodplain where t he Coonambidgal Clay is thin or absent are zones of localized recharge which cause displacement of in situ groundwater into the floodplain s treams. This hypothesis should be tested in further work.