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

We report the results of a study into the interaction between floodwaters and an unconfined alluvial aquifer in the Chowilla anabranch system of the River Murray during a large flood. Data on watertable elevation, groundwater chemistry and unsaturated zone salt storage were collected before, during and after a flood in 1990 which inundated all but the very highest points of the floodplain. These data, combined with analytical 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 limited teaching of salt from this zone to the groundwater occurs. This appears to be due to a reduction in the infiltration capacity of the sodic clay surface soils of the floodplain which disperse and swell when wetted with the low salinity floodwater. This suggests that the unsaturated zone is not the major source of salt which enters the River Murray following floods. However, from previous studies it is clear that the floodplain is an important source of saline groundwater which is added to the river following floods. An alternative hypothesis to explain the observed salt accessions is that areas of the floodplain where the Coonambidgal Clay is thin or absent are zones of localized recharge which cause displacement of in situ groundwater into the floodplain streams. This hypothesis should be tested in further work.