Conditions of the last glacial maximum (LGM) centred on 20ka were dramatically different from those of today. While evidence of widespread dune building and lake floor deflation contributes to evidence of aridity, such evidence varies depending on the location from which it is derived.

Throughout the Murray Basin in southeastern Australia, conditions during this interval were greatly influenced by a system of groundwater-surface water interaction substantially different from that in the same area today. Groundwater levels were regionally high following a period of positive (wet) hydrologic conditions in which components of both local and regional recharge combined to maintain regionally high water tables. Such conditions persisted well into the period of the LGM even though local rainfall may have been much reduced.

Lake basins marginal to upland catchments provide evidence of a relative abundance of water during this interval, with levels often exceeding those in the same systems today. These conditions contrast with records from basins more remote from catchment sources where lake floor deflation and pelletal clay or gypseous dune building dominated over surface water controls. While Lakes Tyrrell, Frome, Eyre and the those of the Willandra system were subject to basin deflation at this time, Lake Urana (Murrumbidgee catchment) and lakes adjacent to the Grampians in western Victoria retain evidence of shoreline dune building under high water-level conditions.

The apparent contradiction between arid conditions in systems far removed from effective catchments and those marginal to catchment sources is a function of improved catchment efficiency during this period of greatly reduced temperature regimes. Major changes in catchment coefficients, associated with increased run-off from slopes often affected by periglacial environments, were sufficient to maintain at least ephemerally high magnitude events in streams feeding adjacent basins. Additionally, depressed evaporation regimes meant that relatively little annual discharge was necessary to maintain local lake levels.

By contrast, in areas distal from catchments of increased efficiency, surface recharge was insufficient to replace evaporative loss. These lakes fell, many becoming groundwater discharge systems with consequent production of saline deflation products compared to the clean quartz sand beaches and dunes derived simultaneously from catchment margin basins, a feature of special relevance to groundwater hydrology. The contrast in surface water budgets coincides with zonal changes from groundwater recharge to discharge systems across the plains

In this system hygrologic gradients from catchments to plains were greatly increased. The existence of pluvial systems near the foothills contrasting with high groundwater, arid conditions on the plains provides a salutary warning to those of us who seek simple climatic solutions. The dilemma of simultaneous existence of environments both wetter and drier than today is thus more apparent than real. Both co-existed, but in substantially different hydrologic settings.