The southern Wimmera region of western Victoria is situated in the semi-arid southwest Murray Basin, at the confluence of the Mallee Limestone hydrogeological province and the Southern Riverine hydrogeological province. This confluence is occupied by a ~100 km long, 4-5 km wide, relict stream valley. Except at Mnt. Arapiles, where Grampians Group quartz arenites outcrop in a monolith which rises to > 200 m above the plain, the palaeochannel is approximately 30 m lower than the surrounding landscape. It is called the Douglas Depression (sensu Brownbill et al., 1995) to the south of Mnt. Arapiles where it consists of a thin veneer of sediments (<30 m) draped over an apparent extension of Grampians Group (basement) from the main range. The water table lies within five metres of the surface over much of the Douglas Depression, or outcrops in a chain of brackish to hypersaline lakes which are groundwater dominated in solute terms. Lakes are particularly abundant around the margins of Mnt. Arapiles because it is a recharge area.

At Mnt. Arapiles, rainwater enters the groundwater system through fractures in the sandstone or through the porous colluvium at its margins. These two groundwater components can be differentiated hydrochemically. Significantly, the monolith provides a driving force (high heads) which sustains many of the lakes at its periphery. Depending on lake position, groundwater moving northward up the depression can also enter the lakes. During wet periods, Mount Arapiles is to some of the lakes what a roof is to a rainwater-tank; a large, mostly impermeable surface, which captures and conveys surface-waters. These surface waters have the effect of diluting solutes which have been evaporatively concentrated in the lake basins. Holocene palaeosalinty records for two the lakes in the region (Jacka Lake and North West Jacka Lake) are interpretated keeping the above hydrological model in mind.

The palaeosalinity records were constructed from an ostracod-based (shelled micro-crustaceans) weighted-averaging transfer function using CALIBRATE (S. Juggins, unpublished program), which relied on a 179 sample paired ostracod-hydrochemistry lake study. The records are supplemented with other biological evidence (Campylodiscus clypeus (diatom), charaphyte oogonia, Coxiella (gastropod), Elphidium (foram), Daphniopsis ephippia (water flea), and brine shrimp faecal pellets) which are used (non-quantitatively) in the absence of ostracods, the d ¹⁸O of ostracod values, and with >130 m m quartz counts. At NW Jacka Lake it is believed that most of the quartz is wind-blown and is derived from the Lowan Sands which cap the NNW-SSE trending still-stand dunes located to the immediate west of the Depression. At Jacka Lake, which has a surface water conduit and therefore a much bigger effective catchment than Northwest Jacka Lake, quartz can also be waterborne. Quartz samples were dated by optically stimulated luminescence using the single aliquot regenerative protocol, and these dates supplement radiocarbon-based chronologies.

The most important feature of the palaeosalinity records at Jacka Lake and Northwest Jacka Lake is their broad correspondence to the lake level curve at Lake Keilambete (Bowler, 1981). Relatively wetter conditions are evidenced by lower salinities between ~10 000 cal. years BP to ~ 6000 cal. years BP. However, mutually exclusive high and low salinity ostracod communities contribute to the ostracod assemblages over this time intervol suggesting more variability, in terms of effective precipitation, than implied at Lake Keilambete. Moreover, drier conditions are evident from ~8200 cal. years BP to ~8600 cal. years BP. This dry interval includes a wind-blown quartz peak at ~8400 cal. years BP (OSL dated at 6750 ± 800 cal. years BP), and may be a southeastern Australian expression of an "abrupt early to mid-Holocene climatic transition registered at the equator and the poles" (Stager and Mayewski, 1997). A drier climate was firmly in place by about 4500 cal. years BP, and is marked, at the groundwater dominated NW Jacka Lake, by a rise in windblown quartz, and by a salinity increase inferring less dilution by fresh surface- and groundwaters. A switch from surface-water to groundwater-domination occurs at Jacka Lake at this time. Jacka Lake has a higher sedimentation rate and provides a much more detailed account of the late Holocene than Northwest Jacka Lake. An important feature of the late Holocene record at this site is a switch to variable conditions at ~1000 cal. years BP. During this time NW Jacka Lake probably entered a regime of winter filling with summer precipitation of a halite crust that has carried over into the present.