What are the main characteristics of Lake Bolac?
Lake Bolac appears to have been a complex wetland system, with areas of open water, a lunette on the south-east side, with areas of steep banks and low-lying marshy ground. It is fed by the Fiery Creek, and flows into Salt Creek and away to the Hopkins River. The lunette on the south east of the Lake Bolac basin is formed from wind-blown sand that came down the Fiery Creek from the hills in the catchment. This lunette naturally partly blocks the outlet, and allows the water to be retained in the lake, until the rising water breaches it at its western edge, and water then flows away south in the Salt Creek.
The lake was characterised by an episodic nature, similar to a seasonally opening and closing estuary system, as its exit was sometimes blocked by lunette formation and sand accumulation. When the lunette was breached, the lake could drain away, and is likely to have eroded its own channel before spreading out into wetlands below the lake. This means that the lake would dry out occasionally. We know that it did from settlers records and the presence of the wind-blown lunette. Lunettes can only form when a lake is dry. When the lake was dry wind would develop and enlarge the lunette, and accumulating sand would block the exit, cutting off the flow to Salt Creek. When flow returned to the Fiery creek the lake would fill up, and eventually breach the lunette again. It is likely that the position of the outflow varied from time to time and event to event, depending on the amount and location of the accumulated sand. The indigenous saying ‘Long time wet, long time dry’ probably describes the water regime well.
In the filling part of the cycle the lake would have backed up and flowed out over low-lying areas where the Fiery Creek enters, creating an extensive wetland system adjacent to the areas of open water. These swampy wetland areas, the low gradient of the creek at the inlet area, the deposition of sand as the water slows down, would have provided habitat for lush, productive and reedy wetlands. The water plants and the residence time of water would have meant that the water was substantially filtered of particulate matter and nutrients before it entered the lake. The surrounding wetlands would have provided an amazing habitat for birds and fish, and we know it supported a large indigenous community (as evidenced by the archaeological deposits in the lunette and elsewhere around the lake, and setttlers records).
Since the channelization of the inflow and artificial building of the barrier on the outflow (‘The Overflow’), water is likely to have had a longer residence time in the lake. Fine clay sediments from the erosion of the banks and wetlands in the catchment have been deposited in the lake and are dispersed in a layer across the lake bottom. When the lake dried out it was possible to detect the original lake floor as a harder, grainer layer beneath the fine, sloppy mud.
Lake Bolac is a eutrophic (high nutrient) warm, polymictic (turns over many times = well mixed) lake that develops a temporary thermocline (distinct temperature gradient from top to bottom) on a daily basis when the weather is warm and still (days in summer). The mixing of the lake is via wind, the fetch of the lake is extensive for all wind directions. It does not experience ice cover, and naturally dries out during extended periods of low rainfall.
The preservation of good water quality of the lake and the variation in the abundance of algae would have been dependent on the natural filtration of the water through adjacent wetlands in the catchment and around the inlet, the growth of aquatic plants (weed beds) within the lake, the diversity of phytoplankton and predatory zooplankton, fish and birds, and the mixing of the lake that prevented dominance of the blue-green algae that favour warm, still water characterised by a more permanent thermocline.
The current situation of poor water quality, algal blooms and midge infestation is a consequence of high nutrient loading, and an unbalanced food-web. The abundance of midges is a consequence of the high nutrients leading to high algal productivity and the paucity of natural controls (i.e. predators of larval and adult midges).