Hydrology

The Lachlan River Valley has a catchment area of 84 700 km2 and incorporates 413 520 ha of recognized wetlands of which Lake Cowal is a part. The Lachlan River has an estimated mean annual flow of 1,270,000ML at Forbes. Lake Cowal is situated on a the fluvial Jemalong Plain which is bound by Jemalong Range (east), Manna Range (west), Lachlan River (north) and tributaries of the Bland Creek system (south). Jemalong Plain is 20 - 30 km in width and elevations range from 203 – 220 m. Landform features include relict stream ridges, stream beds, levees and floodplains.

Surface water inflow to Lake Cowal comes primarily from the Bland Creek in the south and the Lachlan – Lake Cowal floodway in the northeast. The Bland Creek Catchment has a catchment area of 400 000 ha upstream of the Lake. Inflows also occur from the Lachlan River via breakout flows that occur during major flood events in the Lachlan River causing back flooding into Lake Cowal. The Lachlan – Lake Cowal floodway is a modified, breakout channel of the Lachlan River. In flood conditions Lake Cowal drains through Nerang Cowal, Manna Creek, Manna Swamp, Bogandillon Creek, Bogandillon Swamp and finally back into the Lachlan River.

The lake is substantially inundated seven years out of ten, but has been known to dry for extended periods. In the 1930’s Lake Cowal remained dry for 30 years. Without inflows, Lake Cowal dries from evaporative losses, which usually takes three years from full storage

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