Flow patterns in the Lachlan River floodplain are complex, particularly during large flood events. The catchment area to Jemalong Gap is approximately 19,800 square kilometres. Flood runoff from the Lachlan River flows through Jemalong Gap and continues west to Condobolin via a braided network of channels and floodways. Jemalong Gap is about 1.5 km in width. Most flood flows have to pass through this constriction. For the largest floods, a small proportion of the flow is diverted via Little Plain to Gunning Gap, about 9 km to the north. Flood flows pond upstream of these gaps, attenuating downstream flood peaks and prolonging the duration of inundation. In the vicinity of Condobolin, Wallamundry Creek runs parallel with the Lachlan River, but approximately 10 km to the south. To the north, Goobang Creek flows westwards, parallel with the Lachlan River and eventually joins the river at Condobolin.

The study objective was to define flood behaviour at Condobolin in terms of flows, levels and flooding behaviour for flood frequencies ranging between 5 and 200 years average recurrence interval (ARI), as well as for the Extreme Flood event, which for the purposes of this study has been taken as a flood with the ordinates of its discharge hydrograph equal to three times those of the design 100 year ARI flood. This results in a peak flow for the Extreme Flood which is three times that of the 100 year ARI flood.