Most inland rivers on the alluvial plains of eastern Australia decline in size downstream due to inherent reductions in trunk stream discharge and stream power. Water leaving the main channel/s inundates the floodplain and sustains wetlands that are biodiversity and cultural hotspots. The Great Cumbung Swamp (GCS) are Holocene-age wetlands on the lower Lachlan River in south-western New South Wales (NSW). The GCS hosts ~12.5 km2 of Phragmites reed beds; some of the largest in the Murray-Darling Basin. Geomorphic mapping and topographic analysis of high-resolution digital elevation data reveal pronounced features associated with channel breakdown and reformation in the GCS. The meandering single channel entering the swamp has numerous breakaway points that allow water to exit before bankfull flow stage is reached. The channel is straighter near the centre of the swamp, upstream of the terminus, where water is lost through breakaways and through vegetated channel banks, spilling across the low alluvial ridge into nearby depressions and lakes. Overland and flood out flows dominate in the unchanneled swamp downstream of the terminus. Return channels at the downstream margin of the swamp pathways for water to join the Murrumbidgee River. Sedimentation processes dominate the central swamp, while erosion processes dominate the outflowing channels, leading to topographic gradients that influence inundation. The modern wetlands overlie a palimpsest of palaeochannels, which provide subtle impediments or conduits to flow, while coalescing gilgai depressions funnel water to low points. Understanding landforms and geomorphic processes that characterise the GCS can support water management and wetland conservation.