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Wetlands occupy broad, gently sloping valleys characterised by net deposition. Consequently, gully incision is viewed as an agent of wetland destruction. However, little attention has been given to the role of gullies in natural wetland formation. This paper determines if gully cutting and filling cycles have led to wetland formation in both an extant eroding wetland and a semi-arid environment that once supported wetlands. At both sites, the valley fill was investigated to identify infilled gullies, and a C-14 date was obtained from the gully fill. The longitudinal slopes of the valley floor prior to incision, the valley bedrock floor, and the gully fill were also measured. In the Krom wetland, four infilled gullies (5-8.2m deep) were found adjacent to alluvial fans and dated to 470-7060bp. Ten infilled gullies were found at Zuurplaats (Karoo), which were more recent, although one gully started filling 1260bp. At both sites, large gullies were grounded on bedrock. Alluvial fans deposited by tributaries cause a reduction in mainstem valley width and locally increase longitudinal slope, leading to gully initiation. The slope of the gully beds was less than the surface prior to erosion, and the slope of the gully fill was comparable with the valleys’ regional slope. The bedrock on both valley floors had a near-horizontal cross-sectional profile, suggesting that repeated gully incisions had led to valley floor planing. A new model of wetland formation is proposed where gully cut-and-fill cycles laterally plane valley bottoms and contribute to valley widening and longitudinal slope reduction.