Key factors affecting the initiation and progress of gullying in dambos in parts of Zimbabwe and Malawi

Accelerated gullying of dambos (seasonally waterlogged bottomlands) on commercial farms in Zimbabwe has resulted in legislation curtailing dambo cultivation, even in indigenous fanning areas. The transfer of indigenous cultivation to interfluve areas and the resultant concentration of cattle grazing in dambos initiated similar gullying. Facilitated overland flow was perceived to be the cause of gullying both in the case of dambo cultivation and dambo grazing, this overland flow re‐excavating what was believed to be fluviatile infill. Geomorphological evidence is assembled to show that dambos are not ‘fossil’ fluviatile systems, nor is their clay infill alluvial. They are the ‘lows’ of a landsurface irregularly lowered by differential leaching. The infill forms by precipitation of materials leached from the interfluve profiles and discharged into the dambos. The fluviatile‐like configuration of dambos is attributed to integration of sub‐clay groundwater movement, resulting from landsurface incision. Accelerated gullying in Zimbabwe is deduced to be caused by breaching of the dambo clay by subsurface water movement. The particular vulnerability of Zimbabwe dambos is attributed to: (1) the watershed situation; (2) shallow weathering and (3) the thinness of the dambo clay. Evidence from Malawi indicates that interfluve deforestation and cultivation of shallow‐rooted crops conserve water, raising the groundwater level. Such an increased head of groundwater also appears to have occurred in Zimbabwe; this would strengthen upward discharge in the dambos, promoting gullying. Urgent land and water policy revision is required, allowing restoration of dambo cultivation and reafforestation of interfluves, if dambos are to be conserved and the resources which they offer satisfactorily utilized. Copyright © 1990 John Wiley & Sons, Ltd