HYDROCLIMATIC CONTROLS ON DAMBO GEOMORPHOLOGY AND SOIL FORMATION IN UGANDA

Dambos—seasonally saturated, channelless valley floors—serve as important hydrologic, fertility and geochemical reservoirs throughout East and Southern Africa. Yet surprisingly little is known about the formation of dambos; in particular the past and future response of dambo systems to rapid changes in climate and hydrologic regime. For this study on weathered granitic gneiss in central Uganda, we characterized 1st and 2nd order dambo transects by augering 13 and 16 profiles respectively at 10-cm depth increments to a depth of 2-3 m, with additional profiles characterized elsewhere in the catchment. Sloping dambo margins were generally sandy, and a more complex profile was observed on the flat dambo floor, starting at the soil surface: (i) organic-rich fine sand, silt and clay at the surface grading into coarser sand with some gravel; (ii) an abrupt transition into clay and sandy clay; (iii) a gravel lag surface; (iv) saprolite. Given the lack of datable organic materials, we employed Optically Stimulated Luminescence (OSL) to date sandy sedimentary strata marking high-energy fluvial events at ~ 35, 33, 16, 10.4, 8.5-8.3, and 5.9 ka (cal yr BP). Most of these events can be correlated with rapid global and regional climatic transitions—both arid to humid and visa versa. Using Visible and Near-Infrared (VNIR) diffuse reflectance spectroscopy, we estimated relative smectite content for hundreds of augered soil samples, then determined clay mineralogy in more detail for selected samples using X-Ray Diffraction (XRD). The abundance of secondary smectites in lower dambo positions and the persistence of micas indicate high soil solution [Si(OH)4] during the humid Holocene period due to weathering and leaching of silica from upland soils and saprolite. Valley floor morphology, grain-size analysis, mineralogy and optical dating suggest that dambo seepage zones or “bottoms” mark relic channels, with the most recent channel incision correlated to the global “8.2 ka” event. Dambo margins are more stable, with sands dated at ~ 35 ka lying just 24 m away from a relic channel. Dambos are formed and continually reshaped by the complex interaction of (i) hillslope sediment erosion and deposition; (ii) channel flow and sediment transport; and (iii) upland weathering and leaching leading to geochemical enrichment in dambo soils.