The Okavango fan of northern Botswana is one of the largest alluvial fans on earth, and is characterized by gently undulating topography with local relief generally less than two meters. This area is subject to seasonal flooding by annual influx of flood water from subtropical Angola to the north. Sedimentation occurs on the lower lying areas due to the accumulation of fine clastic material derived from the flood water and precipitation of fine grained amorphous silica from the ground water.