GEOMORPHOLOGICAL IMPACT OF HIPPOPOTAMUS ON THE NORTHERN OKAVANGO, DELTA, BOTSWANA: SPATIAL ANALYSIS OF FORAGING TRAILS AND ISLANDS

The common hippopotamus (Hippopotamus amphibius) is a large semi-aquatic animal that produces a diverse suite of traces, including those with substantial zoogeomorphic impact. In the Okavango Delta of northern Botswana, the hippopotami act as key ecosystem engineers, creating networks of channels and trails that branch from primary wading pools to nearby foraging islands. This study used satellite imagery (GoogleEarth Pro^TM) to document and quantify the zoogeomorphic impact of H. amphibius over an area of 7.1 km². Twenty temporally persistent trails (length: 150-750 m; width: <5 m) were nearly straight (average sinuosity of 1.03), whereas the delta distributary channels were wider (>10 m) and had a mean sinuosity of 1.17. In addition to the main trails, 122 secondary trails were mapped with a total surface area of ~56,300 m² (0.8% of the total study area). These were subdivided into four categories based on the context of their termini: island to island (57% of total trail area), island to trail (22%), wading pool to island (13%), and trail to trail (8%). In the summer of 2007, hippopotamus trails led to 49 islands, which had a near-round planimetry and a total area of ~565,600 m² (8% of the study site). By the late fall of 2018, the total foraging island area decreased to ~445,900 m². This study demonstrates that hippopotami may be potentially responsible not only for the formation (avulsion) and maintenance (long-term connectivity) of multiple inter-distributary hydraulic pathways, but may also affect the size and morphology of the emergent landforms within the delta through trampling and consumption of vegetation, as well as active sediment reworking.