National Geographic : 2004 Dec
the soil and groundwater beneath them, trees of about a hundredth of a degree) that water fol- not only keep the water in the delta fresh but also expand the size of their island plat forms. Thus what the termites start, the trees continue, engineering not just a landscape but an entire ecosystem. Just as termite mounds are nuclei around which islands form, hippo paths are the precur sors of water channels. Most channels in the delta have a life expectancy of about a hundred years. During that time sandy sediment gradu ally raises the height of the channel bed, slowing lows such randomly created corridors. The path of least resistance turns out to be the path the hippos have trod. Termites, hippos, and papyrus-these three biological influences are part of a system as intricate and responsive as any on Earth. Yet the delta is not immune to human disturbance, even to eventual destruction. The chief threats lie upstream, in the two countries with which Botswana shares the inflowing water. Angola REFLECTION OF AFRICA, an elephant trumpets above Okavango's looking-glass waters. the current and allowing the fringing stands of papyrus (which are not rooted in soil but linked together in floating mats) to spread into the channel. Clumps of papyrus eventually break off and jam the channel until, like a clogged artery, it becomes completely blocked. At this point the hippos come to the aid of the delta's circulatory system, breaking through papy rus )ams and forming new channel connections. It is only because the delta is so flat (a gradient 66 NATIONAL GEOGRAPHIC * DECEMBER 2004 and Namibia both experienced long, brutal wars in the latter part of the 20th century and now look to rivers to help build their economies. Two aspects of development, the increased use of agricultural fertilizers on riverine land and the production of hydroelectricity, could have disastrous downstream effects on the delta. While neither threat is imminent, their potential impact on so finely balanced an ecosystem has many people worried.