The first half of June had a story of a mysterious floating island on Lake Victoria. Since it was reported, the island has migrated from one site to another, mainly between Ggaba landing site, Port Bell – Luzira and Miami Beach. This is not the first time this phenomenon is reported – it has happened before, and will recur for as long as Lake Victoria exists, and conditions are ripe for land to detach from the mainland and float. The level of certainty pertaining to the recurrence of floating islands on Lake Victoria demonstrates the view that there is nothing superstitious about these events.
However, the fact that some people have named this particular island Jaja Magezi, presumably in an attempt to give the superstitious school-of-thought some credit, shows how desperate many of us are to provide a suitable answer to this “mystery”. I am told one academic was lost for words when he learnt that this island floated away and returned to anchor itself to its original location. Of course, this is a little complicated for most of us to wrap our heads around, but there are realistic explanations.
Floating islands on lakes are not a new thing. In fact, man-made islands have been made to float on lakes for different purposes. For example, on Lake Titicaca in Peru, there are about 70 floating islands. The science of freshwater restoration has also seen the invention of floating treatment wetlands or eco islands, which come in many forms. This, and related technology, have been around the Western world since the 1980s, and is fast spreading to the rest of the world to restore the integrity of freshwater systems.
However, events in nature can also shape masses and cause them to drift on lakes, provided they are buoyant. Innumerable cases have been reported. A strange case of floating islands is reported on a “pond” found on a carbonate landscape in northeastern Mexico. Here, the action of subterranean water on the limestone bedrock led to several caverns. These collapsed to form holes, one of which is Zacatón, a somewhat circular hole. It has 15 roughly circular islands, which float across the surface as wind blows them around.
Closer to home, an investigation of fish dispersal in Lake Malawi was carried out more than 30 years ago (Oliver & McKaye. 1982. Floating Islands: A Means of Fish Dispersal in Lake Malawi, Africa. Copeia 1982 (2): 748-754). This work offers a glimpse on the origin of masses on this Lake. Observations made in the 1940s show that the floating masses originate from swamps surrounding Lake Malawi, which break off due to wind and waves. It is, therefore, not surprising that they are common during the rain season when conditions are ripe to loosen swamp vegetation and soil.
Similar floating masses have been reported elsewhere, notably in Florida. To understand the origin of floating islands, we need to know their composition. They are characterised by (i) organic soils, which vary in depth from a few inches to a few feet, and (ii) a colony of aquatic and sometimes upland plants. This suggests that they are aquatic in origin, and are created in different ways, some of which include the following:
Floating islands are common on lakes where water levels vary seasonally. At low tides, plants at the shore anchor onto loose organic soil, knitting their roots around the soil to create a firm mat. As water levels rise, the plants pull much of the soil material, hence giving rise to a floating mass complete with soil and vegetation. This is similar to the way floating masses are created on lakes that suffer from the effects of prolonged drought. For these, the exposed sediments dry out following a dry spell, and so, only the highly buoyant peat and sprouting vegetation will rise up consequent upon rapid saturation.
Some lakes have bottoms whose sediments are rich in organic matter. Ebullition from the sediments is usually accompanied by detachment of decomposing fibrous materials, which float and coalesce at the shores. Here, they are colonised by both wetland and upland plants, but the masses once formed are only fairly stable to resist wave action.
Wave action and winds destroy vegetation along shorelines. Seasons of destruction create a thick and compact decomposing layer of vegetation that floats on the water. Onto it plants emerge. Using their roots, they hold the mat of decomposing plant materials, benefitting from the accumulated organic layer and soil. In this way, floating islands are also created.
In the case of the floating islands on Lake Victoria, we know that Lake Victoria is a pretty old freshwater body, whose main hydrologic input is rain that falls on this vast water body. That over the years, swamps have colonised its shoreline to create a thick mat of intertwining roots within an organic soil matrix that thins towards the upland. We also know that the lake’s water level dramatically fell after the drought that gripped the region. This also happened at the time another dam (Kiira Power Station) was constructed across the only outlet of the Lake. The result of this disturbance exposed the hitherto inundated land, resulting in sediment desiccation, acceleration of decomposition in places, and colonisation of locales by opportunistic upland plants. The relatively dry environment also attracted cultivation and human occupation of originally very wet areas. I suppose seasonal water level surges, especially during wet seasons, occasionally find land with failures that allow it to give way after bouts of wave and wind encounters.
It is interesting to understand the trends of occurrence of this phenomenon. This is important because of the projected changes in rainfall intensity and seasonal variability, yet rain seems to impact the prevalence of floating islands on Lake Victoria. It is also interesting to determine: (i) that part of the lake which is prone to floating island formation; and (ii) the spatial extent to which the islands can get.
Further, it is important to understand the impact these islands have on the fishing villages in their path. Reports which show the potential of tourism have been made, but the extent to which floating islands interrupt livelihoods on Lake Victoria’s shores remains unknown.
Dr Lugumira is a soil-landscape modeler, wetland scientist, environmental policy analyst and a lecturer of Geography, Gulu University. email@example.com