Banhine Inundation

Banhine National Park in Gaza province, Mozambique gets its name from an extensive wetland system (called Banhine by the local population) similar in function to the Okavango swamps in Botswana. The water for the wetland has its origins far to the north in the higher ground along the Zimbabwean border near Gonarezhou National Park.The water is transported to the park in drainage systems such as the Chefu River, as well as numerous minor ones, and enters the park in a delta system. This water inundates approximately ten thousand hectares of wetland as well as occasionally inundating the extensive grassland surrounding the wetland, approximately one hundred thousand hectares, and by so doing maintaining these open grasslands.

It would seem as if the level of the water in the wetland system is controlled by an alluvial plug in the north east of the park. All overflow water flows from the wetland, via the plug, into the Changana River and on into the Indian Ocean east of Xai Xai. The wetland is also regenerated by occasional cyclones that penetrate sufficiently far inland to fill the wetland. These occurrences are infrequent, but play an important role in the functioning of the wetland as they are responsible for depositing large volumes of fresh water into the wetland in a very short time frame.Banhine falls within a very arid region and only receives approximately 400mm millimetres of rain per annum. The park is also very low-lying and has a flat topography that slopes from the north west (198m above sea level) to the south east (68m asl). The combination of these two factors results in a high evaporation rate affecting the water level in the wetland by means of evaporation. The high levels of evaporation also have an effect on the water quality within the wetland as well as within sectors of the wetland.

As water is evaporated out of the wetland so the dissolved salts become more concentrated and the water becomes more saline. The salinity can also vary from one location to another within the wetland; the exact processes involved in these differences in salinity are not fully understood and are thought to relate to the geology and underlying water table, which is generally saline.Over the past four to five years Banhine and the surrounding area has been experiencing a very severe drought and particularly so over the past two years when only about thirty percent of the expected annual rainfall was received. The extensive drought conditions resulted in the wetland drying up completely as well as all but two of the fresh water pans in the park. This resulted in the temporary extinction of all fish species (there are 17 species recorded) as well as the absence of many aquatic bird species for which Banhine is known.

It was with great surprise therefore that on returning to the park on January 17, I found the wetland full. This water had come from the north, from central Mozambique and north eastern Zimbabwe. These areas have been experiencing heavy rainfall as a result of the current La Nina phenomenon in the south Atlantic which occurs approximately every four years and results in above average rainfall occurring in some areas of southern Africa.The water had flowed down the drainage lines into the park and on encountering the alluvial plug had backed up, flooding the wetland, even though Banhine and surrounding area has had very little rain over the same period.Superficially there is very little difference from the dry Banhine to the flooded Banhine as the water has inundated the area passively and merely lifted much of the vegetation. Some of the grass species are adapted to growing in water and these grasses grow over the surface of large portions of the wetland thus concealing its inundated status.

The same goes for many of the weeds and vegetation that grew in the wetland during its dry status. There are, however, patches of open water scattered along the course of the wetland.But it is the frogs and birds that give away the flooded status of the wetland. Almost overnight vast numbers of aquatic birds descended on Banhine and the air is constantly filled with the chatter of lesser moorhens and jacanas as they fly low across the wetland and drop into the tall vegetation.

Large flocks of reed cormorants fly back and forth along the wetland and flocks of as many as 18 black crowned night herons are seen at a time. Pygmy geese are common as are pochards, fulvous and whitefaced ducks. Red knobbed coots are seen in the open water as well as redbilled teals while purple and lesser gallinules (a nest of the latter was found) are also present, but it takes more effort and perseverance to locate them. Large numbers of African and black crakes inhabit the flooded grass areas and add to the cacophony of calls. Spotted crakes are also suspected to be present, but this needs to be confirmed.

It is expected that the combination of birds will change as the habitat changes. It is expected that much of the present vegetation will decompose resulting in more open water, water lilies and reeds. It is also expected that as this happens so the species composition of birds will change and those that prefer open water will increase. With time as the water increases in salinity we can expect species such as flamingos to make an appearance. Both species have been recorded in the park previously. Pelicans are also expected to make a re-appearance. The confirmed Banhine bird list now stands at 282 species.

The presence of large numbers of cormorants (360 were counted flying past the research camp in an hour) suggests the presence of fish and a few fish have been seen, mostly tilapia or Oreochromis, but generally the impression is that there is still only a very low fish population. People living along the wetland maintain that it takes three months from the time of inundation before they can start harvesting fish on a regular basis. A monitoring programme is planned to determine which species return to the system, seeing that the entire wetland dried up. This has happened a number of times before and the fish have returned. What will be interesting to see is whether the previously unknown Barbus species found in 2005 will return and if so where the residual population is located. This also emphasises the resilience and incredible dynamics of the Banhine system.