Wetland geomorphology and floodplain dynamics on the hydrologically variable Mfolozi River, KwaZulu-Natal, South Africa.

Abstract

Wetlands in southern Africa can be considered a rarity, forming despite a regional negative water balance and a continental background of wide scale incision. These particular characteristics lead to southern African wetlands generally forming on drainage lines, where incision has been momentarily paused and water is locally abundant. The exact evolutionary history of valley bottom and floodplain wetlands is varied. However, their development follows four main themes; 1) those that evolve due to resistant lithologies outcropping on a drainage line and acting as local base levels, 2) those that occur on the coast, with current sea level preventing drainage line incision, 3) those that arise from a particular relationship with a trunk or tributary channel that blocks a drainage line with sediment, and finally, 4) those that occur in a region of dramatic loss of confinement, resulting in the formation of a wetland alluvial fan. Despite varied histories, all wetlands share a common thread, developing along a continuum from small and steep unchanneled valley bottom wetlands to large and flat floodplain wetlands. Incision in valley-bottom wetlands is controlled by a geomorphic slope threshold, whereby for a given wetland size, a particular slope may be considered stable. Wetlands exceeding the particular slope for their size are most likely already incised, or are vulnerable to incision in the near future. This thesis examines the general evolution of drainage line wetlands, followed by a detailed study of a large coastal floodplain, the Mfolozi River Floodplain, located on KwaZulu-Natal’s northern coastal plain. The Mfolozi Floodplain is one of South Africa’s largest at 19 000ha and is located just south of the world heritage site of Lake St. Lucia, with the St. Lucia and Mfolozi River mouths occasionally joining at the coast. Although once a mosaic of Cyperus papyrus and Phragmites australis permanent and seasonal wetland, approximately 60% of the floodplain has been reclaimed since the 1920’s for large-scale sugar cane cultivation. A smaller percentage is used for subsistence farming, while the remaining lower portion falls in the Greater St. Lucia Wetlands Park (which was renamed iSimangeliso Park in November 2007). The formation of the large coastal valley in which the Mfolozi Floodplain now sits was created during a period of incision during the last glacial maximum 18 000 BP when sea level was 120m below the current level. The lowered sea level resulted in regional river rejuvenation and valley down cutting. The Mfolozi River valley became deeply incised resulting in the formation of incised meanders upstream of the Lebombo Mountains. Below the mountains, less resistant lithologies of the Maputaland and Zululand Groups allowed the development of a wide coastal valley. Following the last glacial maximum, sea level rose, reaching its present level approximately 6000 BP. As sea level rose, coastal valleys were drowned and began to infill with sediments. Above the floodplain, the Mfolozi River follows a meandering course in an incised confined valley. Upon passing through the Lebombo Mountains, the valley widens considerably from 915 m to over 6 km in just 1.15 km. This rapid change from confinement to a broad floodplain setting results in a reduction of carrying capacity of the Mfolozi River, creating a node of large-scale deposition at the floodplain head in the form of an alluvial fan. Deposition in this region causes a local oversteepening of the valley’s longitudinal profile, with a gradient of 0.1%. Contrastingly, the mid- floodplain is almost flat, with a decrease in elevation of just 1 m over almost 6 km (0.02%). The lower floodplain, where gradient is completely controlled by sea level, has a steeper gradient of 0.05%. The reason for the rather drastic slope break in the mid floodplain is currently unknown, although it may be related to faulting in the underlying Tertiary aged Zululand Group, which is currently concealed by Quaternary deposits. In addition, tributary drainage lines that once flowed into the Mfolozi River have been blocked by long-term sediment accumulation on the floodplain. As a result, these drainage lines have become drowned and provide local conditions for the formation and accumulation of peat. Besides geological setting, hydrology is commonly recognized as being the other most important factor in valley evolution. Flow in the Mfolozi River has been characterized as highly variable relative to the rest of the globe. The Black Mfolozi has the lowest Coefficient of Variation (CV) at 61%, followed by the White Mfolozi at 69% and the Mfolozi River at 79%. In addition, catchment precipitation was shown to be variable, especially when compared to global values. As a result of variable rainfall and discharge, the Mfolozi River shows hysteresis in sediment concentration on an annual scale, and there is an indication that hysteresis may also occur on a longer time scale during wet and dry rainfall cycles. This however, needs to be confirmed with a longerterm data set. Variable discharge and sediment transport leads to different floodplain processes and dynamics than would be expected for a river of regular flow. Since flow is generally very low in the Mfolozi River, and is characterised by a series of extremely large outlier flood events, the persistence of flood features is likely to be large. In addition, it is likely that extreme flood events are the primary drivers of floodplain evolution and dynamics in such variable settings. The Mfolozi Floodplain wetland study throws light on floodplain process rates, and the forces behind floodplain dynamics in such hydrologically variable settings.