Floodplain chronology of the Stillerust Vlei, Mooi River floodplain wetland, in western KwaZulu-Natal, South Africa

Wetlands have become a focus of conservation efforts worldwide following the international Ramsar Convention (The Convention on Wetlands of International Importance) established in 1971. Wetlands are areas where the soil is saturated most of the time. They are biodiversity hotspots, providing habitat to plants and animals that are adapted to the saturated soil conditions. In addition to understanding the ecological and hydrological aspects of wetlands, it is also important to understand the sedimentological and geomorphic factors that affect wetland development because these factors govern the physical structure and long term stability of wetlands.

Many floodplain wetlands form adjacent to rivers as a result of lateral channel migration, overbank sediment accumulation, and regular flooding. Meander migration largely constructs floodplains; as channels erode cutbanks on the outside of meanders, they deposit sediment on the inside of the meanders as point bars. Subsequent floods deposit fines overbank and on top of point bars, gradually building the floodplain as the channel migrates toward the cutbank side. As a meander bend becomes overly sinuous, the bend may become cutoff through neck or chute cutoff. When meander bends become cutoff, they leave an oxbow shaped depression that accumulates sediment and organic matter during overbank flows and sedimentation. Many floodplain wetlands are characterized by complexes of oxbows and abandoned channels that hold ponded water from either precipitation or overbank flows for extended amounts of time.

The climate of the eastern interior of South Africa is sub-humid, with annual potential evaporation (~1400-2000 mm) in excess of maximum annual rainfall (~1200 mm). Floodplain wetlands in the region are important resources, particularly for supporting habitats of a large number of migratory birds and other wildlife. The Mooi River floodplain wetland, also called Stillerust Vlei, is part of the Kamberg Nature Reserve in western KwaZulu-Natal (Fig. 1). This floodplain wetland is characterized by a channel with a low slope that flows within a broad sandstone-floored valley up to 2 km wide. The channel has alluvial banks and transports a sediment load of mud, sand, and minor gravel. Bedrock is exposed in most of the channel bed and is discontinuously mantled by a thin layer of sediment (<0.3 m) thick. The floodplain wetland adjacent to the channel is characterized by oxbows, levees, ponds, alluvial ridges, and an impounded tributary channel (Fig. 2, 3). The floodplain is constructed primarily through meander migration and cutoff and to a lesser degree by vertical aggradation as fine overbank sediment is deposited on top of the floodplain during floods.

To begin to understand the time that it has taken to build the floodplain of the Mooi River at Stillerust Vlei and to estimate the lateral migration rate of the Mooi River, we used optically stimulated luminescence (OSL) to find the depositional age of sediments from of a series of nested oxbows that have a clear relative age sequence. The outermost (oldest) oxbow is located in the western part of the floodplain, and the innermost (youngest) oxbow is located closer to the modern channel (Fig. 3, 4). Adjacent to this oxbow sequence, a partially-infilled chute cutoff is present (Fig. 3, 4).

OSL is a dating technique that enables measurement of the last time that a silicate mineral, such as quartz, was last exposed to sunlight. Thus, the burial age (the time when the quartz sands were deposited and then buried by subsequent sediment deposits) of the basal sediments in the oxbows of the nested sequence was measured. The OSL ages from the oxbows fall in the expected chronological order given their spatial arrangement. The oldest oxbow was abandoned following neck cutoff at ~1.22 ka and later neck cutoffs occurred at ~0.68 ka and ~0.22 ka (Fig. 4). The OSL ages are complemented by interpretations of aerial photographs that show that the small chute cutoff event occurred between 1944 and 1977, as the channel migrated to its current position. The sequence of neck and chute cutoffs has resulted in episodic channel migration with an average lateral channel migration rate across the valley of ~0.33 m/yr during the late Holocene (Fig. 4).

Compared to other streams worldwide, the floodplain reworking rates of wetlands in the eastern interior of South Africa are relatively slow, and fluvial processes and forms have been remarkably stable during the late Quaternary climatic changes that have affected the region. This contrasts with many rivers worldwide where climate change has caused alterations to the balance between discharge and sediment supply to such a degree that rivers have changed patterns (for example from braided to meandering) or have had drastic changes in channel dimensions. The stability of many rivers and floodplain wetlands in the eastern interior of South Africa are due to a combination of very low slope (<0.002) and stream power (<15 W/m²), low sediment supply, and stable local base levels established by local lithology.

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