Hydrology, sediment transport dynamics and geomorphology of a variable flow river: The Mfolozi River, South Africa

Abstract

The co-efficient of variation for inter-annual streamflow of the Mfolozi River is extremely high at 79%. An analysis of flow frequency indicated that streamflow is skewed towards low-flow values, with a number of extremely large flood events occurring as outliers on the histogram. Streamflow variability in the Mfolozi River may be linked to multiple factors including a large catchment size, a seasonal climate of a dry winter and wet summer, evergreen vegetation in the catchment, variable precipitation and the occurrence of regionally pervasive climatic oscillations. This research aimed to address how streamflow variability impacted upon sediment transport and thus, geomorphology. It was found that sediment transport variability occurred at the intra-and inter-annual scale. Analysis of mean monthly sediment concentration and discharge showed a hysteresis effect, such that sediment concentration peaked prior to discharge in the early wet season. During the late wet season, peak discharges often had unexpectedly low sediment concentrations. Furthermore, data suggested the existence of long-term hysteresis that may be related to decadal-scale climatic oscillations that alter sediment availability and stream capacity, resulting in discharge peaking in 2000 and sediment concentration in 2005. However, more data are required to confirm this relationship. Variability in streamflow appears to share a causal relationship with sediment transport variability, as both are linked to variation in precipitation and the resultant impacts on vegetation growth and evapotranspiration rates. The variability of streamflow and sediment transport has implications for stream and floodplain geomorphology, and the hydrology of variable rivers should be considered when interpreting their geomorphology.

Assessing the Effects of Land Use on Surface Water Quality in the Lower uMfolozi Floodplain System, South Africa

“…The dominant vegetation types in the floodplain system include Cyperus papyrus , Phragmites mauritianus , Phragmites australis and Ficus trichopoda . The uMfolozi River has a highly variable flow regime [ 36 ], characterized by a low base-flow and a few large but brief floods each year. Floods are a feature of the river and they have a profound influence on the geomorphology of the floodplain [ 37 ].…”

“…The clearance of land for cultivation of sugarcane and commercial forestry dates back to 1911 [38]. By 1960, more than 50% of the floodplain had been converted to sugarcane farms whose establishment was made possible by the construction of canals, diversion of the uMfolozi River and the clearing of indigenous wetland vegetation [31,36]. The floodplain also supports subsistence farming of other crops wherein the dominant ones include sweet potato (Ipomoea batatas), banana (Musa acuminate), and amadumbe (Colocasia esculenta).…”

Stoichiometric multitrophic networks reveal significance of land-sea interaction to ecosystem function in a subtropical nutrient-poor bight, South Africa

“…A POC concentration of 2.57x10 10 g y -1 was calculated using the sediment concentration and the assumption that POC was 8.4% of TSS [59]. DOC imports were calculated based on an average global estimate of 6 mg L -1 [59], a January flow rate of 6x10 10 L and a June flow rate of 6x10 9 L for the Mfolozi River [61]. A summary of imports can be found in Table 1.…”

Estimating the sediment trap efficiency of intermittently dry reservoirs: lessons from the Kruger National Park, South Africa

“…has shown that this relationship is likely to vary on different timescales owing to changing sediment availability along drainage lines and variable sediment delivery from inter-drainage areas (e.g. Walling, 1977;Grenfell and Ellery, 2009). However, the different scenarios corresponding to values for the exponent b of 0.5, 1 and 1.5 represent a range of possible relationships that likely includes a scenario that can be considered realistic (Syvitski et al, 2000).…”