>Magister Scientiae - MSc / Non-perennial rivers are characterised by a discontinuous and variable hydrological flow regime which may retreat to form isolated pools along the watercourse during prolonged dry periods. The resulting spatio-temporal variability in hydrological characteristics provides support for a variety of ecological habitats which promote species richness and biodiversity. It is well established that groundwater may offer flow supplementation to perennial river flow throughout the year as baseflow, while fewer authors have unpacked the nuances of the importance of groundwater in dynamics of water persistence and the conditions that determine non-perennial pool reoccurrence. This study explores river-aquifer interaction of the Verlorenvlei catchment within the Western Cape Province of South Africa as a case study in order to create an improved hydrogeological understanding of groundwater’s role in non-perennial rivers to improve of water management practices. A multi-method approach was designed to fulfil this aim. In addition to desktop literature review and in-field sampling of water for environmental tracers, a water presence, groundwater level, and geophysical survey was conducted in order to develop a conceptual understanding of the multi-scale interaction occurring within the Verlorenvlei basin. Results of the isotopic and chemical analysis of water sources revealed the water origin and groundwater flow dynamics for the Verlorenvlei. The contribution of groundwater from Table Mountain Group related, fault-driven flow to the groundwater balance of the Verlorenvlei creates regional gaining conditions. Local gaining conditions within the Verlorenvlei river are created through lateral input of upwelling groundwater which moves downgradient with the topography as evidenced by the hydrogeological and geophysical survey. Using the Verlorenvlei as a case study, a contribution is made to the knowledge of the role of groundwater in non-perennial rivers. The results presented in this study indicate that where basin hydrogeology allows, groundwater may play an important role in the supply of water to non-perennial pools, especially during periods of minimal rainfall. The interaction mechanisms of this groundwater contribution within non-perennial rivers are site specific and spatially variable. Basin hydrogeology, subsurface stratigraphy and water availability are key limiting factors to interaction in non-perennial rivers. Future research aimed at generating robust information on discrete zones of water presence along non-perennial rivers may allow for better assessment of the potential vulnerability of these areas to water loss. Where these areas are fed by groundwater, to accommodate for their vulnerability, groundwater capture maps may allow for investigation of the local impact of groundwater use on these areas.

>Magister Scientiae - MSc / Globally climate variability and anthropogenic effects are causing more perennial rivers to become non-perennial rivers. Non-perennial rivers are distinguished by their isolated pools which serve as refugia for aquatic organisms, water birds, and riparian vegetation. The literature on non-perennial rivers demonstrates that pools are poorly understood in terms of their location, nature, and geomorphic persistence. Therefore, this study examines the relationships between the spatial distribution, morphology, and substrate characteristics of pools in reaches of the Prins and Touws rivers in the Klein Karoo. A greater understanding of pools will facilitate better management, monitoring, and restoration strategies for pool ecology since the geomorphology of pools provides a key part of the ecological template. Worldview-2 satellite imagery (2017) and orthorectified aerial photography (2014, 2013, and 1944) were used to assess the effects of major flooding events on pools over time. A DGPS (Differential Global Positioning System) was used to survey the pool widths, lengths, depths, and valley widths, cross-sections, and longitudinal profiles of the river. Sediment samples and Wolman pebble counts were used to assess the grain size and organic matter content of each pool in the study area. Detailed descriptions of the characteristics of each pool in terms of position in the channel, valley form, and obstruction presence and type were also assessed. Results indicate that most of the large pools occur at bedrock outcrops of the valley margins, and smaller pools are associated with Vachellia karroo debris bar features. Larger and highly persistent pools are associated with valley confinement and smaller less persistent, scour pools occur mid-channel where the valley expands. Analysis of the results shows that the valley width is the dominant control on these forced pools. The type of obstruction also plays a role in the formation of the pool as large woody debris results in smaller pools whereas, bedrock outcrops result in larger sized pools. A significant relationship was found between the grain size and organic matter content of pools. Aerial photography of the spatial distribution of the pools revealed that before a major flood, the pools were small and patchy, whereas afterward, they were larger and more elongated. It is suggested that at the bedrock outcrops, major scouring and eddy processes drive the formation of larger pools during large flood events, whereas pool dissection by sediment deposits prevails during intervening intermediate to low flow periods. The results are discussed in terms of the geomorphic controls (valley width, pool dimensions, morphology, substrate, and obstruction characteristics) on the formation and maintenance of pools in dryland settings. A conceptual model is proposed to explain the geomorphic changes of the pools in the four geomorphological zones of non-perennial rivers.