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The morphodynamics of rivers in semiarid environments tend to be controlled by episodic flood events, but the precise nature of sedimentary processes and sediment source areas within catchments affected by episodic floods remains not well understood. This study presents geomorphological, sedimentary and mineralogical data from different river reaches along the semiarid Letaba River within Kruger National Park, northeast South Africa. Geomorphological and sedimentary data were used to evaluate river morphodynamic behaviour and identify the major controls on landform development. Mineralogical data (heavy minerals and trace elements) were examined from surface sediment samples from different river reaches to determine the potential source areas for sediment reworking within the catchment (channel bank, channel bed, tributaries) for flood-transported sediments. Results of this analysis on downstream reaches show catchment-averaged sediment contributions from the channel bed of 42.5%, tributaries of 35%, and channel bank of 22.5%. These sediment contributions likely arise as a result of the different flow conditions experienced in different locations of the river system during floods. However, there is significant variability between samples from individual reaches, likely related to valley width and bedrock vs sediment availability. The contributions of river sediments from different sources are conceptually developed through a model of sediment source variability at different hydrodynamic stages. This highlights the role of floods in reactivating non-perennial tributaries and adding new sediment into the main channel. Measurements of perennial channel storage, therefore, do not give the whole picture of total sediment availability or activation during floods. [ABSTRACT FROM AUTHOR]

The Letaba River is one of several east-flowing rivers in the semiarid region of northeast South Africa. There is strong seasonality of river discharge and patterns of geomorphic behaviour along bedrock- and sediment-dominated reaches in these rivers, and in response to extreme seasonal flood events. This paper presents new luminescence ages on sediment samples from five sites in upper (headwater) reaches of the Letaba River catchment outside of Kruger National Park, South Africa, combined with evidence from reach-scale geomorphology and sediment sample analysis. River reaches are mainly mixed bedrock-alluvial with a patchwork of poorly sorted coarse sand bars overlying an abraded extended bedrock channel system. Luminescence ages from river sediment deposits (n = 13) cluster around three time periods of the last 400 years, 500–1100 BP, and 1400 BP. This suggests different reworked populations are present, which are a result of the partial bleaching of quartz grains and, thus, a mixed luminescence signal as flood-transported sediments are progressively moved from one depositional sink to another. This pattern of luminescence ages is quite different to lowland river systems in the same region where ages on the whole are significantly younger. Flood processes and dynamics in headwater reaches of semiarid rivers are often not considered but can yield a better understanding of system sensitivity to climate and event forcing. [ABSTRACT FROM AUTHOR]

Coastal sediment stratigraphies, their physical properties and microfossil assemblages are commonly used to reconstruct sea-level change. However, coastal sediments are affected by tidal, wave, aeolian, and overwashing processes at a range of elevations both within and above the tidal frame which means that their sea-level signatures may not always be clearly identified. Saltmarsh is a common coastal sedimentary environment along the Atlantic-facing coastline of northwest Ireland where bedrock headlands and an archipelago of small offshore islands have provided accommodation space for aggradational Holocene sediment sequences. A 2.85 m-long sediment sequence was investigated at Cloughglass, northwest Ireland. This shows weathered granite bedrock overlain by glacigenic cobbles and then in situ Pinus stumps within humified peat that is exposed in the upper foreshore. Above this, organic-rich and laminated saltmarsh sediments are arranged in packages (20–80 cm thick) bounded by undulating erosional surfaces. The saltmarsh sediments are unconformably overlain by a palaeosol (30 cm thick) and 1.5 m of recent dune sand. Sediment samples were removed at 5 cm intervals through the entire logged section and analysed for grain size properties, combustible organic content, CaCO 3 content, and microfossil assemblages. Five samples were dated by the AMS 14C method. Results show organic sediment accumulated around the pine tree stumps at ∼4400 cal yr BP but that later sedimentation was episodic, as evidenced by erosional surfaces throughout the saltmarsh part of the succession that may correspond to episodic storms or flood events. Three microfossil biostratigraphic zones are identified which broadly correspond to the sediment stratigraphy. The lowermost zone has poor preservation. The middle zone contains several halophytic species in low abundance but is dominated by the ubiquitous diatom Achnanthidium minutissimum. The uppermost zone is less diverse, with low but consistent abundance of predominately freshwater species and those that can tolerate brackish conditions. Integrating these results suggests that late Holocene coastal erosion and sea-level rise brought the coastline closer to the site over time, with a concomitant increase in wave and tidal influence as recorded in both sedimentary structures and biofacies. Subsequently, the land surface became stabilized with development of a dune palaeosol and decreased abundance of A. minutissimum. This evolutionary history reflects changing patterns of wave exposure and sediment supply as a result of changes in sand accumulation around the surrounding bedrock islands. Thus, the coastal stratigraphy more strongly reflects local sediment dynamics and accommodation space rather than regional sea-level change in the late Holocene. [ABSTRACT FROM AUTHOR]