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Arid ecohydrology relies on sporadic rainfall and flooding to sustain linear river oases, crucial for supporting flora and fauna during the long dry season and drought years. Growing water demands have led to large reservoir constructions, severely impacting the fragile arid ecosystems. We analyse the hydro-geomorphological impacts of the large Neckartal Dam (completed in 2019) on the Fish River oasis based on existing dams, regional studies, field surveys, hydrological data, and modelling. The Neckartal Dam, along with three smaller dams, impounds 80 % of the desert oasis basin (1235 Mm3, 55,870 km2), exceeding the average annual runoff threefold. A gradient of dam-induced changes was identified, varying with distance from the dam wall. Immediate downstream, hydromorphological impacts include channel narrowing (⁓15 %) and reduce mean annual sediment yield (⁓20 %). Also, water distribution losses create chronic low flows, favour for invasive flora and fauna to spread, significantly disrupting the pristine nature of the desert ecosystem. This phenomenon has been observed over a 70 km stretch below the Hardap Dam. This situation is irreversible, as the invasive flora can withstand even large floods. As the distance increases, the dam suppresses low to moderate annual floods and prolongs droughts from months to years or even decades. Based on our flood routing model, only the largest floods (recurrence interval, RI >25 years) can exceed the dam storage, continue downstream, transport sediment and maintain the pool-bar morphology along 245 km of the oasis. The lack of significant flows between major floods means that sediment from smaller, lower arid basins accumulates in the Fish River valley, filling pools and further reducing water availability during dry periods. The Neckartal Dam's impact assessment demonstrates potential changes in arid river systems globally. [ABSTRACT FROM AUTHOR]

Abstract Ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime, mainly dominated by floods. In these environments, floods are a water resource and a potential hazard with important socioeconomic implications. The Fish River (86,600 km2) is the largest ephemeral stream in Namibia and, recently, also the focus of new development plans, including construction of the largest dam in Namibia. The hydrological analysis to support decisions implies large uncertainties owing to spatial-limited and short hydrological records (since 1962). Here we investigate the current and past patterns of extreme floods combining instrumental, historical, and palaeoflood records. Palaeoflood studies were performed at two reaches with preserved sedimentary evidence: at the upper sector (Vogelkranz) upstream of Hardap Dam (~13% catchment area) and in the lower part of the river, in the Fish River Canyon National Park (70% catchment area). In the Hardap reach, the palaeoflood record identified at least eight large floods during the last 350 years, with the largest flood reaching a minimum discharge of 4800 m3 s−1 (150-year return period). In the Fish River Canyon reach, the sedimentary record shows at least 12 large floods over the last 400 years, the largest with an estimated minimum discharge of 8700 m3 s−1. The elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages, associated with discharges of 6400 m3 s−1 and 16,140 m3 s−1 for the upper and mid-lower Fish River respectively. In the upper reach, the flood frequency analysis (FFA) combining systematic and palaeoflood data provided lower discharges (~25%) of the flood quantiles than the ones using only systematic data sets. In the Fish River Canyon reach, incorporation of the palaeoflood data into the FFA results in slightly higher values in the magnitude of the higher flood quantiles (~4–7%). The FFA analysis using an upper limited lognormal distribution function (LN4) fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit. Our flood frequency results suggest that the Hardap Dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria. However, the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations. Graphical abstract Unlabelled Image Highlights • Flood deposits in the largest ephemeral river in Namibia are described and dated. • Frequency of extreme floods over the last three centuries shown low variability. • Palaeoflood based upper bound discharge was used for flood frequency analysis. • Palaeoflood quantiles give lower discharge than re-evaluated for Hardap dam safety. • Flood hydrology in the Fish River required a review for dam-safety. [ABSTRACT FROM AUTHOR]

Quantitative geomorphic field studies and modeling efforts have focused on the margins of southwestern Africa as an example for landscape evolution in prolonged aridity conditions and tectonic quiescence of passive margins. These efforts concluded that this region is a prime example of a steady state landscape, in which relief changes extremely slowly. Using cosmogenic isotopes, these studies suggested overall landscape exhumation rates of 5–10 m Ma −1 over the past 10 5 –10 6  yrs. Slightly slower rates on flat-lying exposed bedrock surfaces and faster exhumation rates along the Namibian Great Escarpment as well as on steep slopes of granitic inselbergs, such as the Gross Spitzkoppe are also documented. Here we explore the state of “steady state” in central Namibia. Concentrations of 10 Be were measured in bedrock and sediment samples collected throughout the watershed of the Ugab River (~29,000 km 2 ), which drains the highlands of central Namibia and flows to the Atlantic Ocean. Samples were collected from the main stem of the ephemeral Ugab River, from the slopes and streams draining the Brandberg, which is the largest inselberg in the Namib, and from smaller inselbergs around it. We also sampled several other formerly large, but currently subdued, inselbergs such as the Messum Crater. 10 Be concentrations in sediment transported along the axial Ugab River indicate that its drainage basin erodes uniformly at 5–6 m Ma −1 and sediment transport from its headwaters source to the ocean is rapid. 10 Be concentrations measured in sediment transported in ephemeral streams draining the Brandberg indicate its erosion at ~4 m Ma −1 . However, slower rates of 1–3 m Ma −1 were measured for bedrock samples collected from (a) flat lying bedrock surfaces within the Brandberg, (b) top of small tors that rise only a few meters above their surroundings, and (c) exhumed and denuded large magmatic complexes such as the Messum Crater. Furthermore, we found that bedrock buried under grus in the hyperarid zone of Namib (<100 mm yr −1 ) erodes at similar rates as the exposed bedrock. This difference between the rate of bedrock erosion and the overall average erosion rate of drainage basins has been previously attributed to the contribution of sediment weathered from underneath transported sediment and soil on the pediments. Our results do not fully support this explanation. Results from this and earlier studies point to two possible sources of relatively low dosed (i.e. more rapidly eroding) sediment: (a) the steep slopes and cliffs of the large inselbergs and the Great Escarpment, and (b) rock buried under soil in the upper, semi-arid, parts of the drainage systems, where soil and vegetation can promote weathering of plagioclase and biotite and the disintegration of granitic bedrock. We therefore suggest that the “steady state” landscape along the Namibian passive margin be viewed as follows: The entire landscape erodes slowly, generally at ~5 m Ma −1 and this maintains the view of steady state. Small differences in erosion rates between the landscape elements result in very slow and only small changes in relief over time scales ≥10 6  yrs. We find that the large inselbergs and the Great Escarpment erode primarily by retreat of steep slopes and cliffs within the drainage basins while preserving relief over considerable timescales. In the wetter upper reaches of the Namibian drainage systems, erosion of buried rock is most likely increased by the vegetation-covered soil. [ABSTRACT FROM AUTHOR]

Namibia, with its passive margin setting, long-term tectonic stability, and long-lasting arid climate is a typical granitic landscape characterized by numerous dominating inselbergs. The Namib has been the focus of quantitative studies on the overall rates of bedrock exhumation, escarpment retreat, and sediment generation, transport, and deposition. Results from these studies indicate steady bedrock erosion rates ranging between 1 and 5mmkyr−1 over the last 105–106 yr. This rate was determined from samples collected mostly from small inselbergs with low relief. How fast the large and “classic” inselbergs, which dominate the Namib Desert landscape, erode, generate sediment, and contribute to the overall sediment supply has yet to be systematically examined. We document erosion rates of the Gross Spitzkoppe (GS), one of the largest inselbergs in the Namib Desert, Namibia, based on measured concentrations of 10Be in samples collected from bedrock, boulders, and surface grus. The results suggest a slow lowering rate (1–2mmkyr−1), while the overall slope processes deliver sediment to the base of the cliff with 5–6×105 atomsg−1 quartz. This concentration is equivalent to an average erosion rate of ~8mmkyr−1, 2–3 times faster than the bedrock lowering rate. Thus, the inselberg has been eroding by cliff retreat. The concentration of cosmogenic isotopes in slope sediment is controlled by exposure time on the slope, weathering of exposed bedrock, and weathering of subsurface bedrock. Sediment continues to accumulate cosmogenic isotopes while being transported on the flat pediments surrounding the inselberg. Within 1–2km from the base of the GS, the concentration doubles to ~10×105 atomsg−1 quartz. The increase is achieved by dosing due to exposure and mixing with highly dosed sediment eroded from near subsurface bedrock. From this area, grus is transported through a network of low gradient channels to the Atlantic Ocean, ~100km away. Cosmogenic isotope concentration in sediment increases to ~30×105 atoms of 10Beg−1 quartz, which is 3–4 times higher than that measured in the grus close to the GS and is controlled by the erosion rate of stable bedrock throughout the entire drainage basin. Dosing due to exposure during transport obviously contributes to the overall measured concentration in the sediment as it approaches the ocean. [ABSTRACT FROM AUTHOR]