Lowland multi-channel alluvial river systems are highly variable in frequency and magnitude of floodplain inundation and are vulnerable to human activities such as damming. In the Yangtze River Basin, the Three Gorges Dam (TGD) has trapped >80% of upstream sediment supply, causing downstream scouring and rapid geomorphic changes in river and its floodplain lakes. Suspended sediment concentration (SSC) is widely used to monitor river morphodynamics, but traditional measurements of SSC are time consuming, costly and difficult to quantify SSC in a large spatial scale. Using Google Earth Engine and in situ observed hydrological data, we created a multiple linear regression model to map SSC in the multi-channel system Songzi River of the Yangtze. The new SSC predictive model achieved high accuracy (R2: 0.87) and showed opposite downstream trends in SSC during peak flood years and normal flood years. For the first time, we found that in the peak flood year of 1998 the study rivers exhibit a downstream increase trend of SSC with an abrupt increase in their middle reach, while SSC in normal flood years experiences a downstream decline with minimal changes. A prominent difference in SSC is also revealed after the TGD with a reduction of 60%. Furthermore, SSC in the closely hydrologically connected lakes is more dynamic than the less connected lakes. Our study demonstrates that the proposed method enables to rapidly quantify the spatio-temporal SSC distribution in the multi-channel systems on the floodplain of large alluvial rivers and highlights the importance of connectivity in regulating SSC dynamics.

Climate change affects cryosphere-fed rivers and alters seasonal sediment dynamics, affecting cyclical fluvial material supply and year-round water-food-energy provisions to downstream communities. Here, we demonstrate seasonal sediment-transport regime shifts from the 1960s to 2000s in four cryosphere-fed rivers characterized by glacial, nival, pluvial, and mixed regimes, respectively. Spring sees a shift toward pluvial-dominated sediment transport due to less snowmelt and more erosive rainfall. Summer is characterized by intensified glacier meltwater pulses and pluvial events that exceptionally increase sediment fluxes. Our study highlights that the increases in hydroclimatic extremes and cryosphere degradation lead to amplified variability in fluvial fluxes and higher summer sediment peaks, which can threaten downstream river infrastructure safety and ecosystems and worsen glacial/pluvial floods. We further offer a monthly-scale sediment-availability-transport model that can reproduce such regime shifts and thus help facilitate sustainable reservoir operation and river management in wider cryospheric regions under future climate and hydrological change. Intensified glacier melt and discharge pulses remarkably increase summer sediment fluxes and threaten social-ecological systems.

The sensitivity of fluvial sediment load to climate change and predictions of future sediment load in cold basins remain poorly investigated, although changes in river sediment transport have important geomorphological, ecological, and societal implications. Here, we adapt a sediment elasticity approach to examine the sensitivity of fluvial suspended sediment load to changes in air temperature and precipitation in the headwater of the Yangtze River (HYR) on the inner Tibetan Plateau. Results show that every 1 °C increase in air temperature can increase the suspended sediment load by 14–27 % by intensifying thermally-driven glacial and permafrost erosional processes, and every 10 % increase in precipitation can increase the suspended sediment load by 16–24 % through enhancing pluvial-driven erosional processes. We predict an increase of 60–85 % in the suspended sediment loads in HYR by 2050 relative to the present-day period under the Representative Concentration Pathway 4.5, as both air temperature and precipitation are projected to increase. Our analysis highlights that smaller upland rivers appear to respond to modern climate change more rapidly and intensively than larger downstream rivers due to the larger glacier and permafrost coverages, poorer vegetation, as well as steeper fluvial relief, and higher sediment connectivity. This study provides a framework and a data-driven sediment elasticity approach to predict climate change and cryosphere degradation-driven changes in future fluvial suspended sediment load in cold basins, highlights the importance of the spatial scale effects in modulating fluvial responses, and has implications for assessing the impacts of climate change on channel morphology and aquatic ecosystems.

The benefits of developing the world’s hydropower potential are intensely debated when considering the need to avoid or minimize environmental impacts. However, estimates of global unused profitable hydropower potential with strict environmental constraints have rarely been reported. In this study we performed a global assessment of the unused profitable hydropower potential by developing a unified framework that identifies a subset of hydropower station locations with reduced environmental impacts on the network of 2.89 million rivers worldwide. We found that the global unused profitable hydropower potential is 5.27 PWh yr−1, two-thirds of which is distributed across the Himalayas. Africa’s unused profitable hydropower is 0.60 PWh yr−1, four times larger than its developed hydropower. By contrast, Europe’s hydropower potential is extremely exploited. The estimates, derived from a consistent and transparent framework, are useful for formulating national hydropower development strategies.

Global warming-induced melting and thawing of the cryosphere are severely altering the volume and timing of water supplied from High Mountain Asia, adversely affecting downstream food and energy systems that are relied on by billions of people. The construction of more reservoirs designed to regulate streamflow and produce hydropower is a critical part of strategies for adapting to these changes. However, these projects are vulnerable to a complex set of interacting processes that are destabilizing landscapes throughout the region. Ranging in severity and the pace of change, these processes include glacial retreat and detachments, permafrost thaw and associated landslides, rock–ice avalanches, debris flows and outburst floods from glacial lakes and landslide-dammed lakes. The result is large amounts of sediment being mobilized that can fill up reservoirs, cause dam failure and degrade power turbines. Here we recommend forward-looking design and maintenance measures and sustainable sediment management solutions that can help transition towards climate change-resilient dams and reservoirs in High Mountain Asia, in large part based on improved monitoring and prediction of compound and cascading hazards.

Rapid atmospheric warming since the mid-twentieth century has increased temperature-dependent erosion and sediment-transport processes in cold environments, affecting food, energy and water security. In this Review, we summarize landscape changes in cold environments and provide a global inventory of increases in erosion and sediment yield driven by cryosphere degradation. Anthropogenic climate change, deglaciation, and thermokarst disturbances are causing increased sediment mobilization and transport processes in glacierized and periglacierized basins. With continuous cryosphere degradation, sediment transport will continue to increase until reaching a maximum (peak sediment). Thereafter, transport is likely to shift from a temperature-dependent regime toward a rainfall-dependent regime roughly between 2100–2200. The timing of the regime shift would be regulated by changes in meltwater, erosive rainfall and landscape erodibility, and complicated by geomorphic feedbacks and connectivity. Further progress in integrating multisource sediment observations, developing physics-based sediment-transport models, and enhancing interdisciplinary and international scientific collaboration is needed to predict sediment dynamics in a warming world.

Today, climate change is affecting virtually all terrestrial and nearshore settings. This commentary discusses the challenges of measuring climate-driven physical landscape responses to modern global warming: short and incomplete data records, land use and seismicity masking climatic effects, biases in data availability and resolution, and signal attenuation in sedimentary systems. We identify opportunities to learn from historical and paleo data, select especially sensitive study sites, and report null results to better characterize the extent and nuances of climate-change effects. We then discuss efforts to improve attribution practices, which will lead to better predictive capabilities. We encourage the Earth-science community to prioritize scientific research on climate-driven physical landscape changes so that societies will be better prepared to manage the effects on health and safety, infrastructure, water–food–energy security, economics, and ecosystems that follow from climate-driven physical landscape change.

Abstract Understanding the mechanisms controlling downstream water-level variations after the operation of the Three Gorges Dam is important for riverine flood and drought management. However, our quantitative understanding of the multiple controls of river morphology, vegetation, and floodplain resistance on water levels in the Middle Yangtze River (MYR) remains limited. Here, we analyze changes in river channels and floodplain resistance in the MYR using 450 cross-sectional profiles as well as data on discharge, water levels, sediment, and satellite images from 2003 to 2015. Results show an overall decline in low-flow water-levels (at a given small discharge) due to severe incisions of low-flow channels caused by a sharp reduction of ?90% in sediment loads from 1950?2002 to 2003?2020. In contrast, high-flow water-levels (at a given large discharge) display minor changes. Our analysis shows that the notably increased floodplain resistance due to vegetation growth is likely the dominant factor elevating flood water-levels, followed by riverbed coarsening and greater fluctuations in the river longitudinal profiles. Our findings further the understanding of downstream geomorphic response to dam operation and their impacts on water levels and have important implications for riverine flood management in dammed river systems.

Abstract Climate change will likely increase the total streamflow in most headwaters on the Tibetan Plateau in the next decades, yet the response of runoff components to climate change and permafrost thaw remain largely uncertain. Here, we investigate the changes in runoff components under a changing climate, based on a high-resolution cryosphere-hydrology model (Spatial Processes in Hydrology model, SPHY) and multi-decadal streamflow observations at the upstream (Jimai) and downstream stations (Maqu and Tangnaihai) in the source-region of the Yellow River (SYR). We find that rainfall flow dominates the runoff regime in SYR (contributions of 48%?56%), followed by snowmelt flow (contributions of 26%/23% at Maqu/Tangnaihai). Baseflow is more important at Jimai (32%) than at the the downstream stations (21%?23%). Glacier meltwater from the Anyê Maqên and Bayankala Mountains contributes negligibly to the downstream total runoff. With increasing temperature and precipitation, the increase in total runoff is smaller in the warm and wet downstream stations than in the cold and dry upstream station. This is because of a higher increase in evapotranspiration and a larger reduction in snowmelt flow in the downstream region in response to a warming climate. With temperature increase, there is less increase in rainfall flow in the downstream region due to increased water loss through evapotranspiration. Meanwhile, the decline in snowmelt flow is larger further downstream, which can negatively impact the spring irrigation for the whole Yellow River basin that supports the livelihoods of 140 million people. Importantly, we find that baseflow plays an increasingly important role in the permafrost-dominated upstream region with atmospheric warming and permafrost thaw, accompanied by decreased surface flow. These findings improve our current understanding of how different hydrological processes respond to climate change and provide insights for optimizing hydropower and irrigation systems in the entire Yellow River basin under a rapidly changing climate.

Dams alter downstream river flow and sediment regimes, causing significant changes in river morphologies. The middle Yangtze River downstream of the Three Gorges Dam (TGD) has experienced rapid erosion in recent years, and the associated morphodynamic changes have negatively impacted the bank stability, navigation waterways and ecological functioning. Earlier studies have analyzed recent channel adjustments in the Yangtze River; however, our understanding of changes in the bar morphodynamics remains incomplete. In this study, we collected and analyzed flow and sediment data (1991∼2016) and river bathymetry data (1975∼2017) and investigated the mechanisms of bar adjustments along the Jingjiang Reach in the post-TGD period. The results indicate that most steady bars with a higher elevation and better vegetation coverage have experienced lateral erosion, while their elevations have remained stable overall. The unvegetated migrating bars with a lower elevation have experienced severe surficial erosion and area shrinkage. A new assessment method of the dominant discharge range on downstream bars is provided after dam closure. The new dominant discharge range that determines bar adjustments corresponds to the flow stages between the submersion of migrating bars (∼12,500 m3/s) and the overtopping of vegetated steady bars (∼27,500 m3/s) in the Jingjiang Reach. A quantitative relationship exists between bar erosion in response to changed dominant discharge regimes and sediment decline. The significant bar erosion after the operation of the TGD can be attributed to the increased flow duration and the sharp decline in the suspended sediment concentration (SSC) of the dominant discharge range, and changes in SSCs play a primary role. The bar area will decrease by ∼0.003 km2 when the annual cumulative duration of the dominant discharge increases by one day and by ∼0.234 km2 when the annual average SSC of the dominant discharge decreases by 0.01 kg/m3. Furthermore, vegetation encroachment and colonization play a positive role in stabilizing bar morphologies and limiting surficial erosion, whereas vegetation cannot prevent the lateral erosion of steady bars. These findings suggest that multiple controls, including flow, sediment and vegetation, shape the evolution of fluvial bars and have important implications for river management and ecological evaluation in response to the operation of large dams.

AbstractApproximately 40% of the Tibetan Plateau (TP) is underlain by continuous permafrost, yet its impact on fluvial water and sediment dynamics remains poorly investigated. Here we show that water and sediment dynamics in the permafrost-dominated Tuotuohe basin on the TP are driven by air temperature and permafrost thaw, based on 33-year daily in-situ observations (1985-2017). Air temperature regulates the seasonal patterns of discharge and suspended sediment concentration (SSC) by controlling the changes in active contributing drainage area (ACDA, the unfrozen erodible landscape that contributes hydrogeomorphic processes within a catchment) and governing multiple thermal processes such as glacier-snow melt and permafrost thaw. Rainstorms determine the short-lived fluvial extreme events by intensifying slope processes and channel erosion and likely also by enhancing thaw slumps. Furthermore, the SSCs at equal levels of discharges are lower in autumn (September-October) than in spring (May-June) and summer (July-August). This reduced sediment availability in autumn can possibly be attributed to the increased supra-permafrost groundwater runoff and the reduced surface runoff and erosion. Due to rapid climate warming, the ACDA has increased significantly from 1985 to 2017, implying expanding landscapes for hydrogeomorphic processes. As a result, the fluvial water and sediment fluxes have substantially increased. In a warmer and wetter future for the TP, the fluvial sediment fluxes of similar permafrost-underlain basins will continue to increase with expanding erodible landscapes and intensifying thermal and pluvial-driven geomorphic processes. Thus, permafrost thaw should be considered as an important driver of past and future water and sediment changes for the TP.This article is protected by copyright. All rights reserved.

Abstract Accelerated glacier-snow-permafrost erosion due to global warming amplifies the sediment availability in cold environments and affects the time-varying suspended sediment concentration (SSC) and discharge (Q) relationship. Here, the sediment-availability-transport (SAT) model is proposed to simulate dynamic SSC-Q relationships by integrating the sediment availability coupled by thermal processes, fluvial processes and long-term storage exhaustion into a sediment rating curve (SSC = a ? Qb with a and b as fitting parameters). In the SAT-model, increased sediment sources from glacier-snow-permafrost erosion are captured by changes in basin temperature, showing an exponential amplification of SSC when basin temperature increases. Enhanced fluvial erosion by the elevated water supply from rainfall and meltwater is captured by the factor of runoff surge, which results in a linear amplification of SSC. The SAT-model is validated for the permafrost-dominated Tuotuohe basin on Tibetan Plateau utilizing multi-decadal daily SSC/Q in-situ observations (1985?2017). Results show that sediment rating curves for Tuotuohe display significant inter-annual variations. The higher parameter-b in a warmer and wetter climate confirms the increased sediment availability due to the expanded erodible landscapes and gullying-enhanced connectivity between channels and slopes. Through capturing such time-varying sediment availability, the SAT-model can robustly reproduce the long-term evolution, seasonality, and various event-scale hysteresis of SSC, including clockwise, counter-clockwise, figure-eight, counter-figure-eight, and more complex hysteresis loops. Overall, the SAT-model can explain over 75% of long-term SSC variance with stable performance under hydroclimate abrupt changes, outperforming the conventional and static sediment rating curve approach by 20%. The SAT-model not only advances understanding of sediment transport mechanisms by integrating thermal- and fluvial-erosion processes, but also provides a model framework to simulate and project future sediment loads in other cold basins.

Abstract The long-term effects of increased temperatures on sediment fluxes in cold regions remain poorly investigated. Here, we examined the multidecadal changes in runoff and sediment fluxes in the Tuotuohe River, a headwater river of the Yangtze River on the Tibetan Plateau (TP). The sediment fluxes and runoff increased at rates of 0.03 ± 0.01 Mt/yr (5.9 ± 1.9%/yr) and 0.025 ± 0.007 ? km3/yr (3.5 ± 1.0%/yr) from 1985 to 2016, with net increases of 135% and 78% from 1985?1997 to 1998?2016, respectively. The increases are primarily due to warming temperature (+1.44°C) and intensified glacier-snow-permafrost melting, with enhanced precipitation (+30%) as a secondary cause. Sediment fluxes are much more susceptible to climate warming than runoff in this undisturbed cold environment. The substantially increased sediment fluxes from the headwater region could threaten the numerous constructed reservoirs and influence the aquatic ecosystems of the TP and its marginal areas.

The Three Gorges Dam (TGD) has altered downstream flow–sediment regimes and led to significant changes in the morphodynamic processes in the Middle Yangtze River (MYR). However, due to the complexity of this large river, the driving forces and implication of the morphodynamic processes remain insufficiently understood. This study selected two typical meandering and bar-braided reaches, the Zhicheng (ZC) and Shashi (SS) reach, to examine their responses to the TGD operation. The results showed that in the post-dam period significant channel erosion occurred with a higher erosion rate in the ZC reach (closer to the TGD) compared with the SS reach. The area of the Guanzhou mid-channel bar (ZC reach) and the Sanba mid-channel bar (SS reach) shrank by 30 and 90% from 2003 to 2015, respectively. The increased fluvial erosion intensity due to the reduction in suspended sediment concentration (SSC) drove the shrinkage of the mid-channel bars, as demonstrated by empirical relationships between bar geometry and fluvial erosion intensity. An increase of 22 days per year in the frequency of post-dam medium-to-high discharges (10 000–25 000 m3 s−1), and associated with the reduction in SSC, jointly led to the greater erosion at the convex (inner) banks than the concave (outer) banks, which has negatively affected the designed navigation channels at the concave banks by decreasing their discharge partitioning ratios. The post-dam water level at a given high discharge (>25 000 m3 s−1) showed no evident change, but the water level at a given low discharge (<10 000 m3 s−1) decreased. The reduction in water levels at low flows can affect water supply and riverine ecosystems in the MYR.