Mountain glaciers significantly contribute to Earth's topographic relief through erosion. The sediment and solutes produced by glaciers are transported to lowlands via ice, wind, and water, influencing landform evolution, ecosystems, and human activities like water management and hydropower. In the subtropical Andes, increased sediment export has negatively impacted reservoir capacity and drinking water supply, while potentially benefiting coastal ecology. Climate warming is anticipated to decrease glaciers' erosive capacity as they thin and retreat, leading to reduced sediment production. However, a transient increase in sediment export, termed 'peak sediment', may occur after deglaciation begins due to increased accessibility of long-term stored sediment within the subglacial hydrological system. The timing of peak sediment is crucial for ecological and societal impact assessments but remains challenging to evaluate due to the delayed and non-linear response of glacier sediment export to climate forcing. This study uses long-term sediment discharge data from eleven subtropical Andean rivers to investigate the future trajectory of glacier sediment export. The researchers analyze the evolving relationship between water and sediment released by glaciers to understand the temporal evolution of glacial sediment availability in response to changing glaciological, climatic, and hydrological factors. The study's focus on suspended sediment concentration (SSC) is justified by its comparability in magnitude to sediment from snowmelt, yet with a higher solid:liquid ratio due to its circulation over a sediment-rich glacial bed.

Existing research highlights the impact of glaciers on mountain erosion and the potential for increased sediment export during deglaciation. Studies have explored the concept of 'peak water' and its analogous 'peak sediment', emphasizing the dependence of peak sediment on climate variability, glacier thermal regime, bed topography, and the mass and distribution of stored sediment. Global projections for the end of peak sediment vary, with estimates ranging between 2100 and 2200 globally, and earlier termination (2070-2100) for certain alpine catchments. The current study builds on this existing work by providing a detailed analysis of long-term sediment data from a specific region, adding valuable site-specific knowledge to the broader global discussion.

This study utilized six-decade-long (1960s-present) sediment discharge series from eleven subtropical Andean rivers (27–35°S). The researchers examined the evolving relationship between water and sediment released by glaciers to gain insights into the temporal evolution of glacial sediment availability. The catchments had varying ice volumes, but the gauging stations were consistently located tens to hundreds of kilometers from glaciers' termini. Measurements focused on suspended sediment concentration (SSC). A sequential regime shift detection method was applied to the annual series of the three glacierized catchments with the most complete records, identifying three distinct regimes based on SSC values: a high SSC regime (late 1960s-mid 1970s), a low SSC regime (late 1970s-2000s), and another high SSC regime (2010s). The analysis incorporated additional catchments to assess the prevalence of the high SSC periods. To understand the causes of SSC changes, the study examined glacier runoff variations in the Maipo River basin using annual estimates of ice melt and glacier area from the TOPKAPI-ETH model. The model was validated using streamflow and snow cover area measurements. Annual averages of total ice melt and ice melt normalized per glacierized area were calculated for each SSC regime. The study also assessed sediment availability by analyzing the temporal variations in the interannual relationship between ice melt and the number of extreme turbidity events (ETE), defined as days with SSC values above the 85th quantile during the warm and dry season. The ETE analysis was primarily conducted for the Aconcagua River basin due to its data completeness and correlation with other regional turbidity series. The relationship between ice melt and ETE frequency was quantified using the coefficient of correlation and linear regression analysis. The study also investigated the role of other secondary sediment sources, including permafrost degradation, paraglacial landslides, and glacial lake outburst floods (GLOFs).

The analysis revealed three distinct regimes of suspended sediment concentration (SSC) in the subtropical Andes: a high SSC regime in the 1970s, a low SSC regime lasting until the 2000s, and another high SSC regime in the 2010s. Importantly, the SSC values during the most recent high SSC regime were roughly half those observed in the 1970s. This difference suggests that the reduced magnitude in the recent high SSC period is due to the depletion of glacial sediment rather than other factors such as reduced meltwater accessibility. The analysis of the Maipo River basin, for which ice melt data was available, indicated that both high SSC periods experienced greater total and specific ice melt than the low SSC period. However, the 1970s period had greater total melt but lower melt per unit area compared to the 2010s, consistent with a larger glacierized area and slightly colder, wetter climate during the 1970s. Further analysis of the relationship between ice melt and extreme turbidity events (ETE) showed a positive correlation during both high SSC regimes and a negative correlation during the low SSC regime. The intercept of the ice melt-ETE relationship, a proxy for sediment availability, was significantly higher in the 1970s and decreased until the present day, when it showed a slight recovery. This decrease in sediment availability, coupled with decreasing regional glacial water flux, strongly suggests that the peak sediment has largely passed its maximum in most of the study area. However, there is an exception in the Pulido River basin, where a high elevation and likely cold based glaciers may have yet to experience the peak sediment. A basin-scale relationship was observed between long-term decreases in SSC and glacier volume, except for the Pulido River basin, indicating that the timing of peak sediment varies based on glacier characteristics and elevations.

The findings of this study challenge previous assumptions about peak sediment in deglaciating regions. The significant difference between the magnitude of the high SSC regimes and the observation that the decline in sediment concentration correlates with glacial volume strongly suggests that the peak sediment has passed its maximum for many of the subtropical Andean rivers studied, although not for those with predominantly colder based glaciers at high elevations. The multidecadal variability observed in the ice melt-ETE relationship highlights the influence of regional climate and the memory of the glacio-hydrological system on sediment transport. The results demonstrate that the timing and magnitude of peak sediment are strongly influenced by a complex interplay of factors, including topography, climate, and hydro-sedimentological connectivity.

This study, using unique, long-term sediment concentration data from the subtropical Andes, significantly advances the understanding of the 'peak sediment' phenomenon. The findings indicate that peak sediment has likely passed its maximum for polythermal glaciers, but not for colder, high-elevation glaciers. This emphasizes the importance of considering glacier thermal regime, topography, and hydro-sedimentological connectivity when predicting sediment export under climate change. Future research should focus on expanding the analysis to other regions and incorporating more detailed modeling approaches to better understand the complex dynamics of peak sediment.

The study focused primarily on suspended sediment concentration and did not directly measure sediment mass flux. The analysis relied on modeled ice melt data for the Maipo River basin and the extrapolation of these results to other basins might not perfectly capture the local variability. The influence of factors such as permafrost degradation and paraglacial landslides was considered, but further investigation is needed to quantify their contribution to the observed sediment fluxes. The study's findings may not be generalizable to regions with different climatic, topographic, or glaciological conditions.