Sinks Before the Sea: Rethinking Source‑to‑Sink Pathways in a Dam‑Stepped River. An example from the Citarum River, Java, Indonesia.

Hydropower is increasingly central to low carbon energy strategies worldwide, yet its geomorphic and ecological impacts remain incompletely understood. While reductions in downstream sediment load below dams is well documented, a key unresolved challenge lies in quantifying how sediment capture by reservoirs alters sediment composition and disrupts source-to-sink pathways. These compositional changes influence nutrient transfer, habitat quality, and long-term landscape evolution making it essential to understand how dams reshape sediment routing.  

The Citarum River, West Java, Indonesia, provides an ideal setting to investigate these dynamics. Flowing ~300km from its source in the volcanic highlands above Bandung to the Java sea, the river hosts three well established dams, Saguling, Cirata, and Jatiluhur, characterised by well-constrained geology and rapid hydropower expansion.  Indonesia aims to produce an additional 16GW of hydropower by 2034 as part of a wider goal to increase their overall renewable energy share. Sediment samples were collected along the Citarum River over two field seasons. The 2023 campaign targeted the main channel, comparing sediments entering and exiting reservoirs with upstream reference sites to evaluate the influence of artificial sediment sinks.  The 2024 campaign expanded sampling to tributaries to contextualise downstream compositional changes and address gaps identified in 2023. Heavy mineral analysis has been completed for 16 samples, providing a sensitive tracer of sediment provenance and transport processes. 

Heavy mineral assemblages are dominated by pyroxene and hornblende, reflecting the volcanic lithologies of West Java. Systematic shifts occur across hydrological boundaries: modern sediments exhibit a low hornblende-pyroxene ratio, whereas ancient deposits within Saguling reservoir show a significantly higher hornblende-pyroxene ratio, potentially reflecting longer residence times and preferential pyroxene alteration under hot, humid conditions. Increased hornblende-pyroxene ratios in modern low-energy settings further support energy-dependent mineral sorting. Reduced diversity in the Cirata reservoir, followed by downstream increases in e.g., rutile, xenotime, and zircon highlight disruptions in source-to sink-connectivity linked to reservoir trapping, tributary inputs, and/or channel erosion. Hydrological and sediment-transport modelling in MATLAB with TopoToolbox and CASCADE Toolbox, incorporating discharge, rainfall, and runoff datasets is underway to evaluate the influence of Java’s strong seasonal hydrology on sediment transport and composition. Integrating these models with heavy mineral stratigraphy will help assess the reliability of mineralogical signals as proxies for dam-driven perturbations to sediment routing.  

This combined stratigraphic and modelling approach provides new insight into how hydropower infrastructure reshapes sediment pathways from source to sink, with implications for future hydropower development in Indonesia and other rapidly urbanising, energy‑intensive regions.