A global rise in alluvial mining increases sediment load in tropical rivers.

Increasing gold and mineral mining activity in rivers across the global tropics has degraded ecosystems and threatened human health1,2. Such river mineral mining involves intensive excavation and sediment processing in river corridors, altering river form and releasing excess sediment downstream2. Increased suspended sediment loads can reduce water clarity and cause siltation to levels that may result in disease and mortality in fish3,4, poor water quality5 and damage to human infrastructure6. Although river mining has been investigated at local scales, no global synthesis of its physical footprint and impacts on hydrologic systems exists, leaving its full environmental consequences unknown. We assemble and analyse a 37-year satellite database showing pervasive, increasing river mineral mining worldwide. We identify 396 mining districts in 49 countries, concentrated in tropical waterways that are almost universally altered by mining-derived sediment. Of 173 mining-affected rivers, 80% have suspended sediment concentrations (SSCs) more than double pre-mining levels. In 30 countries in which mining affects large (>50 m wide) rivers, 23 ± 19% of large river length is altered by mining-derived sediment, a globe-spanning effect representing 35,000 river kilometres, 6% (±1% s.e.) of all large tropical river reaches. Our findings highlight the ubiquity and intensity of mining-associated degradation in tropical river systems.

Rapid changes to global river suspended sediment flux by humans.

Rivers support indispensable ecological functions and human health and infrastructure. Yet limited river sampling hinders our understanding of consequential changes to river systems. Satellite-based estimates of suspended sediment concentration and flux for 414 major rivers reveal widespread global change that is directly attributable to human activity in the past half-century. Sediment trapping by dams in the global hydrologic north has contributed to global sediment flux declines to 49% of pre-dam conditions. Recently, intensive land-use change in the global hydrologic south has increased erosion, with river suspended sediment concentration on average 41 ± 7% greater than in the 1980s. This north-south divergence has rapidly reconfigured global patterns in sediment flux to the oceans, with the dominant sources of suspended sediment shifting from Asia to South America.

Spatially coherent regional changes in seasonal extreme streamflow events in the United States and Canada since 1950.

Complex hydroclimate in the United States and Canada has limited identification of possible ongoing changes in streamflow. We address this challenge by classifying 541 stations in the United States and Canada into 15 "hydro-regions," each with similar seasonal streamflow characteristics. Analysis of seasonal streamflow records at these stations from 1910 to present indicates regionally coherent changes in the frequency of extreme high- and low-flow events. Where changes are significant, these events have, on average, doubled in frequency relative to 1950 to 1969. In hydro-regions influenced by snowmelt runoff, extreme high-flow event frequency has increased despite snowpack depletion by warming winter temperatures. In drought-prone hydro-regions of the western United States and Southeast, extreme low-flow event frequency has increased, particularly during summer and fall. The magnitude and regional consistency of these hydrologic changes warrant attention by watershed stakeholders. The hydro-region framework facilitates quantification and further analyses of these changes to extreme streamflow.

Heightened levels and seasonal inversion of riverine suspended sediment in a tropical biodiversity hot spot due to artisanal gold mining.

In recent years, rising gold prices have exacerbated the global proliferation of artisanal-scale gold mining (ASGM), with catastrophic consequences for human and ecological health. Much of this burgeoning industry has occurred in biodiversity hot spots, notably in the tropical forests of South America. While the loss of tropical forests and floodplains as a result of ASGM has been well characterized, ASGM impacts on riverine hydrological properties are less understood. Previous fieldwork on ASGM-affected and gully-eroded tropical streams and rivers has demonstrated that increases in suspended-sediment concentration (SSC) can substantially impact fish diversity and aquatic community structure, yet our understanding of the timing and scope of impacts of such increases is limited by the lack of long-term records of SSC. To address this challenge, we present a 34-y analysis of the direct effect of ASGM on 32 river reaches in the Madre de Dios region of Peru, which has been heavily impacted by ASGM since the 1980s. We evaluate spatial and temporal patterns of impacts using estimated SSC derived from Landsat satellite imagery. We find that 16 of 18 stretches of river impacted by ASGM show significant increasing trends in SSC (P < 0.05), while only 5 of 14 unaffected sites do so. Additionally, ASGM appears to reverse natural seasonal cycles of SSC, which may imperil aquatic species. Overall, our findings indicate that ASGM is fundamentally altering optical water quality dynamics of a critical tropical biodiversity hot spot and provide guidance for future regulation of these activities.