Coastal sedimentation across North America doubled in the 20th century despite river dams

Coastal communities and ecosystems face increasing threats from sea-level rise (SLR) and storms. Intertidal habitats, crucial for coastal protection and estuarine health, require sufficient sediment to maintain their extent in the face of SLR. The widespread construction of dams since 1950 has reduced river sediment loads, raising concerns about coastal sediment starvation. This study challenges this assumption, investigating whether reduced upstream sediment delivery translates to decreased coastal sedimentation. The researchers hypothesize that downstream sediment sources might compensate for sediment lost to reservoirs, leading to constant or increased coastal sedimentation rates. The study uses long-term sediment accumulation data from 25 coastal sites across North America to test this hypothesis, focusing on coastal depocenters (net depositional basins). Understanding these sediment dynamics is crucial for effective coastal management and habitat restoration in the context of accelerating SLR and increasing coastal risks.

Previous research has shown that upstream land-use changes initially increased river sediment loads due to soil erosion, but subsequent watershed management, dam construction, and alterations to river networks have reduced sediment delivery to the coast. This reduction, coupled with SLR and land subsidence, contributes to coastal retreat and marsh disintegration. While the research community emphasizes sea-level monitoring and modeling, studies integrating sediment measurements into coastal risk assessments are limited. The existing literature highlights the importance of sediment supply for coastal ecosystems, particularly intertidal habitats, and the potential failure of restoration projects without adequate sediment supplementation in the face of SLR. However, a comprehensive analysis of coastal sedimentation trends across North America, accounting for the effects of dams and other human impacts, was lacking before this study.

The researchers compiled records of mass accumulation rates (MAR; g cm⁻² yr⁻¹) and sediment accumulation rates (SAR; cm yr⁻¹) from 25 coastal sites across North America using published and new sediment core data. These sites represent a range of geologic and climatic settings, including estuaries and inner continental shelves. The researchers selected depocenters as archives of regional sedimentation changes, focusing on sites where sedimentary processes were relatively constant over time. They used ²¹⁰Pb geochronologies to establish sedimentation rates over the last century. Data on coastal population growth were also incorporated to account for potential human impacts on sedimentation. The researchers analyzed the data to assess whether sedimentation rates changed significantly before and after 1950, when dam construction increased substantially in North America. Statistical tests were used to compare pre- and post-1950 MAR and SAR values, and the relationship between sedimentation and relative sea-level rise was also examined. The study carefully considered potential confounding factors such as sediment sources, surface mixing, bioturbation, and the Sadler effect.

The study found that sedimentation rates in coastal depocenters across North America more than doubled after 1950. This increase was observed despite the widespread construction of dams that reduced river sediment loads. The analysis of MAR and SAR data showed a consistent pattern of increased sedimentation at most sites after 1950. The increase in MAR values was observed across diverse settings, suggesting a broad-scale phenomenon. The researchers ruled out several factors that could have potentially influenced the MAR increase, such as changes in surface mixing or bioturbation. They propose that the increased sedimentation is primarily due to supplemental sediment sources downstream of dams. These sources include erosion from human-modified landscapes, shoreline erosion, and the mobilization of legacy sediments. While many sites show sediment accumulation exceeding or matching relative sea-level rise, rapidly subsiding areas in Texas and Louisiana are experiencing increasing water depths and loss of intertidal habitats. The study specifically analyzed changes in mass and sediment accumulation rates pre and post 1950, which showed a greater than doubling of rates. Specific data points are provided in figures and tables within the paper showing the range of MAR and SAR values observed across various sites. The magnitude of the increase varied across sites, but the overall trend was consistent. Coastal population growth data showed a correlation with increased sedimentation in many areas, further supporting the idea of human-induced changes to sediment supply.

The findings challenge the prevailing assumption that dam construction leads to coastal sediment starvation. The study demonstrates that downstream sediment sources can effectively compensate for reduced river sediment loads. This highlights the complex interplay between upstream damming and downstream sedimentation patterns. The results suggest that human activities, particularly land-use changes and urbanization, significantly contribute to coastal sedimentation, even in the presence of upstream dams. The contrasting experiences of sediment-balanced and rapidly subsiding coastal regions underscore the importance of considering local factors, like land subsidence, when assessing coastal sediment budgets and implementing management strategies. The study's findings have significant implications for coastal zone management, habitat restoration, and risk assessment. The need for comprehensive sediment management strategies that incorporate both upstream and downstream sediment sources is emphasized.

This study reveals a significant increase in coastal sedimentation across North America since 1950, despite reduced riverine sediment loads due to dam construction. Downstream sediment sources, largely attributed to human-modified landscapes, compensate for sediment lost in reservoirs. While many areas maintain sediment equilibrium with relative sea-level rise, rapidly subsiding regions face challenges. Future research should focus on quantifying the relative contributions of different downstream sediment sources and improving predictive models to better inform coastal management and restoration strategies in the context of accelerating sea-level rise.

The study primarily focuses on North American coastal depocenters and may not be directly generalizable to other regions. While the researchers considered several potential confounding factors, uncertainties remain regarding the precise relative contributions of different downstream sediment sources. The analysis relies on existing sediment core data, which may have limitations in spatial and temporal coverage. The study assumes a relatively constant relationship between sedimentation rates and SLR, although this relationship may be affected by factors not fully considered in the study.