The Dire Challenge of Coastal Wetland Preservation: Inadequate River Sediment

by Liam O'Connor
coastal wetland conservation

Erosion and submersion issues plague a Barnstable, Massachusetts, salt marsh due to escalating sea levels, as observed on December 2, 2022 (Erin Peck credited for the photograph).

Efforts to elevate sinking wetlands in the face of rising sea levels have hit a roadblock, despite recent scientific progress towards finding potential remedies.

The increasing interest in large-scale dam removal projects as a means to counteract the diminishing of coastal wetlands – crucial for flood control, water purification, and wildlife habitats – is evident in the context of climate change. However, a new study published in Science posits that this approach is generally ineffective for most American rivers.

Scarcity of Sediment: A Major Obstacle in Wetland Revival

The core issue, as identified by the researchers, lies in the insufficient sediment supply. An analysis of roughly 5,000 rivers revealed that about 75% cannot supply enough sediment to balance the sea-level rise affecting their adjacent coastal regions. Nearly half of these rivers provide less than one-tenth of the necessary sediment.

This pioneering national-scale study investigated the potential of river-borne sediment to fortify coastal areas. Previous research mainly focused on a few larger rivers, like the Mississippi, or steep ones, like Washington’s Elwha, which aren’t typical of the majority in the contiguous U.S.

The study highlights that most American watersheds, being smaller in size, are not significant contributors to sediment accumulation in wetlands. It is in these smaller rivers where the majority of dams are found.

Perspectives on Dam Removal and Sediment Dynamics

Lead researcher Scott Ensign, Ph.D., from the Stroud Water Research Center, emphasizes the unique case of the Elwha River as a successful example of a dam removal project enhancing coastal sediment. However, he points out that rivers, particularly along the East and Gulf coasts, are less steep and carry less sediment compared to those on the West Coast. Consequently, the sediment volumes are insufficient for the larger wetlands in these regions, which are under threat from rising sea levels.

Christopher Craft, Ph.D., a wetland restoration and climate change expert at the University of Indiana, concurs, emphasizing the inadequacy of sediment supplies in most coastal watersheds to support tidal wetlands against rising sea levels.

Study Methodology and Conclusions

Ensign, along with Joanne Halls from the University of North Carolina Wilmington and Erin Peck from the University of Massachusetts, utilized data from the U.S. Geological Survey and the National Oceanic and Atmospheric Administration. They employed ArcGIS Pro technology by Esri to model sediment supply to coastal wetlands, comparing their findings with existing data on tidal wetlands changes across the U.S.

Ensign explains that the primary source of sediment aiding most wetlands against drowning does not originate from upstream rivers. He suggests that dam removal in many East Coast locations will not be beneficial, urging the exploration of alternative solutions.

Wetland Conservation Implications

Geological sciences professor James Pizzuto from the University of Delaware praises the study for its innovative approach to a complex issue. He stresses the need for targeted conservation efforts, focusing on processes other than watershed sediment.

Donald F. Boesch, professor emeritus at the University of Maryland Center for Environmental Science, suggests focusing on retaining more mineral sediments, plant matter, and organic carbon in wetland soils, especially in sediment-deprived areas or those like the Mississippi Delta, where sediment diversion is used to sustain rapidly rising wetlands.

Future Research Directions and Conservation Tactics

Further research is essential to gauge the amount of sediment trapped behind specific dams and its potential impact on downstream tidal wetlands.

Ensign advocates for the most crucial step in preserving tidal wetlands: allowing their migration upslope. This may involve restoring natural water flows and conserving low-lying lands. He also mentions the potential benefits of applying sediment directly and other engineering methods on a local scale.

The study, titled “Watershed sediment cannot offset sea level rise in most US tidal wetlands” by Scott H. Ensign, Joanne N. Halls, and Erin K. Peck, was published on December 7, 2023, in Science (DOI: 10.1126/science.adj0513). Funding was provided by the National Science Foundation.

Frequently Asked Questions (FAQs) about coastal wetland conservation

What is the main issue with coastal wetland conservation as per the study?

The study reveals that the main issue in coastal wetland conservation is the insufficient supply of sediment from rivers. This shortfall makes it difficult to combat the effects of rising sea levels, as most rivers cannot deliver enough sediment to sustain the wetlands.

How does dam removal factor into wetland preservation?

Dam removal projects, especially on smaller rivers, have been considered as a potential solution to enhance sediment flow to coastal wetlands. However, the study finds that dam removal is generally ineffective in most U.S. rivers for this purpose, due to the inherently low sediment levels in these rivers.

What are the implications of this study for future conservation efforts?

The study suggests that conservation efforts should focus beyond just watershed sediment. This includes finding ways to maintain mineral sediments, plant material, and organic carbon in wetland soils, and exploring alternative methods like direct sediment application and other engineering approaches.

Why are smaller rivers and their dams significant in this context?

Most U.S. watersheds are small and are not the main contributors of sediment to wetlands. Since these smaller rivers are where most dams are located, understanding their role in sediment supply is crucial for effective wetland conservation strategies.

What future research is needed according to the study?

Future research should aim to measure the sediment trapped behind specific dams and predict its impact on downstream tidal wetlands. This will help in devising more targeted and effective conservation strategies for coastal wetlands.

More about coastal wetland conservation

  • Coastal Wetlands and Climate Change
  • Sediment Supply in River Systems
  • Dam Removal and Environmental Impact
  • Sea Level Rise and Coastal Erosion
  • Wetland Conservation Strategies
  • River Science and Sediment Dynamics
  • Tidal Wetlands Research
  • Climate Change and Coastal Habitats
  • U.S. Geological Survey: River Data
  • National Oceanic and Atmospheric Administration: Coastal Studies

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Sarah Johnson December 19, 2023 - 12:20 am

i’m not sure if I completely get it, how does removing dams not help? thought that would be a no-brainer for helping the environment

Emily Wilson December 19, 2023 - 12:21 am

this is kind of alarming, we need to do more to protect our coasts, especially with sea levels rising, good to see research on this though!

Kevin Brown December 19, 2023 - 12:12 pm

good read, but there’s some typos in there, like “wetland preservation” think it should be preservation? and some sentences are a bit long, hard to follow

Mike Smith December 19, 2023 - 3:22 pm

interesting article but i think it could use more on the impact of climate change on coastal areas, seems like a big factor?

Jane Doe December 19, 2023 - 7:14 pm

wow, this is pretty insightful, didn’t realize how big a deal sediment was for wetlands, makes me think more about how we manage rivers


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