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For thousands of years, coastal marshes and mangroves have kept pace with the sea, building themselves up through biological and physical processes as sea level rose. As sea-level rise accelerates, they are increasingly at risk of drowning. That matters: coastal wetlands, salty and fresh, protect against floods, clean water and reduce shoreline erosion, making them among the most valuable ecosystems in the world. To know which ones are falling behind, scientists measure their elevation over time — but until this U.S. Geological Survey fact sheet, no one had pulled together where that is being measured.
How wetland elevation is measured
The Surface Elevation Table–Marker Horizon (SET-MH) method is the global standard: a nondestructive way to make highly precise measurements of a wetland's surface elevation over long periods, relative to a fixed point.
- Marker horizons are layers of clay, distinct from the wetland soil, laid on the marsh surface. Coring later shows how much sediment and organic matter have built up on top.
- The rod surface elevation table is an arm mounted on a fixed benchmark rod; nine pins lowered from the arm, in four directions, measure the surface height.
Together they show not just whether the surface is rising or sinking, but why.
Where the stations are
Thousands of SET-MH stations are thought to exist along the coasts of the lower 48 states, run by many organizations, so no single map has existed. USGS combined four published datasets of 980 stations — measured for 9 years on average (3 to 30), in tidal freshwater forested wetlands, salt marshes or mangroves, and not experimentally manipulated — and overlaid them on the Protected Areas Database of the United States (PAD-US), the official inventory of protected land and water.
The relative density of SET-MH stations on protected lands; most are concentrated along the Gulf Coast. Credit: U.S. Geological Survey.
| Stations on protected lands | 646 of 980 |
|---|---|
| Managed by | Federal agencies: 380 · state agencies: 117 · multiple groups: 112 · 13 entities in all, the U.S. Fish and Wildlife Service most of all (279) |
| Wetland type | Salt marsh 55% · subtidal 18% · freshwater forested wetlands and freshwater marshes 13% · mangroves 6% · tidal flats 3% · other 5% |
*Figure 4.Bar graph organizing Surface Elevation Table (SET) stations on protected lands (x-axis)
by wetland type (legend shading) described by the National Wetland Inventory Type
(U.S. Fish and Wildlife Service, 2019).*
Because the selection rules exclude experimental stations, stations too new to show reliable trends and those beyond tidal influence, the count is conservative.
Why it matters
Long-term elevation records from protected areas can shape management as more coastal wetlands risk being submerged. Knowing which protected lands are losing elevation fastest points to where intervention is most needed. Mapping the stations may also be a first step toward networks of collaborators tracking elevation change over larger areas — and can show managers where monitoring is missing along key environmental gradients.
Sources
Based on "Spatial Distribution of Elevation Change Monitoring in Coastal Wetlands Across Protected Lands of the Lower 48 United States," U.S. Geological Survey Fact Sheet 2023–3039; a work of the United States government in the public domain. It draws on Feher and others (2022), Moon and others (2022), Saintilan and others (2022), Moorman and others (2023) and PAD-US (2022). Pictures an earlier version of this page left out were restored on 2026-09-26.
Licence : CC0 1.0 (domaine public) · Adapté de pubs.usgs.gov
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