Estuaries are subject to change. Where have they been, where are they now, and where may they be in the future? How have their habitats changed over time? A team led by the National Estuarine Research Reserve System (NERRS) explored those questions in 30 estuaries around the country.
This study found regional differences in how estuaries have changed since the 1800s. The elevation-based mapping and historical mapping techniques used were effective across the wide range of geographies represented by the Reserve System, illustrating their usefulness in future studies anywhere in the world.
Why we mapped
Healthy estuaries are economic powerhouses and essential to the well-being of coastal communities. Over time, however, diking, farming, and other kinds of development have changed the extent of some estuaries, shrinking their habitats and making them less productive. Conversely, in other areas, sea-level rise has pushed tidal marshes landward, displacing tidal forests, nontidal forested wetlands, or uplands.
Accurate mapping of estuarine habitats is critical for successful conservation and restoration. However, current maps often do not reflect the extent of estuarine wetlands, especially those that are forested. Understanding an estuary’s past extent—an indication of where it could be again—is even more challenging, but important to setting restoration goals.
In the face of changing water levels and land uses, understanding the extent of an estuary and its habitats is critical to decisions of where and how to invest limited resources
To sleuth which areas are within the potential reach of the tides, we combined tidal- and land-elevation data in a technique known as elevation-based mapping—a powerful way to visualize where the estuary is today, where it was, and where it could be were artificial barriers to tides removed. We modified this approach for the Great Lakes, mapping areas within reach of highest lake water levels rather than tides.
We also conducted historical mapping using “T sheets” (topographic sheets) from 1846 to 1920, which show the distribution and extent of estuarine habitats in times with less development. This yielded maps that can be used, for example to help identify areas for restoration of habitat types that were once common, but are now rare.
We compared the maps generated by both of these techniques to maps produced by the US Fish and Wildlife Service’s (USFWS) National Wetland Inventory.
What we learned
Elevation-based mapping revealed that more than half of the land potentially within the reach of tides at PBNERR has been disconnected from natural tidal exchange.
The reach of our nation’s estuaries extends further than we imagined, and with the strategic investment in conservation and restoration, they could be larger still.
We found clear regional differences in how estuaries have changed over time. Our analysis of historical maps revealed extensive losses of tidal marsh on the Pacific coast, often due to diking. In other regions, however, the expanse of tidal marshes has been more stable, with losses balanced or even surpassed by gains as these habitats migrated landward into adjacent forests.
These findings highlight the need to tailor climate resilience strategies to suit to regional conditions and the diverse dynamics of each estuary. For example, restoring tidal exchange in areas that are currently disconnected would be an effective strategy for restoring some lost habitats. In places where wetland habitats are moving landward, emphasizing the protection and management of migration corridors may be a better approach to ensuring these habitats and their benefits persist in the future.
This study indicates that further research is needed to understand and manage tidal forests.
Nearly two thirds of tidal forests detected by our elevation-based mapping were missed by the USFW National Wetland Inventory, underscoring that further research and attention will be needed to support these habitats. However, each mapping approach missed some habitats, so we recommend using multiple methods for a true understanding of how our estuaries and their habitats are changing over time.
The methods we used proved effective across disparate geographies; with some modifications, they could be applied to estuaries anywhere in the world. The historical maps are an amazing window into the past and a valuable resource for historical ecology. Elevation-based mapping is a powerful tool for mapping areas within reach of tides, and when combined with other approaches, it is particularly useful for finding forested tidal wetlands that are otherwise difficult to map.
Questions about this project?
For general questions: Kerstin Wasson, Elkhorn Slough NERR