For estuaries to persist as sea levels rise, a detailed understanding of sediment supply is necessary: identifying the type and quantity of sediment supplied to estuaries by rivers; measuring the fraction of sediment either retained in estuaries or lost to offshore environments; and calculating sediment transport mechanisms and the timescales over which these changes occur. If managed properly, sediment supply can not only keep pace with sea-level rise but also facilitate carbon sequestration.

Estuaries provide essential habitat for fish and wildlife—especially for valued commercial and cultural fisheries—as well as diverse recreational opportunities and protection from coastal hazards. Estuarine habitats, such as tidal marshes and tidal forests, capture greater amounts of carbon per hectare than many upland terrestrial forests, making them a particularly important component of greenhouse gas reduction strategies. 

Across U.S. coastlines and specifically at the Nisqually River Delta, there is a growing need to restore and enhance estuary and tidal marsh ecosystems threatened by sea-level rise. The Nisqually River Delta also represents an opportunity to apply adaptive management practices that can ensure the success of large capital investments in ecosystem restoration.  

Beginning in 2009, the USGS, Nisqually Indian Tribe, U.S. Fish and Wildlife Service, and partners conducted comprehensive studies to 1) evaluate sediment supply to the Nisqually River Delta’s vulnerable tidal marshes; 2) identify potential opportunities for recovering sediment supply; and 3) quantify the carbon sequestration potential of a restored delta. These efforts are part of the largest estuary restoration project in Puget Sound.

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Can Coastal Habitats Rise to the Challenge? 

In healthy estuaries, sediments traveling downstream approach the river mouth and are slowed by shallower riverbeds and marsh vegetation. In the era before extensive coastal development, much of this sediment was deposited and retained across floodplains and estuary marsh, building up the elevation of coastal lands above the shoreline. Today, river channelization, flood protection levees, and dikes direct more sediment offshore, leaving floodplains and estuarine lands lower, or subsided in response to lost sediment supply. What sediment does reach the shore and subsided lands today is often vulnerable to waves that can it wash away, where it is lost offshore in deep water.

Rising sea levels threaten estuaries by eroding sediment and eventually submerging habitat. If an estuary’s sediment supply is restricted— by dams, for example, impounding its rivers upstream—the rate at which sediment is retained might not keep pace with sediment lost to erosion, leading to eventual loss of habitat. Similarly, if tidal marshes are bound by levees and prevented from migrating inland as sea level rises, those habitats will be submerged. 

As part of a 2019 ecosystem services analysis in the Nisqually River Delta, USGS and USFWS gathered constituents from the community, local government, and the Nisqually Indian Tribe to understand which ecosystem services were most important to them. The top answers were recreational access, fishing access, and carbon sequestration. 

“From the community perspective, most people in the surrounding area go to the [Billy Frank, Jr.] National Wildlife Refuge for recreation: birdwatching, hiking, and to be in nature. To them, maintaining trails and access is important, and that's important to Refuge staff as well,” said Monica Moritsch, former USGS Research Ecologist and lead author of the report, “Can Coastal Habitats Rise to the Challenge? Resilience of Estuarine Habitats, Carbon Accumulation, and Economic Value to Sea-Level Rise in a Puget Sound Estuary”. “A major reason the Refuge requested this detailed sea-level rise assessment was to understand how they could maintain the mosaic of habitats needed to support their mandate of providing resources for wildlife of many different species, including the salmon protected under fishing treaty with the Nisqually Tribe.” 

Restoring estuaries, therefore, must account for habitat restoration as well as sediment supply in order to be resilient in the future. 

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Assessing Sediment Supply at Nisqually River Delta 

The Nisqually River in Washington State begins along the glaciated slopes of Mount Rainier and travels 78 miles (125 km) to empty into Puget Sound. Its delta is home to migratory fish, birds, and other wildlife. Much of the delta lies protected within the Billy Frank, Jr. National Wildlife Refuge, managed by the U.S. Fish and Wildlife Service (USFWS). 

Beginning in 2009, a coalition of groups, including the Nisqually Indian Tribe and USFWS, removed 100-year-old dikes from a portion of the delta called Brown’s Farm, starting what would become the largest estuary restoration project in Puget Sound. 

The 2009 Brown’s Farm Restoration area reconnected tidal connectivity to 308 hectares of historical wetlands that had been modified for grazing since the mid-1800s.  The restoration achieved many goals toward recovering tidal and stream connectivity to lost salmon habitat, but full restoration of several habitat traits—including water temperature and salinity, water depths, and recolonization of marsh vegetation—remained incomplete, and related directly to uncertainty in the delta’s sediment budget and transport dynamics. 

 

To address this, a study led by USGS researchers at the Pacific Coastal and Marine Science Center evaluated the sediment budget of the Nisqually River Delta, the vulnerability of the largest estuary restoration project in Puget Sound at the Billy Frank Jr. Nisqually National Wildlife Refuge, and the role of coastal hydrodynamics and potential restoration alternatives for recovering sediment delivery to its marshes. 

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“Our findings show that only a small amount of fluvial sediment load accumulates in the restored estuary, and because of that, it will take more than 20 years to recover elevations for desired marsh plants,” said Eric Grossman, USGS Research Geologist at the Pacific Coastal and Marine Science Center and lead author of the study “Assessment of Vulnerabilities and Opportunities to Restore Marsh Sediment Supply at Nisqually River Delta”. “Part of the reason is that Alder Lake traps about 90 percent of the Nisqually River sediment load before reaching the delta. Another reason is that waves are able to erode and redistribute sediment across the subsided restoration area, which leads to the export of fluvial supplied material and its movement offshore to deep settings where it cannot return.” 

The study’s goals were to (1) characterize the hydrodynamics and sediment transport processes that influence sediment delivery and (2) assess adaptive management strategies aimed to recover sediment to areas of concern. 

“I was lucky enough to have some amazing conversations with the late Billy Frank, Jr., who understood that in order to improve degraded coastal habitats at Nisqually River and across Puget Sound, there were critical infrastructure barriers that needed to be addressed, such as dikes, levees, and the Interstate 5 freeway that cuts through the Nisqually River Delta,” said Grossman. “These barriers are commonly found to adversely affect restoration outcomes and ecosystem functions people depend on. In many cases, these barriers are also projected to exacerbate flooding expected with sea level rise. For estuary restoration to be successful and resilient, it will require public will to evaluate the benefits and costs to restore those habitats. I find myself now, several years into this kind of research, hoping to help inform that process.” 

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