Estuaries and large river deltas in the Pacific Northwest Active

Sources/Usage: Public Domain.

Water supply and flood control can be controversial issues in large river systems like the Skagit River delta in the Puget Sound area of Washington State. The challenge can be getting all of the separate interests to the table—farmers, ecosystem restoration practitioners, tribal communities, and dozens of land-use managers. During town hall-style meetings where communities voiced their concerns, USGS geologist Eric Grossman informed them about the science of climate change and its impacts in the region. He’s watched a room of apprehensive residents, regional managers, and other stakeholders grow to a full house in a few years. Some now eagerly share their knowledge and data as they seek solutions for problems like abating coastal hazards and improving conditions for salmon and shellfish, which can benefit several groups. Bringing together diverse interests in this Pacific Northwest landscape has spotlighted opportunities to collaborate, and has fostered a more integrated understanding of the scientific and societal issues and what the USGS is doing to address them. 

The Issues

Sources/Usage: Public Domain.

Human development of the Pacific Northwest since the 1800s has significantly changed stream and tidal flow in deltas and estuaries. Making land surfaces impervious to water and rerouting river channels through dikes and levees have caused extensive and ongoing disturbance to ecosystems important for the salmon life cycle. Although the construction of levees and dikes in river deltas protects lives and infrastructure—allowing cities and livelihoods to develop—it’s been at the expense of an iconic ecosystem that also provides valuable benefits to people.

Climate change adds a new dimension, and the story becomes largely about sediment and its journey to the coastal zone.

Geologically young coastal mountains and steep active volcanoes characterize the Pacific Northwest, and experience lots of erosion from some of the highest rainfall in the country. Forecasts for this area indicate even more rainfall and stronger and more frequent river flooding, which can increase erosion. Moreover, a rising snowline and glacier melt across the Pacific Northwest—now evident from climate change—expose a greater surface area to erosion. A larger quantity of sediment flowing down this “sediment superhighway” is expected to fill stream channels and increase flood risk, especially when a rising sea level slows the flow of fresh water near the river mouth and pushes the ocean-river convergence farther upstream. This convergence also traps sediment, sometimes clogging channels and river mouths, and makes the river system more likely to overtop its banks and levees during times of high flow. This clogging can also cause the river to change course and amplify the flood risk to nearby developed areas.

Aside from contributing to a greater flood risk, sediment shapes estuarine environments critical to fish spawning and rearing; several species of aquatic vegetation such as eelgrass; valued shellfish like crab, clams, and oysters; shorebirds; and a diverse set of small invertebrates that feed the rest of the food web. Shellfish and salmon are firmly entrenched in the culture of the indigenous peoples and those who identify with the Pacific Northwest. Indigenous people have already lost many stocks of fish and shellfish, and concern grows that a changing climate and rising sea level will continue to adversely impact indigenous people tied to coastal reservations.

Sources/Usage: Public Domain.

Land-use managers and farmers are concerned about the combined effects of sea-level rise and coastal floods, which hamper drainage. These managers want to know, for instance, where the shoreline will be in future decades, and the extent of flooding in coastal lowlands. As sea level rises, wetlands essential for salmon recovery could become submerged, and important farmland for national seed crops could be lost to pooling groundwater and saltwater intrusion. These are significant reasons to investigate how estuarine areas will evolve, and what risks people and aquatic species face—particularly as sediment flow fluctuates. Sophisticated models need to be developed to forecast coastal change, and they require data that capture a dynamic and ever-changing environment.

What the USGS is doing

Sources/Usage: Public Domain.

USGS research covers six of the major watersheds in Puget Sound. By gathering high-resolution coastal elevation and habitat data using lidar, sonar, underwater photography, and an ultra-precise GPS, scientists reconstruct how shorelines and estuaries have shifted through time. They also take a series of overlapping photographic images and create seamless onshore-offshore elevation models in 3D, revealing landscape and habitats that can influence and be influenced by storm surge and waves.

Sources/Usage: Public Domain.

They gather detailed measurements of river and ocean levels to map how the annual water flow from tides, rivers, and storms fluctuates up and down the coast. For the first time in Washington State, USGS scientists are placing river-monitoring instruments in sections of estuary and beach environments to capture a comprehensive picture of what transpires when rising seas meet flooding rivers.

Sediment traps, current meters, wave and acceleration sensors, sonar, and underwater video also record how much and what type of sediment moves with specific tides, currents and wave conditions. Sedimentologists work with biologists and ecologists to study how sediment disturbs habitats and organisms, and its cascading effects throughout the food web.

Sources/Usage: Public Domain.

USGS also helped create an online tool (Puget Sound Coastal Resilience) for the region’s six watersheds to visually demonstrate current and future effects from the joint occurrence of projected sea-level rise, storm surge, and river flooding. Additionally, USGS developed tools with the Swinomish Indian Tribal community and Skagit River System Cooperative to understand how future sea-level rise will impact their natural food resources and overall community well being.

To inform salmon recovery efforts, USGS modeled how water moves through the Nisqually Delta National Wildlife Refuge, the largest estuary restoration project in the Pacific Northwest. Similar models show how oyster larvae disperse in Fidalgo Bay Aquatic Reserve, and how marsh vegetation influences waves and traps sediment in Port Susan Bay.

To model future wave energy and its potential impacts on coastal communities and ecosystems, the scientists use an innovative photographic method to capture how marsh vegetation influences water flow and sediment. Instead of spending hours in the field measuring individual plants, they photograph plants from the side to capture such traits as plant heights, leaf widths, shoot densities, and biomass, all of which impede the flow of water and sediment across the marsh. Hundreds of photographs can be taken in the short 2- to 3-hour window of low tide and analyzed overnight. Documenting the intimate ways that vegetation influences water flow and sediment helps highlight the value of restoring natural habitat to combat coastal hazards.

Sources/Usage: Public Domain.

USGS researchers have teamed up with Coast Salish Tribes and First Nations on two projects aimed at understanding and ameliorating the pressures that culturally important species like salmon and shellfish face. The first project is a wave model that forecasts storm surge and waves to examine their influence on habitats for juvenile salmon and shellfish. The second project involves tribes collecting water quality data across the entire Salish Sea—which encompasses Puget Sound, the Strait of Georgia, and the Strait of Juan de Fuca—by towing water quality probes behind traditional ocean-going canoes during their annual Tribal Canoe Journey. These probes measure such aspects as temperature, salinity, and pH throughout the water column and down to the seafloor. This unusual approach reveals variations in water quality conditions today across the vast area of the Salish Sea, and will track indicators of climate change in the coming years.

To read more about other studies in Puget Sound and how they inform ecosystem restoration, visit Coastal Habitats in Puget Sound.

What the USGS has learned

Sources/Usage: Public Domain.

Flooding rivers in the Pacific Northwest bring tons of sediment to the coast each year. In fact, more sediment flows into the basin here—enough to cover a football field to the height of six Space Needles—than flows into the larger Chesapeake Bay. With projections of rising air temperatures due to climate change, USGS studies indicate that sediment moving downstream from the glacial area of the North Cascade Range during larger floods will increase 3 to 6 times by 2080, potentially leading to more flood damage, greater costs to furnish clean water, and impacts to wildlife habitat and ecosystem restoration. In addition, the increase in rain and temperatures could also weaken steep mountain slopes and glaciers, delivering additional sediment from landslides.

Sources/Usage: Public Domain.

“Replumbing” the rivers with dikes and levees means sediments no longer build up in areas like floodplains and marshes, where extending the land would help reduce vulnerability to sea-level rise. Moreover, seawalls built along the coast to defend against ocean waves block sediment that would otherwise supply those marshes and beaches. Instead, sediment accumulates in channels, increasing flood risk, or is transported offshore, where it buries fish habitat and disrupts food webs.

Sources/Usage: Public Domain.

Working with several indigenous communities, USGS has installed a network of sensors to monitor how sediment moves through rivers and estuaries, and calculate how much arrives on beaches. By integrating coastal inundation models with studies of shellfish and traditional harvesting practices, the USGS has determined that future sea-level rise is likely to reduce suitable shellfish areas by 20 to 25 percent by 2100 in some areas where seawalls disturb the shoreline.

New analysis reveals that storm surge raises water levels in the Salish Sea 1 to 3 feet above predicted tides 10 to 13 percent of the time, most often when king tides coincide with high river flow. Other research illustrates that marsh restoration reduces the impact of those waves and storm surge in coastal lowlands. Green infrastructure like marsh restoration and rebuilding oyster reefs helps counteract the local impacts of climate change while restoring essential ecosystems for juvenile marine species like salmon, forage fish, and Dungeness crab.

To help promote societal awareness, USGS has recently joined the Washington State Coastal Hazards Resilience Network and leads a community citizen science effort. Washington’s shoreline is not always publicly accessible for gathering data or placing instruments, so in the spirit of engaging locals and landowners, nearly 50 volunteers help document the effects of coastal flooding. After USGS scientists survey the area, both USGS and citizen-collected information help validate and shape the models that provide a predictive snapshot of how vulnerable or how resilient the coast will be.