The Dutch Slough Tidal Marsh Restoration Project site, located in the Sacramento-San Joaquin Delta near Oakley, California. Photo by Florence Low / DWR

BAY DELTA SCIENCE CONFERENCE: Delta Adapts: Assessing Ecosystem Vulnerability to Climate Change

The loss of most ecosystems in the Bay-Delta has heavily compromised the functioning of the remnant ecosystems, which are anticipated to be further impacted by climate change. Therefore, it is critical to establish a solid understanding of current Bay-Delta ecosystem vulnerabilities to climate change and identify opportunities for adaptation.

To understand specific regional climate vulnerabilities, the Delta Stewardship Council conducted a climate change vulnerability assessment, called the Delta Adapts project, that estimates the exposure, sensitivity, and adaptive capacity of dominant ecosystem types to sea level rise, increasing air temperature, and changes in local precipitation within the Delta and Suisun Marsh.  The first phase of the Delta Adapts is the vulnerability assessment, which has been completed.  The Delta Science Program is now working on the second phase, an adaptation strategy.

At the 2021 Bay-Delta Science Conference, Dr. Dylan Chappelle with the Delta Science Program and one of the technical leads for the ecosystem chapters of the climate vulnerability assessment, presented the study’s findings related to ecosystem impacts.  He acknowledged the others who contributed to this work:  technical lead Annika Keeley; co-authors Corey Copeland, Andrew Schwartz, and Kaylee Griffin from the Delta Science Program; and the team from AECOM, which included Kirstin Tremain Davis, Tayler Tharaldson, and Kerstin Kalchmayr.

Climate change risks facing today’s Delta are closely related to the system’s history of ecosystem loss. Historically, the Delta was a vast hydrologically connected system dominated by over 450,000 acres of tidal freshwater wetlands. Seasonal floods would often fill the Central Valley in wet years, and salinity in the system would increase during drought periods. In these extensively connected habitats, aquatic, terrestrial, and avian species evolved to adapt to changing conditions and could better find refugia from the extremes.

The San Francisco Estuary Institute’s historical ecology study looked at how the construction of levees to support farms and communities completely reconfigured the Delta.

Today’s Delta is heavily controlled and fragmented, compared to its past state,” said Mr. Chappelle.  “This has major implications for ecosystem health energy, which continues to decline by many measures. Climate change has the potential to exacerbate this decline if decisive actions are not taken. Ecosystem restoration and management will be critical for increasing the resilience of the Delta’s ecosystems.”

The Delta Adapts study considers the impact of climate stressors and hazards on assets in the Delta and Suisun Marsh and is intended to be a guide for regional planning and resource management.  The ecosystem chapter, the subject of this presentation, focuses on climate change exposure of existing terrestrial and intertidal ecosystems in the region and, specifically, the impacts of gradual changes to mean sea level, episodic flooding events, and changes to temperature and precipitation.

The first step in the process was to characterize existing ecosystems.  They utilized vegetative data for the Delta and Suisun Marsh, cross-walked the vegetation cover types with the San Francisco Estuary Institute’s Delta Transformed report, and then combined riparian categories into a single category.  To reflect hydrologic connectivity or lack thereof, they divided the landscape into leveed and unleveed ecosystems, including tidal, fluvial, and floodplain areas.  Ecosystems in the Yolo Bypass were included in the unleveed category.  This resulted in eight different ecosystem types spread across two landscape categories.

Most of the acreage – 132,000 acres of terrestrial ecosystems identified in the Delta and Suisun Marsh are protected by levees; over 38,000 acres are unleveed.

The landscape was divided into five regions for analysis; more information about how these regions were determined can be found in a technical memorandum accompanying the vulnerability assessment.

Unleveed ecosystems are directly connected to water and therefore required a different analysis approach than the levee overtopping results and flood maps presented in the flood risk study (See: BAY DELTA SCIENCE CONFERENCE: Delta Flood Risk Under Climate Change: Key Findings from the Delta Adapts Flood Risk Analysis), which only considered events when peak water levels overtop levees. For tidal wetlands and riparian areas not protected by levees, the projected mean water levels across the nodes of the DSM 2 water level network were used.

For our analysis of tidal wetlands, we wanted to capture the ability they have to create mineral and organic sediment which can allow them to keep pace with moderate rates of sea-level rise,” said Mr. Chappelle.  “So to account for this, we partnered with Kevin Buffington and Karen Thorne of the USGS to adapt to the WARMER wetland accretion model to explore the potential risk to freshwater and brackish tidal wetlands. The Warmer framework models changes to marsh surface elevation based on the emergent marsh plant productivity, salinity regime, mineral sediment accretion, tidal datums, and other factors.”

The model is designed to run as a point model on a high-resolution digital elevation model or DEM; for the study, the outputs were adapted to run on a polygon scale.  Given this, it is important to note that the results are coarse and at the system level; more detailed models will be needed for site-specific decision-making.

Because wetland vegetation distorts DEM surface elevations and vegetation corrected DEM is not currently available at the legal Delta scale, they assumed a starting elevation of one foot above mean sea level for each freshwater wetland polygon.  For the Suisun Marsh area, a mean surface elevation two feet above mean sea level was generated for tidal brackish wetlands based on the vegetation corrected DEM developed for the Suisun Marsh region.

Using those starting elevations, they ran field-calibrated regionally-specific models for each wetland polygon across a range of sea level rise scenarios modeled across the DSM 2 water level network. These regional models were run using field data from Rush Ranch for Suisun Marsh, Browns Island for the Central and South Delta, and Miner Slough for the North Delta and the Cache-Yolo Complex.

We determined the risk to tidal wetlands in two ways,” said Mr. Chappelle.  “First, we looked at changes when marsh surface elevation fell below the local mean sea level, which represents the transition between high/mid marsh and low marsh. This transition can have major implications for ecosystem function, as well as habitat support for target species.  And second, we looked at complete drowning, where the marsh surface elevation fell below the mean low or low water, which indicates a transition to mudflat.”

Looking at these two types of risks to tidal wetlands, the table shows the sea level rise scenarios with one foot and two feet by 2050; with two feet and three and a half feet by 2085, and 6 feet by 2100, and the resulting impacts on the Delta’s current freshwater wetlands and Suisun’s existing brackish wetlands.  The dark green shows the scenarios where the high marsh is likely to persist; the light green shows areas where models predict a transition to low marsh; and the black shows scenarios where complete drowning and transition to mudflat is predicted.

If we see two feet of sea level rise by 2050, you can see that freshwater wetlands in the Delta may transition to low marsh, but brackish wetlands may be a little more resilient and able to persist as high to mid marsh,” said Dr. Chappelle.  “Under the 3.5 feet by 2085 scenario, both wetland types are likely to transition to low marsh.  Under six feet by 2100, all of the marshes, both freshwater and brackish, are projected to drown.”

Under more moderate rates of sea level rise, so one foot by 2050 and two feet by 2085, high marshes should be resilient,” he continued.  “We did run a number of additional scenarios to test the influence of sediment supply. We found that our results were minimally sensitive to these different scenarios. We also tested a low marsh starting elevation scenario, and we did find that for both freshwater and brackish wetlands, starting at low marsh elevations led them both to drown completely under 3.5 feet of sea level rise by 2085 and above.”

Tidal wetland connections to upland areas are key for improving their ability to keep pace with sea level rise and essentially migrate upland in response. The map on the right shows the potential for upland transition, shown in light green color, does exist in the region, but the current wetlands, shown in light blue, often lack adequate connection to migrate upland, Dr. Chappelle said.

Dr. Chappelle noted that this is well documented by other efforts as well.  The map on the left shows that the Cache Slough Complex and Yolo Bypass have the extensive potential for wetlands to migrate to uplands, but the Liberty Island wetlands, shown in the light blue, have minimal direct connections to upland areas.  Browns Island, Winter Island, and Sherman Lake wetlands in the central Delta are extensive but are almost completely lacking in any upland connection that will allow them to keep pace with sea level rise.

Thus, restoration projects that plan for sea level rise will be critical for increasing the resilience of the system, and thankfully, many planned projects do show promise in this regard,” he said.  “One important assumption of this effort is that the D-1641 freshwater flow requirements stay in place, so we didn’t analyze any changes to salinity that may come with sea level rise and drought. And it is important to note that if salinity does increase in the system, this could lead to significant plant community shifts that do create additional ecosystem risk.”

They also considered the leveed ecosystems using the probabilistic flood maps developed for the flood risk analysis and determined that by 2050, 55%, or more than 70,000 acres are at risk with a 10-year storm under this 2050 scenario, and 79% or over 104,000 acres are at risk under the 2085 scenario.

If the levees protecting these flooded areas are repaired relatively rapidly, these ecosystems will be likely to persist,” said Dr. Chappelle.  “But it’s also important to note that our results for levee ecosystems don’t incorporate any future levee improvements, and they also don’t include other types of levee failures besides overtopping.”

Looking at the risk of flooding impact to wet meadows, seasonal wetlands, and non-tidal freshwater wetlands, which cover approximately 20,000 total acres in the region, under the 2085 scenario, 100% of the seasonal wetlands and 80% of the freshwater non-tidal wetlands are at risk.  

For some of these critical leveed ecosystems, there is still quite a bit of flooding risk,” he said.  “But when these areas are restored and managed with subsidence reversal projects, they actually reduce the risk of levee failure by increasing surface elevations and taking pressure off of the levees. And so these ecosystem types, while at risk, are also a key natural adaptation measure for dealing with the impacts of sea level rise and flooding.”

Managed wetlands in the Suisun Marsh are another example of leveed ecosystems at high risk of flooding even under current conditions.  These areas are generally less subsided than those deeply subsided islands in the central Delta, and extensive water control structures exist to manage some degree of flooding.

However, higher water levels and more frequent flooding events will increase the risk that the levees required to manage these water levels will degrade, and this would really put the entire system at risk,” he said.  “These ecosystems are important for birds and other migratory species. And so there is a lot of ecological benefits to making sure that they are resilient to sea level rise.”

Changes to temperature and precipitation, in addition to some of the sea level rise and flooding impacts, will also have implications for the Delta’s ecosystems. Temperature and precipitation changes were modeled by the AECOM team using Cal Adapt with these three emission scenarios over these three-time horizons.

The results show that increases in air temperatures will adversely impact ecosystem health, extreme heat days, in particular, putting species at risk,” Dr. Chappelle said.  “More frequent droughts will limit water availability for ecosystems, and shorter, rainy seasons will increase stress for both plants and wildlife. This risk will be particularly pronounced for ecosystems like seasonal wetlands that depend on local seasonal rainfall to function and provide habitat for the species that occupy them.”

Key findings

Dr. Chappelle then recapped the key findings.  “At a high level, this effort demonstrates that the ecosystems in the Delta and Suisun Marsh are certainly at risk from sea level rise, flooding, increases in temperature, and changes to precipitation patterns.

Nature-based solutions, including restoring tidal wetlands with upland connections and creating subsidence reversal wetlands will be critical for reducing the vulnerability of the system and increasing ecosystem extent.  Rapid implementation of these efforts will be required to allow natural processes to develop well ahead of the worst impacts of sea level rise.

To the considerable ecosystem extent that we do have protected by levees, it’s important to note that in addition to protecting farms and communities, maintaining this levee infrastructure will also reduce the risk for leveed ecosystems.

Creating a healthier Delta ecosystem under climate change will not be easy, but it is possible. And the work that many of you are doing today is critical for building a more resilient Delta future. I want to acknowledge the tireless work that goes into planning, permitting, and implementing restoration projects like Dutch Slough, that point towards a more vibrant and climate-resilient future for the Delta’s ecosystems.

As for the next steps, the next phase of the Delta Adapts project is developing an adaptation strategy, where they will be taking the higher-level findings from this study and translating them to concrete action in different scenarios on the ground.

QUESTIONS & ANSWERS

QUESTION:  Have you translated your results for communicating with different audiences, and if so, how did you go about that? I know the Delta Adapts project has a pretty broad set of stakeholders that you’re trying to reach.

Dr. Dylan Chappelle:  “I think that the translation of these results for a variety of stakeholder groups and making sure that the points are clear was one of the more interesting parts of putting this together. There’s a lot of information that we are working to group into one report. There’s an enormous amount of detail in the technical memorandum, for example, looking at each ecosystem type and risk under different scenarios. So one of the key things to honing the message of this effort over time was the iterative presentation to multiple stakeholder groups while our analysis was still in process, so we had a technical advisory group that was very helpful for us in determining that. And then, multiple agencies and other groups were able to ask us clarifying questions and highlighted what they found most interesting. So that really helped us put together what information was going to be the most useful.”

COMMENT: If there’s an upside to sea level rise and salinity intrusion into the Delta, maybe it’s a decrease in submerged aquatic vegetation.

Dylan Chappelle: I think about that a lot. It is interesting, though, because those benefits depend on what happens with flow releases and the maintenance of the hydraulic salinity barrier. We also might see those benefits concentrated in different regions. So we could see a future where Suisun is proportionately saltier, but the Delta itself stays in a relatively similar range. And I think that could be a really interesting potential outcome.  I feel that in some cases, we have a bias towards looking at the negative impacts of climate change. But there’s quite a bit of potential details like that aquatic vegetation detail where we might see some things change for the better as well. However, that’s not typically put at the front of our reports, because by and large, we’re looking at avoiding the negative consequences, but excellent point.”

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