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Engineering the Delta Marshes

Engineering the Delta Marshes of Tomorrow

Victoria Schlesinger

Fish Habitat, North Delta Arc, Suisun Marsh Scenarios, Yolo Bypass
See-Session-PowerpointsSOE2013-cover100px2The curving swath of waterways and marshes that stretches from Suisun Marsh, northeast up the Sacramento River—referred to by presenters on Tuesday afternoon as the North Delta Arc—are prime spawning, growing, and foraging habitat for native estuarine and freshwater fish. Yet fish as well as waterfowl in the area will face challenges as sea level rises, other effects of climate change take hold, and people continue to modify the region. Three presenters from UC Davis’s Center for Watershed Sciences looked ahead at how to best support the region’s ecosystem in the decades to come.

The North Delta Arc region is a rarity within the Bay Delta system because it offers superior habitat conditions for native fish. Image credit: Amber Manfree
The North Delta Arc region is a rarity within the Bay Delta system because it offers superior habitat conditions for native fish. Image: Amber Manfree

Research led by John Durand with UC Davis has examined what makes the North Delta Arc—including Cache and Lindsey sloughs and Suisun Marsh—successful habitat for fish. Their preliminary research identified several mechanisms critical to supporting resiliency in the region’s ecosystem, including habitat complexity, tidal interaction with the shore, water quality dynamics, and interconnected habitats. They described the North Delta Arc as a mosaic of riparian habitat, mudflats, rip-rapped levees, tule stands, and emergent vegetation bathed in highly variable tides. Tidal exchange of sediment and phytoplankton in the water help maintain a variety of habitats that may be beneficial to both native and desirable invasive species. The researchers noted that phytoplankton growth appears to be most concentrated in the upper reaches of dead-end sloughs, possibly contributing to the local food web. Because these sloughs are well connected to the main channels of the estuary, aquatic life can readily move between them in search of food or refuge. UC Davis researchers aim to learn how these aspects of the ecosystem function in order to inform plans for tidal marsh restorations throughout the Delta said Denise Decarion, a project collaborator who presented on behalf of Durand.

Integral to the North Delta Arc’s past and future is historic Suisun Marsh—the largest remaining estuarine tidal marsh on the West Coast. Presenter Amber Manfree, coeditor of the forthcoming book “Suisun Marsh—Ecological History and Possible Futures,” outlined different approaches for managing the wetlands. Nearly all of the marsh has been affected over the past 100 years by the construction of dikes, canals, roads, floodgates, and upstream diversions. For this reason, Manfree said, heavy human intervention is required to maintain the current state of the marsh. Pressures from sea level rise and changing precipitation patterns will demand new approaches to managing the marsh over the coming century. According to one model, the 100-centimeter rise in sea level anticipated by 2100 could render most of Suisun Marsh a subtidal zone. The marsh could be preserved as it is today with the same goals as the Suisun Marsh Plan—maintaining current levels of duck hunting, open space, recreation, endangered species protection, and urban area buffers from sea level rise. This would require traditional levees with tidal gates to keep the rising waters literally “at bay.” But over time, the amount of diking required to maintain such a “fortress marsh” would likely become financially unsustainable. Another future could be a flooded marsh, which would ensue if any of the dikes breached in an earthquake or extreme storm. Such an event would transform much of the marsh into tidal or sub-tidal habitat. But if soil-building processes could keep pace with the rising sea level, tidal marsh habitats would eventually develop. A third option would be to manage the area as a tidal marsh, with diverse habitats, and planned wildlife corridors. This would require significant funds for conservation and limiting duck hunting activities. A final option, which Manfree calls an “ecomarsh,” would prioritize the needs of wildlife and employ every means to conserve habitat over the long term, including building up soil. “Addressing expected changes sooner than later,” said Manfree, who is completing her PhD at UC Davis, “leaves more options open for us.”

When flooded during the rainy season, the agricultural fields of the Yolo Bypass provide a place for native fish to fatten and waterfowl to rest. Image credit: Carson Jeffres
When flooded during the rainy season, the agricultural fields of the Yolo Bypass provide a place for native fish to fatten and waterfowl to rest. Image: Carson Jeffres

Some 40 miles upstream of Suisun Marsh lies the Yolo Bypass, which diverts floodwaters from the Sacramento River into the Sacramento-San Joaquin River Delta and ends near Cache Slough. As part of the Bay Delta Conservation Plan, the proposed Conservation Measure #2 would allow more floodwaters to reach the Bypass and adjacent Yolo and Solano counties. UC Davis’s Robyn Suddeth has modeled the impacts of such increased flooding on regional agriculture, wildlife, and recreation. While critical for managing floodwaters, the bypass is important to aquatic species that migrate along the waterway. Chinook salmon and splittail find plentiful food here. Waterfowl and migrant birds stop here during their journey along the Pacific Flyway. Because the bypass already supports so much wildlife and covers such a large area, it is often pointed to as a rare place within the Bay Delta system with great potential for habitat restoration. Though extremely valuable to wildlife, the floodplain is equally prized by farmers. Rice is the primary crop, but this landscape is also used for grazing and to grow corn and tomatoes. Like Suisun Marsh, the bypass is popular among duck hunters, and contains a state wildlife area. But as flood patterns change, said Suddeth, the question is whether “it’s possible to have an ecologically functioning flood plain that’s also heavily engineered and managed.” To help planners and decision makers answer this question, Suddeth has plugged expert surveys and numerous other data sources into her model. Her results indicate there will be trade-offs associated with different management scenarios. As an example, she showed how one could set the model to flood in a manner that would optimize habitat for fish or waterfowl, and what this might cost farmers in terms of yields and limits on crop choices.

Where the Wild Things Aren’t


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Durand Slides

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Manfree slides

Credit: Amber ManfreeCredit: Amber ManfreeCredit: Amber ManfreeCredit: Amber Manfree

 

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