Learning from the Past to Inform the Future
Using Historical Analysis of the Delta Landscape to Inform Restoration Strategies
Researching the past helps scientists understand the present and envision the future. That’s what Robin Grossinger and Alison Whipple of The San Francisco Estuary Institute-Aquatic Science Center (SFEI-ASC) are hoping to accomplish in their historical ecology study of the Delta. Their study, funded by the Department of Fish and Game, will provide understanding of what the Delta probably looked like and how the Delta functioned physically and biologically before the arrival of Europeans.
“In projects like this, we’ve often found opportunities in the landscape that haven’t necessarily been recognized before,” Whipple said. “We’re going into the historical record to piece together the historical habitats, habitat types and mosaics of the Delta, giving us a sense of the whole that has been largely absent for over a century. We’re getting a sense of where different features of the landscape were located so we can understand how the system functioned historically, which helps our understanding of how species used the Delta.”
The goal of the project is not to create a literal template from which to recreate the historical Delta since the Delta has undergone extensive changes over the years. Rather, the objective is to better understand ecosystem function and how it varied throughout the system in response to identifiable physical gradients (e.g. salinity, tidal range). This can inform large-scale restoration strategies currently being considered for the Delta. The goals of the project are to:
Describe historical habitat characteristics
Develop landscape-level understanding of historical ecological patterns
Document former ecological, hydrological, and geomorphic processes
Provide information for application to current restoration efforts
Using a diverse range of data, including maps, drawings, textual accounts, and early photography spanning about two centuries, the historical ecology team is documenting historical characteristics of landscape components (also referred to here as habitat types), including channels, tidal and nontidal perennial wetlands, seasonal wetlands, ponds and lakes, and riparian forest. This process involves the compilation and synthesis of the data into a geographic information system (GIS).
GIS is a mapping, data comparison, and spatial analysis tool that allows SFEI-ASC to bring historical information together for comparison across time and space including overlaying that information with modern conditions. The mapping provides a way to convey how habitats were arranged and how water flowed through the systems, all in a versatile dataset that is easily used by scientists and restoration managers.
“For instance, in GIS we can place old maps in space using modern coordinate systems and map the orientation of historical river channels, which we can then compare to the river’s orientation today. We’re able to make estimates of how many acres of different habitat types persisted historically, i.e., how many acres of riparian forest. One way we use this is to estimate the magnitude, extent, and type of habitat loss,” Whipple said.
“Importantly, the historical data also paint a rich picture of habitat complexity at many spatial as well as temporal scales, where we can begin to see the Delta not as a single place, but as many, each with a different set of characteristics,” she added. “As scientists have discussed, this complexity in the landscape made the Delta the incredibly rich place it was.”
Scale is very important to consider when thinking about how a system functions. Whipple and the historical ecology team are beginning to see from their research that there was not only a lot of local-scale complexity in the Delta-it’s not only tule marsh-they’re also seeing distinct patterns in how that complexity was expressed at larger scales within the historical Delta.
In considering how to restore important ecological functions that have been lost, scientists and managers have recognized the importance of thinking at larger scales. “For instance,” Whipple said, “a restored tidal marsh might provide really different ecological functions depending on whether it is five acres or one hundred, whether it is adjacent to riparian forest, or whether it receives winter flood flows. We need to think about more than just habitats as single units, but how habitats are arranged into mosaics (complexes of different communities of species), and how those mosaics fit together in the landscape.”
One way to think about landscape is to compare, at the broadest scale, three primary landscapes of the historical Delta: the flood basins (North Delta), tidal islands (Central Delta), and distributary rivers (South Delta). Each primary landscape can be distinguished by characteristics such as vegetation patterns, relative tidal influence, channel width and density, stability of features, and flooding influence.
The historical North Delta flood basins, the Yolo Basin being the largest, were characterized by broad zones of freshwater emergent wetland, not influenced by tides, grading into tidal freshwater emergent wetland. Much of these wetlands were dominated by dense stands of tules, which could reach heights of 14 feet. Perennial ponds and lakes also were found here; places known as great hunting grounds for ducks. Seasonal river floods played an important role in this system.
“The Sacramento River channel did not have the capacity to carry flood flows, so water spilled over the natural levees and into the basins that ran parallel to the river,” Whipple said. “At times, more water would be flowing on either side of the river than in the river channel itself. So, basins like the Yolo Basin were the historical equivalent of flood retention basins. Consequently, they retained much of this water into the dry season. This was valuable floodplain habitat. Riparian vegetation also was an important part of the landscape, occupying the natural levees that reached above tides and most floods.”
In the Central Delta, the landscape historically consisted of large islands, many of which exist today, and were surrounded by the branching sloughs of the main rivers. They were occupied by freshwater emergent wetland habitat laced with many sinuous tidal channels that branched off into tidal marsh. Unlike the basins in the north, historical sources and studies on remnant wetlands suggest that willows, grasses, and even ferns, were a significant part of the wetland matrix in addition to tules.
The islands closer to the Delta mouth were more influenced by tides than elsewhere. Within the Delta roughly 250,000 acres were likely regularly influenced by tides, although well over 100,000 additional acres were probably influenced by tides at certain times of the year or at extreme tides.
The South Delta can be described as a distributary (branches of a river that flow away from the main stream) rivers landscape, where the San Joaquin River met its Delta and divided into three main branches. This was a locally complex and dynamic part of the Delta. Patches of grass and areas devoid of vegetation as well as tule stands were frequently found as one traversed the landscape. Some ponds dried up in the summer and many small channels along the plain of the marsh only carried floodwater during the wet season.
“This is not to say that this area wasn’t entirely inundated in the wintertime,” Whipple said. “Gold miners found it entirely impassable during the spring snowmelt. Some of the land between the distributaries was elevated above tide levels, while the waterways around those channels ebbed and flowed with the tide. This was a landscape on the edge.” At this edge, the riparian forest marched downstream and slowly dwindled as the natural levees reached tide level. Some of these waterways recruited large woody debris in the channels, much to the frustration of early explorers who encountered large ‘rafts’ while trying to navigate the waters of the South Delta. These waters, however, appear to have been popular salmon fishing grounds, which is perhaps linked to the fact that woody debris creates excellent fish habitat.
One important point in historical ecology is that it can help develop a landscape level perspective in ecosystem planning, Whipple explained.
“As we piece together what the Delta looked like from the historical record, our documentation of habitat characteristics and governing physical and biological processes lead to the larger-scale understanding of how things looked and functioned,” she said. “Thus, we get some ideas about the details that might be useful for restoration planning, that is, how deep were the ponds, how wide were the sloughs. But, just as importantly, we better understand how these details fit together to provide all those functions that have since been lost to a great degree. Both the details and the larger-scale perspective are valuable to restoration planning efforts.”
“As scientists and managers start to envision what is possible and what is desired for the future Delta, we hope that such reconstructions of the historical landscape can help make those visions more concrete and help set priorities and goals that have better chances of success.”
How can this perspective help us today? “Knowing these landscapes and better understanding why things looked the way they did gives us ideas for how things might fit together as we integrate the historical knowledge with the contemporary environmental, social and political landscapes,” Whipple said. “The historical information is only valuable to the extent that it is integrated with both our current understanding of ecosystem function and with the larger Delta planning process in general.”
“As scientists and managers start to envision what is possible and what is desired for the future Delta, we hope that such reconstructions of the historical landscape can help make those visions more concrete and help set priorities and goals that have better chances of success,” she said.