Rising Seas and The Delta Part 3: The Future For Wildlife
An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with kcet.org/baydelta for all the project's stories.
The best available science says the planet may see a meter's rise in sea levels by the end of this century, and another meter or more by the end of the next century. About 1,150 square miles of the Delta is below current sea level, protected by levees whose durability is open to question, and most of which would soon be overtopped by a meter's sea level rise.
Which means that unless Californians are both incredibly lucky and incredibly diligent, within a century and a half the state is very likely to have a large inland sea, perhaps as much as 1,200 square miles in extent, more than three times the size of the Salton Sea. That flood will be incredibly expensive in economic terms, and will uproot people who have lived in the Delta for generations.
But could there be an upside to nature's reclaiming of the Delta?
As we said in Part 1 and Part 2 of this series, the historic Delta was a sea-level complex of freshwater and brackish marshes and tidally influenced sloughs, with a network of natural levees and other pieces of high ground in between. The marshlands were the source of the Delta's rich soils, built as tules and other native plants grew, died, fell into the water, and decomposed anaerobically. Over the 6,000 years or so in which the historic Delta existed as an ecosystem, those peat soils grew very thick.
And then came the American settlers, some of whom noticed the Delta's fertile soils and commenced to draining and cultivating them. When peat or other soil organic matter is exposed to the air, aerobic microorganisms start eating it, releasing the carbon into the atmosphere as carbon dioxide. Soils have less organic matter, which means they shrink, which means ground level drops.
The Delta has roughly 1,150 square miles of land that's dropped below sea level, some of it more than 15 feet. Those lands are protected by levees that, at best, rise three feet above the level expected in a so-called "200-year flood," which is defined as a flood with a half a percent chance of happening in any given year.
And as we discussed in Part 2, with sea levels expected to rise four feet by 2100, and given that even a one-foot rise is expected to make 200-year floods happen on a regular basis, it's not hard to do the math.
In 100 years, or 150, those parts of the subsided Delta that are still low and dry will be the ones that Californians put their backs, and their wallets, into saving. Some of the decisions will be pragmatic, some sentimental. Some of those decisions will come to nothing as the flood waters overtop the rebuilt levees anyway. If it doesn't happen by 2100, it will by 2250.
Islands that have already succumbed, such as Frank's Tract in the 1930s and Mildred Island during the Reagan White House years, are now popular fishing holes. Others, such as Liberty Island, are being managed as potential nurseries for beleaguered native fish such as the Delta smelt. If rising seas reclaim the Delta back from its human owners, wouldn't that be a boon for wildlife species sorely in need of habitat?
The answer necessarily relies on speculation, if not arm-waving. But it's relatively safe to say that some species will benefit, and others won't so much.
There are a couple of big issues at play here.
One is pollution. The Delta is the site of intensive agricultural activity, and many of the submerged islands have local stockpiles of substances that aren't particularly benign when wildlife comes into contact with them. They range from herbicides and insecticides to soil fumigants, fertilizer, fuel oil, gasoline, and motor oil, and painting supplies and industrial cleansers. Oil and petroleum product pipelines cross the Delta, often along the levees, and would be at risk of breach if those levees fail.
And then, of course, there are homes on the islands as well, and flooding them would expose Delta wildlife to the whole range of potentially toxic substances in those homes, from engine degreasers to oil paints to -- in some abandoned houses -- the byproducts of making methamphetamines.
If our likely eventual abandonment of Delta islands before the flood were to be carried out in an organized, systematic manner, then a lot of these toxic substances could be cleaned up. But it's far more likely that evacuation from flooded islands will happen by surprise after the levees break, and immediate efforts will be quite rightly focus on saving lives rather than removing diesel fuel from tanks or cleaning out random containers from garages.
And what's left in the water will be left for wildlife to deal with.
But what kind of wildlife will there be? The post-sea-level-rise Delta will be radically different from the one American settlers found in the 19th Century.
The peat soils that underlay the historic Delta were built, a few inches in a good year, by plants growing in standing water -- "emergent vegetation," in the ecologists' jargon. In the Delta that's mainly tules and cattails, which cannot grow in more than a couple feet of standing water. When levees fail in the west and central Delta, the water will race into islands whose surfaces are 15 or 20 feet below sea level. And the force of the inrushing water will scour out the soft soil, creating holes that might be as much as 60 feet deep.
Frank's Tract, which flooded in 1938, provides an interesting example of what might happen to other subsided Delta islands after levee failure. Subsidence wasn't the only reason Frank's Tract lay below sea level: the island was an active peat mine for some years.
It's been 77 years since the island flooded, and the majority of it is still open water. Emergent vegetation grows around the shores of the 3,500-acre lake, but any peat soils that vegetation creates are promptly eroded by winds and wave action. That's not to say there isn't vegetation in the water at Frank's Tract: the lake is dominated by invasive Brazilian waterweed in summer, and its floor is covered in dense colonies of the invasive Asian clam Corbicula fluminea. Both invasive species tend to reduce the amount of plankton available for native fish and other desirable species. But you can still land a good-sized striped bass at Frank's Tract, and waterfowl find the open water an appealing place to rest.
If more of the Delta looked like the open waters of Frank's Tract, that could be good news for those species who like open water of a certain depth.
But some of the species we care most about in the Delta -- the Delta smelt, chinook salmon and steelhead, a range of other struggling fish -- use deep open water only sparingly. Delta smelt thrive best in turbid water with abundant plankton, which a lake full of imported clams will not provide them. Juvenile salmon and steelhead use submerged vegetation, such as seasonally flooded grasslands, as rearing habitat.
Subsided Delta islands filled by levees that collapse under the increased pressure of sea level-enhanced flood waters will probably never offer that kind of habitat.
In 2012, a team of wetland experts published a study of more than 600 wetlands that had been the sites of efforts to restore their local ecosystems. The paper, published in the periodical PLOS Biology, reported that even when restoration projects were engaged deliberately -- rather than restoration by fluke, as a massive flooding of the Delta would be -- the restored wetlands' basic ecosystems were still hurting after some decades -- in some cases, after more than a century.
"In general," wrote the authors, "once disturbed, wetlands either recover very slowly or move towards alternative states that differ from reference conditions." You can read "reference conditions" as meaning "how the place was before it got messed up."
A lot of Californians seem to have a sense that nature is self-healing. Leave a landscape alone to follow the course it's supposed to without human interference, goes the thinking, and nature will restore itself.
And it's true in a way, as long as you accept two very different habitats -- deep water full of a few introduced species and mixed-depth marshland with a diverse assemblage of native species -- as equally valuable.
But restoring the Delta to accommodate the species that are in the most trouble will take a lot more work than sitting back and waiting for sea level to rise. One intriguing studysuggests that the solution may lie in the same thing that's threatening the Delta: flooding. Except that in this case, the flooding would take place a little bit at a time.
Researchers working with the U.S. Geological Survey got hold of a plot of land on Twitchell Island in the western Delta, whose surface has subsided as much as 20 feet below sea level. They divided the land into two 7.4-acre study plots and kept them submerged in 22 and 10 inches of water, respectively. Within a year cattails had colonized both sites, and within two years the shallower test plot had become a solid stand of emergent vegetation, while the deeper plot was a mix of marsh and open water.
Over the years from the study's outset in 1997 until 2005, the elevation of the ground grew in both plots, as emergent vegetation created new peat soil. It wasn't fast: over that eight-year period, the shallower plot created just under nine inches of soil, while the more deeply flooded plot rose by almost 11 inches.
That's a soil-building rate of between one inch and an inch and a half per year. That would have kept Delta marshes above water during the last 6,000 years, during most of which time sea levels were rising at a half a millimeter or so per year.
Deeper water does build soil, both through accumulation of organic matter and through sedimentation, but those soils accumulate more slowly -- and open water allows currents and wave action to erode those soils. Large areas of open water can also inhibit peat soil formation in adjoining marsh tracts, through wave erosion and similar means.
In other words, once a subsided island is filled with 10 or 15 feet of water, it's probably going to stay that way. If we decided to rebuild freshwater marshes in those islands, we could conceivably rebuild those peat soils, chasing the rising sea level at an inch per year. At an inch a year, a 15-foot-deep island would take 180 years to build back up to present sea level -- at which point seas might be six feet higher, or more.
One could short-cut the process by adding fill to the subsided islands, but one would need a lot of fill. Jay Lund, director of UC Davis' Center for Watershed Sciences, points out that 250,000 acres of the Delta are below sea level: filling them back up to within a few inches of sea level for optimal marsh building would take half a cubic mile of material.
And of course, there's a minor detail in the way of dumping a half cubic mile of stuff on the Delta: many of the deeply subsided islands have people living and working on them, and those people already look askance at experts coming into the area and hatching grand plans to upend their lives.
In areas on the periphery of the sunken Delta, where ground elevations are within a few feet of current sea level, flooded islands might well build their own marshes in a few years' time. That's what seems to be happening at Liberty Island, well north of the Central Delta along the Yolo Bypass. After its levees were breached during the floods of 1997, Liberty Island had freshwater tule marsh growing at its north end within a couple of years. By 2010, the island was being hailed as a nuanced and tentative success story for restoration, with new habitat for beleaguered native fish species -- including the Delta smelt, if any of the smelt survive this drought.
The fact that Liberty Island's restoration was in large part accidental offers some hope that -- at least in the fringes of the Delta -- native fish might see some of their habitat restored by rising seas. But unless sea level rise estimates are wildly overstated, the central Delta is probably going to be open water in 200 years.
And if anything, those estimates of a meter's increase by 2100 or two meters by 2200 are likely significantly understated. A study published in the journal Science earlier this month suggests that the last time the earth had CO2 levels like we have now, about 400 parts per million, the oceans were around six meters higher than they are now. That's a little under 20 feet. Six meters of sea level rise would turn the Delta into a body of salt water larger than San Francisco Bay running from Yuba City to Modesto.
It will be habitat for something. It may even be valuable habitat for valuable species. It may be a pleasant place in which to spend time. But it won't be the Delta that existed 150 years ago. And it won't be the Delta we know today.
