Bay-Delta, California, Water Resources

Bay-Delta: Levees, Climate Change, and Water Quality

This is the 2nd post in a series on the crisis facing the Sacramento-San Joaquin River Delta. Click here for a list of all posts in the series.

For several decades following the Gold Rush, almost a half million acres of the Delta, then a tidal marsh, were drained and reclaimed. Since then, the uncovered soil has been made available for agricultural use, but reclamation has led to a profound level of land subsidence. When early Delta settlers drained and lowered the land in order to farm it, they exposed the soil to oxygen. The now oxygen-rich peat soil decomposed and then released carbon dioxide, and the loss of carbon resulted in the subsidence of land. Continued Delta reclamation throughout the late 19th and early 20th centuries compounded the problem. Indeed, most the Delta is now below sea level, and a great deal of the western and central Delta is at least fifteen feet below sea level (see map below). Land subsidence renders unstable the Delta’s 1,100-mile system of protective levees, instigating levee failure and subsequent flooding.

Land Subsidence in the Delta

Land subsidence in the Delta (1995). Original map courtesy of DWR/PPIC.

On June 3, 2004, the levee failed at Upper Jones Tract in the southern Delta, flooding some four thousand acres in both Upper and Lower Jones Tracts; just one example of the resulting damage is pictured below. The levee was repaired throughout the remainder of 2004, but even now, in 2009, it has been leaking. The Jones Tract levee failure garnered a great deal of public attention and concern over the recurring problem of levee integrity — as did Hurricane Katrina in 2005. Although New Orleans is over two thousand miles from the Delta, that catastrophe provided an eerie and sobering glimpse into what could occur in the Delta. But the 2004 Jones Tract incident was far from an isolated occurrence; there has been a long series of levee failures that have required costly repairs. In 1969, Sherman and Mandeville Island levees failed, and in 1972, levee failure on Andrus Island flooded about 1/3 of the city of Isleton, requiring $91 million to repair. In 1980, Webb and Holland Tract levees failed within an hour of each other on the same day, and Lower Jones Tract failed later that year, with total repairs at over $26 million. 1982 levee failures on McDonald and Venice Islands cost a total of $22 million to repair. There were still further failures in the 1980s, culminating in the major Central Valley flood of 1986, in which several levees failed. This was the major event that was needed to call everyone’s attention to the Delta’s collapsing levee system. A subventions program was established in 1988 that allocated local and state funds toward levee maintenance. This influx of money has incrementally improved the condition of the levees, but the program failed to address the problem of water seeping into levee foundations. Moreover, a 100-year flood event will most likely trigger multiple levee failures, notwithstanding improvements that aim to protect Delta islands from just such a flood. This lies in stark contrast to the Netherlands, where protection is built in anticipation of a 10,000-year flood event.

Road damage in Jones Tract

Road damage in Jones Tract; courtesy of DWR.

Global warming will exacerbate the challenges posed by levee failure and floods. Recent approximations suggest that sea level near the Bay-Delta will rise 28-39 inches by the year 2100, which will further compromise the Delta’s fragile, aging levees by putting more pressure on them. The levees could also utterly fail in the event of a serious earthquake, which would disturb their weak foundations. Seismic activity poses a particularly high danger for the western and central Delta — where the levees are closer to faults, the islands are more seriously subsided, and the unstable soil is more susceptible to liquefaction.

Land use in the Delta
The Delta’s diverse land uses; courtesy of UC Davis.

Flooding is further complicated by urbanization. The California Department of Finance projected that the state’s population will increase to 60 million by the year 2050, and the population of the Delta region, currently at about one-half million, is projected to grow more quickly than the state as a whole, more than doubling by 2050. The California Department of Water Resources has gone even further to project that 90 million people will call California home by the year 2100. There is considerable pressure to urbanize the agricultural land in the Delta, and little wonder why: the state must accommodate an increasing population, and the Delta region features several major freeways and rail lines that are attractive growth corridors; it is also centrally located to employment, sandwiched between the Bay Area and the rapidly growing Central Valley. The Delta Protection Act of 1992 limited development in the Delta’s central “primary zone,” but the “secondary zone,” which consists mostly of of land on the fringe of the Delta, was not rigorously addressed. To the extent that the existing urbanization of the secondary zone induces additional development, there will be an increase of population in areas that are particularly susceptible to flooding. Increased urbanization will also force a potentially problematic reorganization of priorities, in that Delta resources will be directed toward building new urban infrastructure, and toward implementing flood management strategies to protect this increased population. Barring a destructive catastrophe, residential use is essentially permanent. Thus, as agricultural land urbanizes, more funding and resources will be diverted away from solving the other significant problems in the Delta.

A significant and ever-present problem is water quality. The basic source of the problem is that water diversions reduce the freshwater flow through the Delta. As a result, saltwater from San Francisco Bay is permitted to enter the Delta, increasing the salinity of Delta water and compromising water quality for urban exports and in-Delta agricultural users. Increased salinity and deterioration of water quality have long been a source of concern in the region. As early as 1920, the City of Antioch challenged upstream diversions from the Sacramento River, and the problem worsened as more water was diverted from the Delta. It was partly the need to manage this problem that gave rise to the construction of the federal and state projects during the first half of the 20th century (described in this previous post); freshwater was released through the Central Valley Project’s Shasta Division to address this concern. But an important consideration here is the effect that decades of draining the Delta watershed, with little or no thought paid to conservation, has had on this region’s once-thriving ecosystem. The state and federal projects have had a detrimental effect on the environment of the Delta for several reasons, including that exports reduce the quantity of water that flows through the Delta, thus allowing the intrusion of Bay saltwater. Upstream diversions continue to have a significant effect on Delta inflows: on average, about 18 million acre-feet of water are diverted each year from the Delta, which amounts to 40% of the water that otherwise would have flowed. About two-thirds of the total diverted water is actually diverted upstream of the Delta, with the remaining one-third diverted by the state and federal projects, and in-Delta agricultural users. The State Water Resources Control Board (SWRCB) — the state agency that administers water rights — can adopt water quality standards that protect the public health or welfare, and it has the authority to regulate water permits that it grants in connection with the SWP and CVP such that those standards may be fulfilled. Nonetheless, our handling of the Delta’s resources has historically not been particularly gracious or sensitive. Water exported from the Delta has increased in the past few decades, particularly through the state project, which serves municipal users up and down California. As the state’s population increases, urban water demand promises to increase as well, which will necessitate further conservation measures.

Sierra Nevada
Sierra Nevada; courtesy of NaturalLight.

In addition to increased flooding, global warming will detrimentally affect water quality, thus challenging our longstanding historical use of the Delta as a source of freshwater. The root of the difficulty is that warmer temperatures will alter the established pattern and timing of runoff. Snowpack is expected to decrease at least 25% by the year 2050; snow lines will fall at higher elevations, and more precipitation will fall in the form of rain. The current runoff pattern from the Sierra Nevada generally focuses on the spring and summer when the snowpack melts; water is stored during this time, and is then delivered later in the year. Climate change will transform this established, slow-paced pattern into behavior that is erratic, uncertain, and variable. The frequency of violent winter floods will increase, putting even more pressure on fragile levees. But another result is that, on the whole, peak runoff will shift to storm season: the winter and early spring. This will result in reduced freshwater flows through the Delta in the spring and summer, which will permit more saltwater from the Bay to enter the Delta, further compromising water quality. At the same time, warmer temperatures will also lengthen agricultural seasons and increase the number of droughts; and all the while, the state is projected to add millions of people in upcoming decades. The final result is that there will be less available freshwater; but there will also be a higher demand for that increasingly limited water supply. Global warming both poses a new challenge and exacerbates existing problems — and it only emphasizes the fact that our current approach to Delta management is not sustainable in the long-term.




  1. Pingback: California: In Dreams Begin Responsibility | - 4 May 2009

  2. Pingback: OPENwater: A reason to go to Alameda | A Better Oakland - 8 November 2010

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