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Subsidence Surveys

One thing everyone can agree on is that the Delta's islands are sinking. Bronwen Wang, a hydrologist at the U.S. Geological Survey, is trying to promote just as much agreement on what can be done to slow the process down or, in some cases, stop it completely.

The Geological Survey has been working in the Delta since the late 1980s, charting historical rates of subsidence and studying the process of peat oxidation, which is the main cause of subsidence. Since the summer of 1993, Wang has been conducting a third study, manipulating water flows into three plots on Twitchell Island to gauge the effects of different water regimes: year-round flooding; winter flooding and summer irrigation; and seasonal winter flooding only. Such studies will also help planners evaluate different habitat development and restoration options for Delta islands - among which Twitchell is a candidate.

As intuition might suggest, Wang has discovered that converting to a permanently flooded wetland environment controls subsidence best. What remains to be seen is how a field subjected to winter flooding and summer irrigation - basically a managed fallow field - fares in terms of moisture content and oxidation. That will be compared to a third site subjected only to seasonal winter flooding.

Although she's hoping for interesting results from her more intensively managed fields by the time her study ends in 1995, Wang says that her biggest surprise so far has been the richness of Delta peat. In some areas, her crew has had to drill through 30 feet of organic matter to reach stable ground.

Contact: Bronwen Wang (916)978-4648 ext. 342

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