Update 11/21/2011: DWR
continues to plan target areas of the Delta for "restoration" based on
elevations, so that must be an important factor in the planning process.
The problem is different agencies and mapping services show different
elevations for the same lands. The 2007 LIDAR survey can be off by 3+
feet due to plants and other surface variations...perhaps that is the reason
for the conflit of information?
For seismic history and risk in the Delta, go to "Delta
Earthquake History" page added August 2012
General comment: Over the years, some of the Delta islands have subsided due
to the soil types found on those islands, and the farming methods.
However, a majority of islands are within 10 feet of sea level, and have
good levees to protect them. (Consider Holland that uses 100 foot
levees to hold out the ocean!) The levees of the Delta have NEVER
failed from an earthquake, to anyone's knowledge. But since the
elevations have become a big issue, this page provides links to maps showing
elevations over time. Please note that islands like Sherman Island,
which has subsided substantially, is owned by state agencies who were
farming the island to lower it at one point. Other islands, like Webb
Tract and Bacon Island, have also been planned to be "in-Delta" water
storage islands, so one would assume the owners of those islands were trying
to get the islands as deep as possible. The fact is, once farmers
realized some of their practices caused subsidence, they stopped farming
that way, and many islands no longer subside. Anyway, here are various
maps you can use to compare for your self. Note that some large map
sets (from 1906 and 1978-1980) will be scanned and added to this page area
to provide good historical records. Copyright note: These maps
are provided for educational purposes and not for sale or other commercial
use. If anyone claiming a copyright on any map below needs to request
that a particular map be removed, please email us.