Originally published on September 24, 2014
It’s been called the largest alteration of the earth’s surface. In the San Joaquin Valley, since the 1920s, farmers have relied on groundwater to varying degrees, and over time, overpumping of groundwater basin has caused the land to subside – over 30 feet in some locations. In this presentation, USGS hydrologist Michelle Sneed discusses subsidence in the San Joaquin Valley, specifically along the Delta Mendota Canal, where there’s been some problems associated with subsidence, as well
She began with a photo that’s very well known for depicting land subsidence. “This is Joe Poland and he’s using a telephone pole or a power pole to illustrate where the land surface was in 1925, where the sign is, 1955, and where he is standing in 1977. Almost 30 feet of land subsidence has occurred at this location during that time period.” She noted that in the picture, he is standing is southwest of Mendota, but today she would be talking about a different area, a new area where they were surprised to find subsidence occurring.
“What we found were 1200 square miles subsided in the northern San Joaquin Valley area in an area bounded by Mendota on the south, Merced on the north, Madera on the east, and Los Banos on the west,” said Ms. Sneed. “The subsidence is occurring at rates ranging from about a half inch a year to almost a foot a year over a 2 year period, from 2008 to 2010.” She noted that surveys done since then by Reclamation and DWR indicate that these rates of subsidence have continued through 2013.
“The problem with subsidence in this area is that there’s a lot of water conveyance infrastructure that’s getting impacted by the subsidence,” she said. “This includes the Delta Mendota Canal, which is the original study area that we were working on, also the California Aqueduct, the East Side Bypass, which is the most important flood control channel east of the San Joaquin River, the San Joaquin River itself in the restoration area, and many local canals.”
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