North Central Region Office collects, analyzes, and stores flow, stage, and tide information from
rivers and streams throughout its district boundaries, as well as operating and maintaining
a network of continuously monitoring tide stations in the Delta. Surface water conditions in the Delta
are an issue of particular importance and data collected within the Delta is presented
separate from the surface water information collected from other rivers and streams.
Datum Change in the Delta
In 2002, DWR conducted a Global Positioning System (GPS) survey of over 100 vertical
control benchmarks in the Sacramento-San Joaquin Delta. The purpose of this survey was to
improve the accuracy and usability of our surface water data collected by our network of
tide monitoring stations. The data collected in the 2002 GPS survey was processed by the
Department with assistance from the National Geodetic Survey. The vertical datum that our
tide monitoring stations in the Delta are set to were updated during the summer of 2005
from the old National Geodetic Vertical Datum 29 (NGVD29) to the North American Vertical
Datum 88 (NAVD88). This change in vertical datum impacts both State and local flood alert
systems as well as hydrodynamic and environmental studies in the Delta. To eliminate potential
confusion, the Department initiated a public outreach and education program to assist data users in
transitioning from the old NGVD29 datum to the new NAVD88 datum. The Department fully adopted
the NAVD88 datum standard on October 1, 2006.
Click here to
view a detailed explanation of the datum change and how it may affect
local water operation procedures.
Click here to view
a map of the Delta and all of the Department's tide monitoring
stations that were affected by this datum change:
lists all of our Delta tide monitoring stations along with latitude/longitude and correction
factors to convert from the NGVD29 to NAVD88.
The Change to the new NAVD88 datum will affect the tide stage forecasts for four Delta
locations (Rio Vista, Antioch, Mallard Island and Venice Island) and river stage forecasts for
three official river forecast points (Mokelumne River at Benson's Ferry near Thornton, San
Joaquin River at Mossdale Bridge, and Yolo Bypass at Lisbon). In addition to affecting stage
forecasts, the NAVD88 change requires modification to the monitor stage, flood stage, and
danger stage values at four of these locations. Table 2
summarizes the datum changes for all seven of these stations.