San Diego Creek - San Joaquin Marsh

The San Joaquin Marsh is a long and narrow, approximately 500-acre (2.0 km2) man-made wetland that occupies the lower 1.4 miles (2.3 km) of the San Diego Creek stream course. Typically only 150 to 300 feet (46 to 91 m) wide, it is separated into a series of ponds by several low water rock dams, which also regulate streamflow. The marsh utilizes an ancient, perhaps once abandoned, river channel that discharges into Upper Newport Bay. Roughly 100 migratory bird species currently use the marsh as a resting area, 20 nesting bird species are found, and a total of 200 different bird species have been cataloged. Although crossed by a number of bridges and fed by polluted water, the marsh also functions as an important facility to treat San Diego Creek water before discharging into the bay.

The marsh, which is currently the largest freshwater marsh in Orange County, was originally a severely polluted and degraded expanse after use as farmland and a duck-hunting area. Before the 1950s and 1960s, San Diego Creek had followed a much shorter, channelized course to the bay, leaving the marsh without a source of water. Agricultural runoff and later urban runoff also polluted the site, leaving a remaining trickle of wastewater running through the marsh. In July 2000, the City of Irvine, which owns 300 acres (1.2 km2) of the marsh in the San Joaquin Wildlife Sanctuary, contributed $1.5 million USD to its restoration, also with contributions from the Audubon Society and the State of California. The remaining 200 acres (0.81 km2) belong to the University of California and have been designated as the San Joaquin Freshwater Marsh Reserve.

A spreading ground, currently out of use, borders the marsh on the right bank, and the UC Irvine campus is on the left bank. Geologically, the marsh is an incised meander in the creek, bending southwards. The old diversion channel bypassed the meander, and while that is still in use, it is only used to pass water around the marsh section during high flows.