San Lorenzo River - Flooding and Water Pollution

Flooding and Water Pollution

Following devastating floods in December 1955, the Army Corps of Engineers built flood control measures along the San Lorenzo River and Branciforte Creek through downtown Santa Cruz. No flooding from the San Lorenzo River was reported when a flow event of nearly equal magnitude occurred in January 1982, but the water levels were much higher than one would expect, based on the original design. That was because a large amount of sediment had re-deposited in the channel after it was built. The original project design (1956) required periodic dredging of the bed sediment, which is both expensive and destructive to aquatic and riparian habitat. The flood control channel was not maintained regularly, presumably for those reasons. The levees and floodwalls were rebuilt in 2004, but the design for those changes still assumed that the channel bed would be maintained by dredging. Although the latest project calls for less dredging the original project, any dredging at all is economically and environmentally unfeasible for the City of Santa Cruz, the local sponsor of the federal project. The current and likely future levels of flood protection provided by the project, without dredging, was being studying by the Army Corps of Engineers as of August 2011.

A habitat restoration project has been underway since 1985. Recent counts of fish show the population is slowly rising to approximately 3000. Tidal influence wanes between the Broadway and Soquel Avenue bridges in the city of Santa Cruz, where some sand bars are visible.

As early as the 1970s Santa Cruz County began to create a hydrology transport model of the San Lorenzo River, retaining a group of scientists and engineers from Earth Metrics Inc., who were to create the first mathematical model of the river, with the objective of analyzing bedload transport and downstream sedimentation. The model was also used to predict transport of nitrates and other water pollution, and would be eventually used to study a variety of different land management and flood control strategies.

Another problem for the river and its habitat is the water pollution from the city of Santa Cruz and the upstream communities of Felton, Ben Lomond, Brookdale, and Boulder Creek. The San Lorenzo River Watershed has the highest septic system density of any comparable area in the state of California. Bacteria levels were dangerously high in the 1980s but have improved over the last few years. Still the bacteria levels in many areas barely meet the standards for safe swimming, and the nitrate levels are extremely high and are a threat to the drinking water supply for much of the county.