Living on an Urban River: Water Quality

The San Lorenzo River, 2010. Photo © J. Klinger

The San Lorenzo River, 2010. Photo © J. Klinger

What does it take to keep a river clean? The intrinsic beauty of northern California’s San Lorenzo River camouflages the challenges and efforts of maintaining the health of the river ecosystem. The river has been changed, altered, and influenced by people as seen by the presence of levees and dredging operations. Runoff from roads into storm drains carry pollutants and bacteria to the river system sure to degrade water quality.

The Drudgery of Dredgery

The native residents of Santa Cruz knew better than to build permanent structures on the banks of the San Lorenzo River. Spanish settlers learned their lesson and built the Mission and Branciforte Villa on high ground surrounding the San Lorenzo River, using the floodplain only as grazing area for farm animals. But eventually, we made our way down in elevation from the safe high grounds. In fact, downtown Santa Cruz was built right on the banks of the San Lorenzo River and right in the middle of the river’s floodplain.

Ecologically speaking, floodplains are naturally occurring areas that help transport and store flood waters. When a floodplain is developed with homes and businesses it can no longer function as the effective flood relief system nature intended without being a risk to the people that moved in. Due to recurring severe floods, most notably the Christmas Flood of 1955 (read about it here), the Army Corps of Engineers built flood control levees along the urban San Lorenzo River in 1959.

Imagine the San Lorenzo River surging during a winter storm and you can see the danger of living within a river’s floodplain. Photo courtesy and © of Santa Cruz Museum of Art and History.

Imagine the San Lorenzo River surging during a winter storm and you can see the danger of living within a river’s floodplain. Photo courtesy of and © the Santa Cruz Museum of Art and History.

Sediment buildup is common in sections of levees. The segment of the river where the Santa Cruz Riverwalk meets Josephine Street is wider and less steep than portions of the river upstream. When river water hits this area it slows down and deposits sand, silt and clay. For a long time, the levees were maintained with ongoing sediment removal (dredging). The goal was to increase the speed at which water could flow through the river during a flood.

Frequent dredging is costly and is only a temporary fix since sedimentation continuously fills in river beds. The last dredging event occurred in the 1980s. As part of that project, a channel bar in the middle of this portion of the river was built to act as a floodplain and a habitat for riparian vegetation. Since that time, different flood control tactics have been implemented like raising the levees and bridges.

The Water In An Urban River

Think about your neighborhood. Do you ever see someone washing something in the driveway with soap? Do you ever see puddles from oil leaks or an escaped piece of trash blowing in the wind? All of these pollutants from city streets, sidewalks and landscapes are washed into storm drains as it rains polluting your local creek or stream.

Ten storm drains enter the San Lorenzo River. Water runoff from a rain storm can carry these pollutants that eventually affect water quality. Pollution of this type is called non-point source pollution because it is not from a single point like a factory, but a myriad of sources throughout the watershed.

Debris and litter is just one type of pollutant that can affect an urban river. Photo courtesy of and © Coastal Watershed Council.

Debris and litter is just one type of pollutant that can affect an urban river. Photo © the Coastal Watershed Council.

Also affecting the San Lorenzo’s water quality is sediment. Sediment is made up of fine particles such as dirt, clay, silt, decaying plant matter and even decomposing aquatic insects. Large sources of sediment come from erosion that exposes soil along the sides of creeks and rivers. Sediment in water makes it difficult for fish and other aquatic insects to breathe, as suspended sediments can clog the gills of fish. If you’ve ever experienced smog in the air, it’s like this for fish, but worse. As the sediment settles it can bury gravel beds and smother fish eggs and aquatic insects. Cloudy water also diffuses light and heats up the river water.

Read the rest of the story here at Mobileranger.com to see what harmful substance the San Lorenzo has record high levels of.

Take the Self-Guided Mobile Tour

This piece is part of a tour all about the lower San Lorenzo River by the Coastal Watershed Council. You can download the free app with many tours of the Santa Cruz area and beyond at www.mobileranger.com.

Like the Content? It’s by and © Mobile Ranger. Check out all our blogposts and our free mobile app with sixteen AppTours of the Santa Cruz coast at www.mobileranger.com. Please like us on Facebook!


Julia Gaudinski


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