Citizen Water Quality Monitoring

Water Quality MonitorsWater quality monitoring is crucial to maintaining the health of habitat and water resources. This program identifies contaminant levels in the Arroyo Seco and sets the stage for corrective measures.

Arroyo Seco Foundation volunteers collect water samples from the stream and test the levels of a set of parameters, such as pH, dissolved oxygen, and bacteria. Anyone can participate! If you are interested, please let us know at

The water quality data collected will:

  • be used in the planning and evaluation of stream enhancement activities;
  • promote understanding of the overall function of the Arroyo Seco stream;
  • measure upstream-downstream, seasonal, year-to-year, and long-term changes in water quality and watershed conditions;
  • detect point sources and non-point sources of pollution;
  • educate the public about the stream and its riparian corridor; and
  • develop a sense of community stewardship for the Arroyo Seco and its rich aquatic resources.

Seasonal monitoring is essential for an accurate assessment of the water quality of the Arroyo Seco because the ecological life of the watershed varies with the seasons. Changes in weather conditions have a great influence on water quality. Heavy rainstorms in the winter and early spring introduce large amounts of pollutants due to runoff. In the summer, reduced flows and higher temperatures caused by dry weather alter the conditions. Baseline monitoring data (chemical, physical, and biological) are necessary to identify vulnerable areas to be targeted for improvement.

Water quality in the Arroyo Seco Watershed is degraded in several significant ways. The Hahamongna Basin is a Superfund site due to the presence of volatile organic chemicals and perchlorate. Nitrates from past agricultural practices and septic tank systems are a significant factor in some parts of the watershed. Street runoff from throughout the watershed pollutes the Arroyo Seco stream with trash and contaminants, and the Los Angeles Regional Water Quality Control Board has detected unacceptable levels of coliform from animal waste.

For more information, see Developing a Citizen Water Quality Monitoring Program in the Arroyo Seco, a complete guidebook for water quality testing and improvement in the Arroyo Seco.

The Test Sites

Samples are collected from 11 locations along the main stem of the Arroyo Seco. These sites were established by the Los Angeles County Department of Public Works as being important for determining the condition of the stream in general. You can view a map of them here.

  • Confluence — The site where the Arroyo Seco joins the Los Angeles River, near Downtown Los Angeles.
  • North Branch — A major tributary that flows through Sycamore Grove Park in Northeast Los Angeles.
  • Arroyo Seco Golf Course — The base of the northern end of the Arroyo Seco bike trail.
  • San Rafael Creek — The creek coming from Johnson Lake in Pasadena that enters the Arroyo just north of the South Pasadena city line.
  • BFI Lower Arroyo Park — The low flow streams in Pasadena's Lower Arroyo.
  • Slime Slide — The area under the Colorado Street Bridge where the concrete channel meet the natural soft bottom.
  • Seco Street — The storm drain system that drains much of Northwest Pasadena and enters the Arroyo near Brookside Park, just south of the Rose Bowl.
  • Flint Canyon — The Arroyo tributary that drains La Cañada Flintridge and meets the Arroyo northwest of Devil's Gate Dam in Hahamongna Watershed Park.
  • Altadena Drain — The storm drain outlet from Altadena near the top of the Hahamongna Basin.
  • Millard Canyon — The first major interface between natural and urbanized parts of the watershed.
  • El Prieto — The canyon near the Forest Service's homes above the Hahamongna Basin. This is the most natural site we test at.

Water Quality Paramenters

The presence of certain substances in water indicate conditions that are harmful to individual organisms and a balanced ecosystem. The parameters ASF tests include:

  • total coliform bacteria
  • E. coli
  • dissolved oxygen
  • total dissolved solids
  • nitrogen
  • pH
  • temperature
  • turbidity

For more information on these parameters and why they are important, see our water quality parameters page.