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#SummerScienceFriday | Testing the Waters of the LA River

One of the five questions addressed by the Los Angeles River Watershed Monitoring Program (LARWMP) is “Is it Safe to Swim?” Prior to LARWMP, we knew very little about bacteria concentrations at sites thousands of visitors swim in every year. This question looks at bacterial contamination at recreational swimming and kayaking sites along the LA River and assesses the concentration of bacteria, comparing them to California recreational standards. In order to tease out at the relationship between heavy recreational use and E. coli concentrations, we sample at sites not only on weekdays but also on weekends and holidays, when we see a higher number of visitors. In addition to collecting river samples for E. coli analysis, Council for Watershed field staff also measure water quality parameters such as pH, conductivity, and dissolved oxygen to get a more holistic understanding of water quality and the relationship between water chemistry and bacteria.

Below is a summary of the measurements we take and what they mean for the health of the river.

  • pH: measured with a multi-parameter meter. We place the probe in the water and jot down the number on the meter (picture). pH is a measure of how acidic a solution is. The pH of a waterbody is important because all organisms have a pH range in which they can survive, and for river inhabitants, an extremely low or extremely high pH can be deadly. For example, a relatively low pH, indicating acidic water, can hinder fish eggs from hatching and damage the membranes of macroinvertebrates (water bugs that are food for fish). Amphibians like frogs are also very sensitive to low pH levels.

  • Conductivity: measured with a multi-parameter meter, as described above. Conductivity is a measure of the ability of an electric current to pass through a solution due to the concentration of dissolved ions (different types of salts) in the solution. Conductivity is usually lowest near the headwaters of a stream and increases as a stream picks up salts from land surfaces. Although measuring conductivity can’t identify specific ions in a water sample, large increases in conductivity relative to the baseline can indicate that pollution has entered a waterbody. Freshwater streams should have a conductivity of 150 to 500 µS/cm to support a diverse array of aquatic life.

  • Dissolved Oxygen: measured with a DO meter. Dissolved oxygen is a measure of the dissolved oxygen molecules (O2) in water. Aquatic plants and animals depend on dissolved oxygen in the water for breathe (respiration is the metabolic process that allows organisms to obtain energy from food). Therefore, consistently high concentrations of dissolved oxygen help ensure a healthy ecosystem while low concentrations of oxygen can signal contamination, like too many nutrients in the water. Below are ranges of dissolved oxygen values with the associated ecological effects in freshwater streams.

  • 0-2 mg/L: not enough oxygen to support life.

  • 2-4 mg/L: only a few fish and aquatic insects can survive.

  • 4-7 mg/L: good for many aquatic animals, low for cold water fish

  • 7-11 mg/L: very good for most stream fish



Monitoring in Action! Former CWH Intern and now staff member, Christina Vallejo recounts her time as a water quality monitoring intern.

Being a monitoring intern is a sweaty job. We get to our first site, the beautiful Eaton Canyon in Pasadena at about 7:45am. The sun is shining against the mountains and early morning hikers have just started their ascent. The CWH Science team, which consists of our scientists and water quality monitoring interns head down to the first monitoring pool, and take our first water sample. Each team member has their own role as we measure pH, dissolved oxygen, water temperature, and conductivity - each an important measure of the health and quality of the water. After we finish up our measurements we pack up, hike out, and head to our next sites, Hermit falls and Sturtevant Falls. These two sites feature beautiful waterfalls and are popular for swimming and hiking, so there are always friendly people asking us what we are doing and why. It is very inspiring to be able to share our science with the community right as it’s happening. Finally, we head out of the mountains and toward downtown Los Angeles. Our last two sites are on the LA River itself on the edge of downtown in the Elysian Valley. Here we greet cyclists near the Great Heron Gates and talk about water quality with kayakers about to start heading down the river. We grab our last two water samples and meet up with another monitoring team from our contractor, Aquatic Bioassay Consulting, who will take our samples to the lab. Walking through forests with the birds chirping and water running is the best way to spend your morning.

To learn more about CWH’s intern program head to

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