Protecting OC’s Waterways: Wet Season Monitoring

Orange County’s water resources are our most incredible assets. That’s why we work 365 days a year, in both dry and wet weather, to keep them healthy.

In addition to baseline monitoring efforts, during the wet season (October – April), our team goes out to monitor water quality at storm drain outfalls, creeks, and coastal areas during and after rain events. These efforts help us understand the watershed-level effects of human activity on our waterways and the impacts of our management efforts (learn more about watersheds at our What is a Watershed page).

We test for a range of pollutants, including indicator bacteria (such as E. coli, Enterococcus, and fecal coliform), nutrients (typically from fertilizers), metals, pesticides, and organic compounds.  H₂OC’s monitoring efforts help ensure our water resources remain beautiful and healthy for all to enjoy.

Let’s talk about our storm drain system and how to help keep our water bodies thriving during the wet season.

Wet Season

From October through April, rain is more likely to occur in Orange County. The storm drain system was built to move rainwater from the streets as quickly as possible to protect life and property. As a result, the storm drain system does not treat water. Unlike the sanitary sewer system, which treats water, anything that flows into a storm drain can go untreated into our creeks, rivers, and ocean. During wet weather, anything on the ground that touches rainwater can flow into our storm drain inlets. The primary goal of wet season monitoring is to evaluate the amount of pollutant entering in our waterways during storm events.

It’s up to all of us to keep pollutants out of the storm drain system to help keep our waterways and community healthy. This is achievable by practicing simple watershed-wise habits in our daily lives, such as picking up and properly disposing of dog waste in covered bins, following pesticide label instructions and avoiding application 48 hours before and after rain, and adjusting sprinklers to only water landscaped areas to prevent irrigation runoff.

Let’s dive into all our monitoring efforts to protect OC’s waterways!

Sampling Methods

To keep our water resources safe, we collect water quality samples at numerous locations across the County. This is to ensure we’re getting a representative sample not just on the coast, but for our inland water bodies too.

The techniques we use to sample are composite sampling (done by an automated device which blends samples over time) and grab sampling (collecting a sample at a specific point in time).To learn more about these techniques, read our Dry Season Monitoring Protecting OC’s Waterways blog here.

Composite sampling is used to collect samples for testing nutrients (typically from fertilizers), metals, pesticides, and organic compounds and are timed to collect the first hour of a storm and additional storm flows over the following day(s). These periods typically represent the highest pollutant amounts, so it’s helpful to take regular samples during this time so we have a complete understanding of what is occurring in our waterways. Collecting samples for bacteria testing is done by grab sampling. The indicator bacteria we test for include E. coli, Enterococcus, and fecal coliform which can come from pet waste and raw sewage. Since conditions can change rapidly and dilution can occur in composite samples, grab sampling offers a snapshot of current conditions.

Both sampling methods provide critical information to ensure our water bodies are safe for all.

Differences Between Wet and Dry Sampling

When it is not raining during the wet season, monitoring and sampling continues in dry conditions.

However, when it does rain, there are key programmatic differences between wet and dry season sampling. These sampling programs include coastal, creek, outfall monitoring, sediment sampling, and high stream flow gauging.

Let’s now discuss the different sampling efforts and specific actions taken during wet weather!

Coastal Monitoring

Coastal waters are monitored during the wet season to protect public health and evaluate water quality.

Orange County coastal beaches are monitored weekly throughout the year to help ensure safe recreational activities, including swimming and surfing. The data collected from this weekly monitoring program is available on the OC Health Care Agency’s OC Beach Info webpage, which displays beach monitoring stations and the weekly test results for meeting public health standards.

During wet weather, H₂OC and Orange County Health Care Agency track weather forecasts, and issue public health warnings at OC beaches. These warnings advise residents to stay out of the water due to runoff pollution. The weekly coastal monitoring is also paused for 72 hours after a rain event. Then, if no further rain is in the forecast, beach warnings may be lifted and weekly coastal monitoring resumes.

Bays, harbors, and estuaries, such as Newport Bay and Huntington Harbour, receive stormwater runoff from large watersheds. Stormwater flows from the Santa Ana mountains inland, across the land and through inland waterways, to the coast. Monitoring in these areas includes water chemistry and evaluates potential impacts on aquatic life.

For our coastal waterways, grab sampling is typically conducted from a boat or along nearby estuaries and beaches, both during and shortly after storm events. This is done to collect data on pollutants associated with storm events.

To learn more about these techniques, read our Dry Season Monitoring Protecting OC’s Waterways blog here.

Let’s now discuss creek and inland waterway monitoring!

Creek Monitoring

Inland waterways are key pieces of the puzzle too. Streamflow (creek) monitoring during the wet season focuses on evaluating the amount of major pollutants transported by stormwater runoff to receiving waters like bays, harbors, and the ocean.

Sampling sites are located within Orange County’s major flood control channels. Many are immediately upstream of coastal receiving waters, although some sites are higher in the watershed to characterize land use types or specific sub-watershed areas of interest. This approach helps provide a watershed-scale view of pollutant levels within our channels during storm conditions.

Wet weather monitoring typically uses automatic samplers, programmed to collect water at regular intervals every day during a storm. The samplers are activated by rising water levels. These individual samples are combined into a composite sample, which represents water quality throughout the storm event. Certain pollutants, such as indicator bacteria, are collected as grab samples.

In addition to water quality sampling, flow measurements are recorded using equipment that supports Orange County’s public safety flood-warning Automated Local Evaluation in Real Time (ALERT) system. To learn more about the ALERT system, read our blog post on it here.

Combining flow data with pollutant concentrations allows for the calculation of pollutant loads, which helps characterize water quality conditions and evaluate the effectiveness of watershed management practices.

Most of this water flows through the storm drain system, so let’s discuss outfall monitoring!

Outfall Monitoring

Only clean rainwater should be flowing through the storm drain system and anything else is considered pollution.

Storm drain outfalls (which receive rainwater through the storm drain inlets you see on the street) are the most direct pathway for pollutants from developed areas to reach creeks and coastal waters.  Similar to creek monitoring, storm drain outfall monitoring combines composite samples and flow measurements to calculate total pollutant amounts.

During wet weather, monitoring focuses on characterizing the discharges from a small number of high-priority outfalls. These outfalls are monitored during wet weather because they convey large amounts of runoff from homes, neighborhoods, and other urban areas. Each outfall is monitored during one storm per wet season to help us understand how each outfall potentially contributes stormwater pollutant loads and investigates changes in these loads and flows over many years. This differs from dry season outfall monitoring which investigates persistent non-stormwater flow sources such as irrigation runoff or groundwater and identify potential pollutants.

Sediment can also be a significant pollutant, specifically during wet weather, so let’s talk about it next.

Sediment Sampling

During storm events, large amounts of water flow through OC’s waterways. This water transports sediment (dirt) loosened due to natural processes or urban development.

Development without proper best management practices can lead to erosion and more sediment than would naturally occur during a given storm event. Sediment is itself a pollutant because excess sediment can fill waterways and harbors, choking bottom dwelling organisms and impeding navigation by boats. Sediment can also carry other pollutants such as fertilizers and pesticides which can harm aquatic life.

During large storms (more than one inch of rainfall), we go out and measure the sediment being carried into Newport Bay. To collect sediment samples, we lower a special heavy sampler from a bridge into the creek at several, regularly spaced points across its width (this is called a transect). This is done using equipment such as a bridge board or a truck-mounted crane, as pictured. Sampling at different points helps us capture what’s happening across the entire creek, not just in one place. We take a “depth-integrated” sample, meaning we collect from the water’s surface down to the bottom of the creek. This is important because the sediment doesn’t spread evenly and can vary from the top to bottom.

By sampling through the full depth and across the transect, we get a more accurate representation of how much sediment is moving downstream and may eventually reach the bay.

High flow stream gaging during storms is another critical aspect of the work we do! Let’s talk about that next.

High Flow Stream Gauging

During large storms, OC monitoring staff measure streamflow at its peak. This shows us how much water is moving through waterways during the most intense part of a storm. The measurement of how much water is flowing through a creek is called stream gauging.

This helps us understand how much water is running off the landscape and entering our waterways. Like sediment sampling, high flow stream gauging is done on bridges at regularly spaced points across a transect. We also use the same bridge boards and cranes, but instead of a water sampler, we use a special sensor that measures the speed of the flowing water.

This flow information is used for a variety of purposes including evaluating the adequacy of flood control channel capacity and to estimate the amount of pollutants transported downstream.

Let’s now discuss the safety challenges associated with wet weather sampling.

Safety

No matter the weather, our team answers the call to protect our community. However, they also take the proper steps to protect themselves.

During storms, water bodies can cause flooding which can lead to hazardous sampling conditions. Driving in rainy conditions increases the chance of an accident, so the team practices safe driving speeds and safe following distances with the vehicle in front.

Wet road surfaces can also present potential slip, trip, and fall hazards for our sampling teams, so we make sure to use proper protective gear such as slip-resistant boots. Sampling locations can take place near public roads, so low visibility and traffic can present safety hazards to our teams.

Be on the lookout for traffic control devices including truck-mounted traffic directional lights, traffic cones, and road signs, and always drive at safe speeds.

Before we go out sampling during a rain event, we always have a tailgate safety meeting to discuss any potential hazards or concerns that are specific to each site.

Now, let’s discuss everything you can do to become a waterway protector too!

What the Public Can Do

No matter where you live, you can make an impact to protect our waterways. Here are some ideas to continue your impact all year:

1. Attend cleanups. Coastal Cleanup Day isn’t the only time we come together to keep trash out of our waterways. Volunteer events like this are happening year round! To find a cleanup event near you, check out our events calendar here. 
2. Apply for our RainSmart Rebates (currently only available in South Orange County).  The RainSmart Rebates Pilot Program offers incentives and assistance to upgrade both residential and large landscapes through the installation of rain containers and rainwater treatment features. Whether you have a residential or large landscape, we are stacking rebates on top of those already offered through the Municipal Water District of Orange County, including substantial additional incentives for landscape features to collect rainwater, manage runoff, and help local waterways flourish.   
3. The Good, the Bad, and the Bugly Actions. Pesticides are a common method of keeping pests away. However, they are often toxic to aquatic life and, if improperly applied, can find their way to OC’s creeks, rivers, and ocean via the storm drain system. To prevent this, keep ants and spiders away by using ant bait stations and decluttering your outdoor spaces to discourage spiders from inhabiting areas around your home. Learn more here.
4. Install a rain barrel. Collect rainwater from your roof with a rain barrel! This water can then be used for your irrigation needs to save money and water. Diverting this runoff also helps prevent untreated water from flowing into the storm drain system. Commit to installing a rain barrel here.
5. Leave no trace. While you’re enjoying OC’s beautiful parks and beaches, take everything you brought with you and place all trash in a closed bin.
6. Pick up after your pets. Place all dog waste in a bag, tie it shut, and dispose properly in a closed bin.

Thank you to all of our Orange County residents and businesses!

Orange County’s community members love helping our water resources thrive! It’s one of the reasons we had more beaches on the annual Heal The Bay Beach Report Card Honor Roll than any other county in California! Using rainwater as a resource and placing trash in closed bins are quick actions everyone, old and young, can do throughout the year.

Great work so far this wet season, OC, and keep protecting our waterways throughout 2026!