Feb 10, 2023

January 2023 Field Updates: Heavy Rainfall and High Flows

A series of heavy rain storms hit San Luis Obispo County during January 2023. The intense rainfall led to extremely high stream flows throughout the Morro Bay watershed. Many low-lying areas experienced overflowing channels, flooding, and erosion as a result of the storms.  

Breaking Records

San Luis Obispo County received a record amount of rainfall during recent storms. A rain gauge located near the center of the Morro Bay watershed recorded 9.11 inches of rainfall between January 1st and January 24th. To put this in perspective, our area typically receives about 20 inches of rain during the entire year.

A series of rainstorms can quickly and dramatically influence stream levels. When the ground is already saturated with water, the rainfall that would normally sink into the ground instead runs quickly downstream into the nearest water body. At its highest point, water levels on Chorro Creek at Canet Road reached a height of 22.95 feet on January 9th. This is the highest water level recorded at this site since monitoring began in 1979.  

How Much Water?

Stream gauges tell us how deep the water is at one stationary location. While this information is important, we are interested to know how much water is moving through the creek. To calculate how much water is in the creek, we use specialized equations called “rating curves.” Rating curves help us translate water height in feet into stream flow in cubic feet per second (or “cfs”).  

Using a rating curve specifically designed for the Canet gauge, we estimate that Chorro Creek received a maximum stream flow of 12,456 cfs during the January 9th storm. This peak discharge is more than 50% higher than previous peak flows measured at this site. To better illustrate this, the graph below shows the peak discharges at the Canet gauge between 1979 and January 2023.  

Bar chart showing peak discharge, in cubic feet per second, of water from Chorro Creek at Canet Road. The chart ranges from 1979 to 2023.
The graph shows peak discharges on Chorro Creek from 1979 to January 2023. This gauge is located at Canet Road and is maintained by the County of San Luis Obispo.

Visualizing Streamflow 

A standard basketball is roughly the size of one cubic foot. If the stream flow is 20 cfs, you might picture about 20 basketballs rolling downstream per second. The recent storm brought a peak flow of nearly 12,456 cfs, or 12,456 basketballs moving downstream every second.

Now, imagine that each basketball weighs 62.4 pounds, the weight of one cubic foot of water. At peak flow, those 12,456 basketballs had a force of 777,260 pounds per cubic foot every second!   

Monitoring Creeks Post-Storm

Many of our creek sites look very different following the recent storms. Sites like Pennington Creek experienced significant erosion, where the strong flows scoured stream banks and swept away vegetation.  

Two images side by side, showing the difference in erosion in a creekbed before and after a major storm event.
Pennington Creek experienced significant erosion after recent storms. The photo on the left shows the channel on January 4, 2023 before the record-breaking storm, and the photo on the right shows the same location on January 10, 2023 after the storm.

Chorro Creek Ecological Reserve

In the fall of 2019, the Estuary Program completed a 4.8-acre floodplain restoration project along Chorro Creek at the base of Hollister Peak. This project was made possible with the help of many partners including the California Conservation Corps (CCC), California Department of Fish and Wildlife (CDFW), State Coastal Conservancy, and many more. To read more about the project design and implementation, you can revisit our blog post from last year. 

During late January 2021, the Chorro Creek Ecological Reserve experienced its first large storm event, which you can read more about in this blog post. In both the 2021 and 2022 storms, the project site held up well against some of the highest flows on record. The constructed floodplain allowed sediment to be deposited during high flows rather than being pushed downstream into the estuary. The floodplain also allowed for water to slow down and sink in, which recharges groundwater and provides wildlife habitat. 

Four images showing a floodplain during four different days. The water level is different in each image.
The Chorro Creek Ecological Reserve restoration project site was activated during the January 2023 storms. The time series shows the project site pre-storm (top left); after the site was activated (top right); during the height of the storm (bottom left); and post-storm (bottom right).

Check out the video below for a time-lapse of the project site during the high January flows!

The video above shows the Chorro Creek Ecological Reserve during the January 9th storm. This is the largest flow event that the project site has experienced since construction was completed in 2019.

Unlike the 2021 storm which had over 90% plant survival, the extremely high flows from January 2023 impacted native plantings in the lower floodplain. The Estuary Program is currently assessing plant survival with the support of the California Conservation Corps. We are also looking into areas of erosion and deposition to understand how the project has changed post-storm. 

A panoramic image of a floodplain. There is a noticeable lack of plants, and evidence of erosion.
The photo shows the Chorro Creek Ecological Reserve project site after the high flows of January 2023. Many of the native plants installed to reduce erosion and stabilize the site were swept away.

Volunteers Back in Action

Large storms and high flows can create unsafe situations for staff and volunteers. Turbulent stream flow and unstable banks can quickly sweep even the most river-savvy off their feet. To keep our team out of harm’s way, we assess sites after large storms. When flows are low enough to safely enter, we can resume creek monitoring tasks. 

Two people wearing waders are standing knee-deep in a creek. One appears to be taking measurement, and the other is writing on a clipboard.
Creek water quality volunteers, Jude and Devin, take measurements on San Luisito Creek post-storm.

Help us protect and restore the Morro Bay estuary! 

Thank you for helping our beautiful, bountiful, biodiverse bay!