Nov 05, 2021

Field Updates October 2021: First Rain, Groundwater Recharge, and Runoff

First rain!

The Morro Bay watershed received its first rainfall for the new water year on October 24! A local rain gauge at Canet Road off Highway 1 in the Morro Bay watershed recorded 2.32 inches of rainfall over a three-day period.

Local rain gauge network

The Estuary Program has a local rain gauge network that compiles rainfall data and tracks hyper-local trends. This network relies on citizen scientists to gather rainfall data from their yards, schools, businesses, offices, or any outside space where a small rain gauge can sit undisturbed and collect the rain.


If you’re not already a part of the Morro Bay Rain Gauge Network, now is a great time to join! The Estuary Program provides free rain gauges like the one pictured above. Contact to schedule a time to pick up your rain gauge, and start recording the rain at your home, school, or workplace!

Rainfall influences our creeks

October’s first rainfall generated enough runoff to raise the depth of water, known as stage height, on Chorro Creek by over one foot at peak flow. The stage height prior to the storm was approximately 4.44 feet, and at its highest point, the stage height reached 5.53 feet.

The photos show a section of Chorro Creek that was dry prior to the storm and fully connected and flowing shortly after the storm.

Chorro Creek before and after the storm
Chorro Creek received some much-needed water during the October rainfall event. The photo on the left shows a site on Chorro Creek before the storm and the photo on the right shows the same location shortly after peak flow, which is the point when water levels reach their highest level in the creek as a result of the storm.

Flow returns to pre-storm levels

Although this rain event brought some much needed water to our creeks, the stage height in the days following the storm doesn’t appear to have changed drastically.

The graph below from SLO County’s monitoring site illustrates the stage height at Canet Road from pre-storm to post-storm. The dark blue line shows the recorded stage height compared to the creek bottom, which is indicated by the orange line.

Graph of Canet Stage Height during October 2021 storm
The graph shows the stage height, or water surface level, of Chorro Creek at Canet Road. This data is collected and reported by the County of San Luis Obispo.

Prior to the storm, the stage height was approximately 4.4 feet. While the stage height increased in response to the storm, the stage height returned to about 4.5 feet after the storm receded. This minimal change is likely a result of the long dry period prior the storm, as the dry ground soaked up much of that rain and less of it ran off into nearby creeks.

Rainfall and stage height data, including graphs like the one here, can be viewed in real-time or downloaded from the County of San Luis Obispo Public Works Department webpage.

Rainfall affects groundwater

Even though the stage height at Canet Road didn’t change considerably in response to the October rainfall event, there are other ways the watershed continues to hold water. As rainfall soaks into the ground, it percolates down to the water table and recharges groundwater.

Recharging groundwater is important for a number of reasons. Humans rely on groundwater for drinking water, irrigation, and industrial purposes. Surface water systems like lakes, rivers and wetlands also rely on groundwater to keep them replenished. Groundwater provides a crucial source of water for these systems, especially during the dry season.

Rainfall increases urban runoff

While we always get excited about the first rainfall of the water year, these flows typically have the highest levels of polluted runoff.

Pollutants build up during dry periods in places like streets and yards. Runoff picks up and carries these pollutants to the nearest storm drain, and then into a nearby creek or the bay. This is especially true in areas with a lot of paved surfaces where water isn’t able to sink into the earth.

The first rainfall of the year often pushes accumulated pollutants downstream. This photo shows a storm drain that flows into the bay at the end of Marina Street on the Embarcadero.
The first rainfall of the year often pushes accumulated pollutants downstream. This photo shows a storm drain that flows into the bay at the end of Marina Street on the Embarcadero.

The Estuary Program monitors this runoff at Morro Bay State Park Marina as part of First Flush monitoring. This year’s First Flush monitoring was triggered by the late October storm, and results are anticipated in the coming months.

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