A decades-old stormwater management method in Modesto, known as rock wells, is being scrutinized for its dual role in both recharging groundwater and contributing to frequent street flooding. Unlike many cities of similar size in California that use traditional pipe systems to direct stormwater into rivers or the ocean, Modesto relies on over 9,000 gravel-filled holes that drain untreated rainwater directly into the ground.
“It’s basically a hole that’s drilled straight into the ground with filler rock that creates voids around the rocks and gives a space for the water to go,” said Robert Englent, superintendent of Modesto’s wastewater collection system.
These rock wells help replenish underground aquifers. “The groundwater levels in the Central Valley have been sort of sinking everywhere, but the Modesto subbasin has some of the best groundwater levels around,” Englent said. “Now we don’t know for sure that it was a direct correlation to the rock wells, but we think that that’s a big part of it.”
However, maintenance issues persist. The wells are prone to clogging from debris such as leaves and trash, leading to street flooding during storms. “When the leaves get on the top like that, that’s really what causes a lot of problems,” Englent noted after lifting a grate during a recent storm.
Modesto’s 2050 general plan risk assessment identifies rock wells as a flooding concern because they can take up to 40 hours to absorb stormwater and cannot handle major storms effectively.
Residents report recurring problems. Brian Schardt described rapid flooding near his home: “I wear winter waterproof boots when I go walk my dog when it rains, unfortunately.” Allison Glasscock recalled wading through water reaching her lower calf near Downey High School: “It would spread basically across that entire intersection and go up the block to the corner it was connected to… Depending on the length of the storm, it could take a few days to a few weeks to fully drain out.” She also pointed out risks for parents and students at nearby Rose Elementary who might misjudge water depth.
To address clogs, city workers sometimes use rakes—what they call a “rake-off”—to clear drains manually. More severe blockages require large trucks capable of holding up to 2,500 gallons of water; these vehicles may need multiple trips during heavy flooding events.
Rock wells were initially chosen more than five decades ago as an affordable solution suited for Modesto’s sandy soils. Englent explained: “The cities of Modesto, Turlock and Merced have very sandy soils… So the thought was back in the ‘50s and ‘60s, ‘Why build all these pipes to get rid of the water when if it just goes into the ground it goes away pretty fast?’” Over time though, increased flooding has revealed limitations in this approach.
There are also concerns about contamination since untreated stormwater flows directly into underground aquifers via rock wells. This runoff can carry substances like gasoline or chemicals. In response, city rules restrict where portable toilets can be placed and encourage residents not to let hazardous materials enter drains.
Crews avoid vacuuming up gasoline-contaminated water due to explosion risks. The city is required by law to monitor these wells and educate residents about preventing surface contaminants from entering groundwater.
Stanislaus County still permits new rock well construction but now favors dry wells as a more resilient alternative. Dry wells filter out solids before draining water underground—a process requiring annual maintenance but offering higher capacity than traditional rock wells.
Modesto abandoned earlier plans for an extensive pipe-based drainage system due to cost constraints. About 13 years ago, efforts began shifting toward replacing aging rock wells with dry wells in flood-prone areas; so far 91 have been installed citywide.
Englent summarized this gradual transition: “What you’re looking at here with this 91 is our attempt to address the worst flooding spots in the city, and once we deal with the worst, then we go to the next worst.”
