A Watershed Moment

Rain gardens can help cities adjust to a changing climate.

- By Jackie Flynn Mogensen

In 2021, on San Francisco's Wettest October day on record, an "atmospheric river" dumped a stunning 4.02 inches of rain downtown, causing highways and neighborhoods in the area to flood. Cars were stranded in standing water. And the city's sewers, which carry both stormwater and sewage, overflowed in the low-lying Marina neighborhood. Officials estimated that 1.4 million gallons of untreated water could have escaped into the bay. For the approximately 700 municipalities in the United States with combined sewers, overflow can happen during periods of heavy rainfall and can lead to polluted waterways, closed beaches, and tainted drinking water. And with climate change, heavier storms are on the horizon. So about 10 years ago, San Francisco began to turn, in part, to a simple solution: planting dozens of public rain gardens.

At the most basic level, rain gardens function like sponges. They are typically made by digging 5 or so feet into the ground, adding layers of rock and soil mixes designed to absorb and filter water, and topping the layers with flowers, trees, and shrubs. A finished rain garden should dip like a bowl about half a foot below ground level so that when it rains, the garden can temporarily fill up, allowing water to percolate into the ground rather than run into the street.

And, research shows, the gardens are remarkably effective at capturing runoff. When San Francisco's Public Utilities Commission, for instance, installed 30 new rain gardens along a 12-block strip in the city's Outer Sunset neighborhood, the corridor reduced the amount of stormwater entering the sewer by 95 percent for the area, taking in about 6 million gallons-more than the volume of the iconic Lincoln Memorial reflecting pool in Washington, DC-per year.