It was wonderful to see the massive pool of water behind Devil’s Gate Dam in Hahamongna Watershed Park after the brief storm last Wednesday. It was a flashy storm. The flow in the mountain watershed of the Arroyo Seco climbed from six cubic feet per second to almost 600 cubic feet per second in a few hours. Within eight hours more, the current dropped back below 100 cfs. But that brief storm left 700 acre-feet behind Devil’s Gate Dam for a few days. That’s about 228 million gallons, worth more than $1,000,000. But then the County pulled the plug, opening the dam’s valves even though there is no storm on the immediate horizon.
It’s deeply ironic that the Los Angeles County Flood Control District, which has a stormwater program that annually provides $280 million of taxpayer money for local capture and beneficial use, quickly drained Devil’s Gate Dam, sending nature’s gift downstream to the Los Angeles River and ultimately to the Pacific Ocean. And Pasadena is planning to spend $80 million of the county money on a series of “stormwater” projects that won’t capture as much water in a year as nature provided in a single brief storm. This is exactly the kind of local water supply we should be holding, infiltrating, and putting to work—consistent with flood safety and modern weather forecasting techniques.
The Hahamongna area is a natural filter, and Devil’s Gate Dam is a natural funnel. Rain and runoff from the San Gabriel Mountains flow into the Arroyo Seco and Los Angeles River system and, during storms, collect behind the dam. That basin is more than just open space; it’s one of the most important groundwater recharge areas in the region. When stormwater is slowed, spread, and allowed to infiltrate, it becomes local supply—stored in the aquifer, protected from evaporation, and available during drought years.
The case for saving stormwater is simple: local capture is cheaper than buying more imported water, and it provides multiple benefits at once. Birds, wildlife, and people love it. Most of the water will percolate into the long-starved Raymond Basin that underlies Pasadena. That’s water we don’t have to purchase, pump over mountains, or fight over in interstate negotiations. Conservation reduces pressure on ratepayers, and it strengthens our independence in a world where water politics are only getting tougher.
This is not a “single-storm” issue; it’s an annual pattern that has persisted for years, when storm forecasting was more of a mystery than it is today. Hahamongna and Devil’s Gate Dam commonly see ~700 acre-feet or more per storm, and we get 5–10 storms each winter/early spring. We’re effectively wasting thousands of acre-feet and 2 billion gallons a year when we should be optimizing capture and recharge, consistent with flood safety, at a value of $1,500 per acre-foot.
Saving stormwater behind Devil’s Gate Dam is the most practical, cost-effective way to strengthen Pasadena’s water future while improving the health of the Arroyo Seco. We already have the “hardware”: a major flood-control facility and a large natural spreading grounds complex at Hahamongna. What we need now is the civic will on the part of Pasadena and Los Angeles County to treat stormwater as a resource, not a nuisance to be rushed out to sea.
Los Angeles County, for its part, cannot credibly champion stormwater capture—backed by a $280 million program—while simultaneously releasing stored stormwater at Devil’s Gate Dam. If the County expects cities and nonprofits to maximize stormwater capture and beneficial use, it should hold itself to the same standard at one of the region’s most important recharge locations.
Tim Brick is the Executive Director of Stewards of the Arroyo Seco.
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