(AP) -- Riffling through old maps while researching a history project for San Francisco public schools, landscape architect Bonnie Sherk made a discovery: a century ago a creek coursed where two school campuses stand today.
"There was Islais Creek, running where Balboa High School is now," said Sherk. "All of a sudden it made sense: the school's cafeteria had been flooding and the basements of homes in this neighborhood flood during heavy rains because they're in this large watershed."
Two wells have been drilled on the school campuses since Sherk's discovery a decade ago, tapping the hidden creek's water to irrigate community gardens, parks and street vegetation, while hopefully reducing the threat of floods.
Now, as part of an estimated $4 billion sewer upgrade, Islais Creek and other streams that last saw daylight more than a century ago could flow openly once again through neighborhoods of one of the country's most densely built cities.
Such "daylighting" of urban creeks is being embraced in cities throughout the world. Seattle, Portland, Ore., Yonkers, N.Y., Providence, R.I., as well as Zurich are among many places reopening long hidden waterways. Resurrecting old creeks can help remove hundreds of millions of gallons of storm water from sewer systems each year - meaning fewer sewage spills and cleaner water.
Covered up during and after the Gold Rush when the city's booming population created demand for housing, San Francisco's many creeks were diverted and sent underground into the sewer system - parts of it still utilizing 1850s-era brick pipes. The water is mixed with the waste and sent to a treatment plant before being expelled into San Francisco Bay or the Pacific Ocean.
Each year, these rain-swollen creeks often overload the system - and about a dozen times a year raw or partially treated sewage spews into the bay and sea.
Currently, Islais Creek, once the city's largest, can only be seen in a park in the southern part of the city, where it flows into concrete sewer pipes.
The only place people can catch a glimpse of Mission Creek is in the basement of the San Francisco National Guard Armory and Arsenal building, a massive brick structure not open to the public and currently owned by a pornography film company.
"We want to partially restore the natural hydrology of San Francisco," said Tyrone Jue, spokesman for San Francisco's Public Utilities Commission, the agency in charge of the project. "It helps the sewer system by reducing flow, beautifies neighborhoods and can bring back natural wildlife that may once have lived in an area."
San Francisco is studying the best sites for unearthing these historic creeks, and officials say the first phase of projects would likely start in five to 10 years.
Islais Creek, which starts in the city's Glen Park neighborhood, and Mission Creek, which runs beneath the trendy Mission and South of Market districts, are likely the first candidates.
Both creeks flow toward the bay through densely packed neighborhoods, which could expose the water to pollutants such as auto runoff and garbage.
Berkeley in the 1980s opened a stretch of Strawberry Creek in a public park. But after heavy rains the creek filled with a lot of debris.
Still, regulators say with proper monitoring and natural filters, opening the creeks can actually improve overall water quality by reducing raw sewage overflows.
"Growing plants and vegetation along the creek banks can be very efficient in filtering pollutants and making sure the water going into the bay is better quality," said David Smith, manager of Clean Water Act permits for the U.S. Environmental Protection Agency office in San Francisco. "There's a lot of environmental benefit in turning streams back into living systems."
Seattle has many urban creeks, most of which have had some sections reopened or restored in recent decades.
These projects have improved water quality and reintroduced wildlife into a populated urban area, said Judith Noble, a strategic adviser for Seattle Public Utilities.
Yet the process has been vexing at times, and full of challenges, Noble said. Some creeks have become healthy habitat for spawning salmon and other wildlife, yet other creeks have been polluted from urban runoff, increasing fish deaths. Federal fisheries managers are studying the fish deaths, Noble said, trying to figure out the toxins responsible.
"Our story is both positive and puzzling," she said. "Daylighting is an admirable venture, but it can also get very complicated very quickly."
Though San Francisco city creeks will never flow as they once did, those working to expose them see benefits for the city and its residents.
"It turns a negative in terms of flooding and property damage into a positive," said Sherk. "The community can use the water for irrigation, help mitigate flooding, improve the environment and have an educational resource for the schools."
On the web:
San Francisco's Urban Watershed Planning: http://sfwater.org/mto-main.cfm/MC-ID/14/MSC-ID/361/MTO-ID/550
A Living Library: http://www.alivinglibrary.org
Seattle Public Utilities: http://www.cityofseattle.net/UTIL
Explore further: Nearly 1 million gallons of runoff, raw sewage spills into San Francisco Bay