Environmentalists call it a creek. The city considers it a drainage pipe.

Whatever your term, Thornton Creek is at the center of a legal dispute holding up expansion of Northgate Mall.

Advocates of restoring part of the creek's south fork brought their case to a meeting of the Sierra Club's Seattle chapter last night. About 125 people attended.

"It's really a no-brainer for a lot of us," said Knoll Lowney, an attorney for the Thornton Creek Legal Defense Fund, which has filed two legal challenges to a developer's permit to expand Northgate Mall.

Thornton Creek now flows under parking lots before emerging as an open creek east of the mall and flowing into Lake Washington. Lowney said the creek has existed for hundreds of years and is a spawning ground for several species of salmon.

"Salmon can make it all the way to the mall," he said.

Lowney maintains the city's own Critical Areas Ordinance calls for "daylighting" buried creeks whenever possible.

He proposed creating an open, 30-foot-wide creek bed to improve fish and wildlife habitat. He also said building a pedestrian and bicycle underpass at Interstate 5 would allow the creek to connect to its headwaters at North Seattle Community College.

The Simon Property Group of Indianapolis wants to nearly double the mall to 25 acres, adding a 30-screen theater, residential units and office and retail space.

Advocates of restoring the creek say expanding the mall is a perfect chance to do so.

"We have the opportunity to be good environmental stewards in our own back yard," Seattle City Councilwoman Heidi Wills said at the meeting.