The Petaluma River is more than just the tranquil backdrop for waterfront restaurants or a place for weekend kayaking and bird-watching.
It's a major Bay Area shipping route that has seen a steady increase in commercial traffic over the past two decades.
Barges the length of football fields sail the narrow passage almost daily, bringing sand and rock mined more than 1,000 miles away in Canada to projects along the North Coast.
Tonnage has increased sixfold since 1987, fueled in part by the closure of quarries along the Russian River.
"People don't know how much commerce is happening on this river," said Christian Lind, vice president of operations for Jerico Products Inc., a family business that has run barges and tug boats out of Petaluma for 40 years. "We're hauling 4,000-ton barges just about every day."
But time may be running out on one of the last remaining commercial uses of the river as its namesake city transforms itself from a crumbling wharf town to a quaint tourist destination.
In recent years, Victorian-era warehouses along the banks have been converted to trendy shops and loft-style apartments. Plans for more riverside homes and stores are in the offing.
At the same time, efforts to build a new asphalt plant at the site of an old aggregate company south of town have run into opposition from locals who say it could pose a threat to nearby wetlands.
And precast cement manufacturer Pomeroy Corp. closed its Petaluma operation in 2006, leaving only two river-dependent businesses inside the city limits.
There could come a day when the forces of redevelopment and environmentalism combine to end a chapter in the history of a town founded on its connection to a river.
Inside a dusty tin warehouse beside the river, Jerico president and family patriarch Mitch Lind talked about the future with sons Christian and Aaron Lind.
"The river is a big asset to the North Bay," Mitch Lind said as the current lapped gently at a large tug tied up outside.
"It's the transportation lifeline to Petaluma and Sonoma County," Chris Lind chimed in.
From its mouth at San Pablo Bay to its headwaters 16 miles upstream, the river winds like a snake past rolling hills and tidal marshes, dairy farms and forgotten moorings.
The lower 12 miles flow through southern Petaluma and the largest salt marsh on the West Coast. It's home to ducks and geese and boasts the largest breeding ground for the California Clapper Rail, which is listed as an endangered species.
The upper four miles are mostly unnavigable, branching off into creeks fed by springs and runoff from Sonoma Mountain.
"It's a treasure," said David Yearsley, executive director of Friends of the Petaluma River. "It's used as a benchmark of what a pristine tidal marsh should be."
The pace of shipping on the river belies its perception as a sleepy tidal estuary, while harking back to a time when passenger ships and steamboats plied the blue-green waters.