A 16-inch water main break in downtown Portland flooded streets, closed the Hawthorne Bridge in both directions and caused water damage in five commercial buildings Sunday.

Repairs to the broken main along Southwest First Avenue are expected to go through the night, according to the Portland Water Bureau. The Hawthorne Bridge was reopened to eastbound traffic Sunday afternoon. Westbound lanes and other closed portions of the street are planned to be reopened in time for the morning commute on Monday.

The cast iron water main that broke was installed in 1958, the water bureau said. The cause of the break is under investigation. Portland crews respond to roughly 200 main breaks a year, according to the agency.

The main break occurred around 11:45 a.m. at Southwest First Avenue and Madison Street, said Lt. Rich Chatman, a Portland Fire & Rescue spokesman. Several inches of standing water soon spread along Southwest First Avenue between Madison Street and Taylor Street.

As of 2 p.m., water valves in the area have been shut off, but Southwest First Street is closed between Southwest Jefferson Street and Southwest Salmon Street, the water bureau said. Southwest Madison, Main and Salmon streets are also closed between Southwest First Avenue and Second Avenue.

It's not yet clear how much damage was caused to parts of the five buildings where water seeped in, according to the water bureau. Some people may have dirty water in their homes, but it should clear up on its own by 4 p.m.

Due to a construction project, the westbound lanes of the Morrison Bridge are only accessible from Interstate 5, so westbound drivers should use the Burnside Bridge or Ross Island Bridge.

TriMet said bus service is temporarily unavailable to Southwest Madison Street and First Avenue, Southwest Main Street and Second Avenue and stops on the Hawthorne Bridge. Buses will instead be using the Burnside Bridge.

Stephanie Yao Long of The Oregonian/OregonLive staff contributed to this report.

-- Everton Bailey Jr.

ebailey@oregonian.com

503-221-8343; @EvertonBailey