Exploratory drilling will begin in August to probe the riverbed under the Healdsburg Memorial Bridge, part of the initial work for a major rehabilitation of the span scheduled for next year.

The drilling is to ascertain soil characteristics that will determine the design of a new centerpiece and foundation for the 91-year-old Russian River crossing.

"We're shooting for the first week of August," Public Works Director Mike Kirn said of when work will start. "There's still a couple of loose ends we need to tie up with regulatory agencies and scheduling with the driller."

The work is expected to take two days, during which the bridge will be closed to motor vehicles. Bicycles and pedestrians will be able to use it.

The closure is insignificant compared to the 18 to 24 months the bridge is expected to be shut to all traffic after major work begins in mid-2013.

The planned $12 million rehabilitation and retrofit involves bolstering the rusting structure to withstand earthquakes, flooding and erosion. It includes sandblasting and painting, repairing damaged structural members, installing a new deck and making railing safety improvements.

Nearly 90 percent of the funding comes from federal programs, but the city must come up with about $1.4 million for its share. Healdsburg intended to use about $1 million in redevelopment money toward the bridge work, funds that are now in question after the state dissolved redevelopment programs earlier this year.

City officials hope some redevelopment money will be available, but the city also could use funds it gets from gas taxes.

Regardless of the source, Mayor Gary Plass expects the project will proceed.

"I feel we have a strong commitment not only from the City Council, but the community. Unless that changes we are doing our best to find funding," he said.

There had been discussion of asking voters for a parcel tax to help maintain the bridge, but Kirn said Tuesday that appears unlikely. "I don't think there will be a ballot item on the bridge," he said.

So far, more than $2.7 million has been approved by federal and state officials for ongoing environmental review and special studies that are expected to wrap up in November.

In a brief City Council update this week on the status of Memorial Bridge, Plass noted that when it closes next year, bicyclists will need to use Westside Road as an alternate route in and out of Healdsburg. Motor vehicles will be able to use the Highway 101 crossing just downstream.

The bridge was completed in 1921 and became part of the Old Redwood Highway. It was supplanted by the more modern freeway bridge to the west.

Preservationists have rallied periodically to keep the vintage, steel-truss structure from being razed, especially after it was given a poor rating in 1979 by Caltrans.

But four years ago, engineers acknowledged that rating was based on faulty calculations and determined the bridge can carry all legal loads.

Last year, the bridge was added to the National Register of Historic Places, validating the efforts of residents who rallied to save it.

You can reach Staff Writer Clark Mason at 521-5214 or clark.mason@pressdemocrat.com.