Drilling of 4.5-inch-diameter holes is taking place at nine locations of future bridge abutments at depths up to 150 feet.
All drilling fluids will be contained in a 5.5-inch-diameter steel casing for proper disposal and the bore holes will be backfilled with a cement slurry or a mixture of cement and bentonite slurry.
One-way traffic control may not last the full three weeks of drilling.
The existing bridge was built in 1940 and is reaching the end of its useful life, according to Caltrans.
The new bridge will include a lane for bicyclists and pedestrians and feature a railing that provides passersby with views of the Smith River.