NACHES, Wash. — A state snowplow driver making a routine winter run on U.S. Highway 12 late Tuesday afternoon made a disconcerting discovery — a 3-foot-by-3-foot hole in one lane of Wildcat Creek bridge.

The bridge was quickly shut down and a short time later, road crews found a second, equally large hole in the other lane of the 77-year-old bridge, located east of the highway tunnel near Rimrock Dam.

Traffic will be restricted on this section of highway until at least Saturday as road crews repair the holes and engineers begin assessing the long-term viability of the bridge, which dates to 1936.

The holes, which completely penetrated the bridge’s 6-inch-thick decking, were likely caused by a combination of wear and tear from winter weather, salt chemicals used to melt ice and the bridge’s age, according to Summer Deery, a spokeswoman for the state Department of Transportation.

On Wednesday, workers placed a plywood barrier under the bridge to catch any debris that could fall into Wildcat Creek below. The immediate plan will be to cut an 8-foot-square patch of roadway in each lane and patch the holes with concrete, said Darin Wilkens, a state Department of Transportation Department bridge supervisor.

In the interim, two 1-inch-thick steel plates are temporarily covering the holes.

The bridge is 14 miles east of White Pass and about 38 miles west of downtown Yakima.

While far from the state’s busiest mountain pass — it gets an average of 1,600 cars and trucks a day compared to Snoqualmie Pass, which sees 27,000 vehicles daily — the route is the only link between Yakima and the White Pass Ski Area. Unlike Snoqualmie Pass, White Pass is rarely closed for avalanche control, which makes it popular with some truckers anxious for a reliable route.

Transportation Department spokeswoman Meagan McFadden said the highway will remain open, but traffic will be restricted during daylight hours to one lane of alternating traffic in the vicinity of the bridge while repairs are made. A speed limit of 25 mph is posted in the work area.

There are three other bridges in the area. The Indian Creek bridge was built in 1939, while two other bridges crossing the Tieton River were replaced in 2011.

• Yakima Herald-Republic photographer Gordon King contributed to this report.