Every Thursday night, a Maryland highway department crew trundles onto the Woodrow Wilson Bridge to plug the holes that have appeared in the roadway the preceding week. These are large holes, big enough to swallow the tire of an 18-wheeler. If you look straight down, you can see the Potomac River.

During a snowstorm two winters ago, a hole on the Cabin John Bridge opened. The hole was so large that a pickup truck had to be parked on top of it to prevent something from falling in.

Some time next year, planning being what it is, both big bridges that carry the Capital Beltway across the Potomac River will undergo major reconstruction at the same time.

The cost will be $10 million for the Cabin John and anywhere from $35 million to $45 million for the Woodrow Wilson. Money is only one big part of the problem. Highway planners know the whole project will be a headache for tens of thousands of commuters. Minimizing delays and inconvenience is just as important as the money.

The Woodrow Wilson Bridge, 5,900 feet long, six lanes wide, dedicated on a December day in 1961 that was so cold the Marine Band could not toot, carries an average of 134,000 vehicles per day. That is 1.2 cars and trucks per second.

The Cabin John Bridge, officially called the American Legion Bridge, is 1,400 feet long and six lanes wide. It was dedicated without as much attempted fanfare in 1963, and carries an average of 105,000 vehicles per day -- 1.22 cars and trucks per second.

Both of the bridges are major bottlenecks because the Beltway in effect feeds eight lanes of traffic onto the six-lane bridges.

"We call that the hourglass effect," said Maryland highway administrator Slade Caltrider. What he means is that, for the driver, time crawls.

Almost every major bridge in the United States of about 20 years of age is being given a new deck. The Cabin John and Wilson bridges are only two of many in the Washington area that require reconstruction.

All of the Potomac crossings inside the Beltway either have new decks or are going to get them. The District of Columbia government alone is planning a $250 million, 15-year effort to repair or replace more than 30 bridges.

The decking problems with all these bridges essentially stem from the same causes, high traffic, freezing and thawing, and most important, corrosion induced by salt used to thaw ice in the winter. The last problem, engineers say, may be overcome by new bridge construction techniques.

The Cabin John and Wilson bridge projects will involve complicated feats of engineering, major environmental problems and ticklish traffic control situations. The Woodrow Wilson Bridge has its own special bureaucratic problem: parts of it rest in Maryland, parts in Virginia, and parts in the District of Columbia. The whole shebang is owned by the Federal Highway Administration.

The reconstruction program would be easier, and much cheaper, if the bridges could simply be closed for the duration. That obviously is impossible. The bridges are vital links on the Capital Beltway, which carries one mile of every 10 that is driven in the Washington Metropolitan area.For thousands of commuters, there would be no realistic alternative to the two big bridges.

"Based on past experience we know we are going to get delays on the Woodrow Wilson no matter what we do," said Robert Nickerson, of the Federal Highway Administration's Maryland office. "We're going to get some delays on the Cabin John from people slowing down, but we're not going to have to close any lanes."

Sad to say, that is not the case on the Wilson Bridge. To the certain ire of motorists, one half of the Wilson will be closed every night between about 8 p.m. and 6 a.m. Traffic will be limited to one lane each way, and the remaining middle lane will be used to store stalled vehicles.

During that 10-hour period, the contractor will knock out about a 25-foot-by-25-foot section of bridge deck and hoist into position a precast concrete replacement span. Another section will be removed and replaced the next night until, at least 18 months after the program is begun, the bridge will have been redecked.

No one is looking forward to the job. "Drivers are nasty out there." Nickerson said. "They just expect that bridge to remain open."

Caltrider recalls that somebody once fired a shot at one of his state repair trucks parked on the bridge. Robert Ambush, a state inspector from the regional office, said that people spit and throw cans at construction workers patching holes in the bridge.

"If we have to go out on Friday night, it's really bad," Ambush said. "I think it's because people have had more to drink."

The Wilson bridge project could take even longer than 18 months. Here is why: Tentative engineering plans call for the huge cranes that will be needed to do the work to be mounted on barges floated in the Potomac. Some dredging will be necessary before even shallow-draft barges can move in close to the piers.

Dredge spoil -- the gunk from the bottom of the river that will have to be moved to deepen the channel -- has to be placed somewhere, and in this era of environmental concern dredge spoil dumping permits are hard to come by.

It is possible that the dredge spoil will contain toxic heavy metals.That is because the Woodrow Wilson Bridge is immediately downriver from the Blue Plains sewage treatment plant, which dumped virtually untreated sewage into the Potomac for decades. A multimillion-dollar project has resulted in a new, efficient Blue Plains that cleans the water, but problems from the past may remain on the Potomac floor. Sewage effluent regularly contains toxic heavy metals.

If the smaller cranes have to be used and the work has to be done entirely off the bridge instead of off barges, the precast replacement sections will be smaller and it will take longer to fix the bridge.

By the time the project is completed, the Wilson bridge will have six new lanes, plus a narrow shoulder where stalled vehicles can be parked. The lack of a shoulder is one of the things that makes a disabled car on the Wilson bridge during rush hour such an exciting event.

Also under study is the possibility of widening the bridge to eight lanes and including a protected bicycle lane. Federal and Maryland officials said, however, that portion of the widening will have to come later, because it involves enormous complications and because the present deck cannot await for those complications to be solved. The complications are mostly on the Virginia shore, where six lanes are crowded between a cemetery in Alexandria and a major apartment development in Fairfax County. Maryland officials, with Virginia's knowledge, are looking at the possibility of double-decking the Wilson or of adding a parallel span. "Alexandria is a historical district," Caltrider said. "The environmental aspects alone may not be surmountable."

It is going to be cheaper and a lot easier to fix the Cabin John Bridge, which actually is two separate bridges. Like the Wilson bridge, only the deck will have to removed and replaced.

To do that without closing lanes, the contractor will first build a new bridge between the two existing bridges. Traffic will be shifted from one old bridge to the new middle bridge bridge and the old bridge will be closed and redecked. The same procedure will be used for the other old bridge.

When that $10 million project is completed, perhaps within a year, the Cabin John will be a nice, wide sixlane bridge on a deck that will have the capacity for carrying eight lanes. The new middle bridge will be part of the entire structure. A protected bicycle lane across the bridge probably will be included.

Before eight lanes can be opened, Virginia would have to widen a short section of Interstate 495 from the George Washington Parkway to the bridge, and Maryland would have to widen a longer section between River Road and the bridge.

Financing on the Cabin John is simple. The bridge is owned by Maryland and is on the interstate highway system. As an interstates restoration project, it will be eligible for 90 percent federal funding. The rest will come from the state.

Financing of the Wilson bridge, the only federally owned interstate highway bridge in the country, will be complicated.

Maryland is happy to act as the agent for arranging engineering and construction work on the Wilson, Caltrider said, but the federal government will have to pay the entire $35 million to $45 million bill.

That will take an act of Congress, and Rep. Herbert E. Harris II (D-Va.) has introduced the necessary bill. It is languishing in the House Public Works transportation subcommittee, but is expected to be tacked onto new federal highway legislation early next year.

The Carter administration has not decided where it stands. "We concede we own the bridge," said Moritmer Downey, assistant transportation secretary for budget. "We have to look closely at that bill to see what we'll get." Downey and other federal officials interviewed agreed, however, that the bridge must be fixed soon.

Right now the annual cost of the Wilson bridge approaches $450,000, split equally among D.C., Virginia and Maryland. Maryland is responsible for maintaining the roadway, the District is responsible for operating the draw span and manning the bridge tower, and Virginia pays the electric bill.

"Cars are not going to fall into the river," said Maryland highway administrator Caltrider, "because the bridges are basically sound. But the situation on the decks is serious, and we don't want to wait too long."