The Chesapeake & Ohio Canal National Historical Park is about to put into place the last piece of flood control upgrades that were envisioned after floods twice devastated the canal in 1996, park officials said this week.

The officials said they hope soon to test the new and reinforced pieces of the historic 1852 stop lock, which had been destroyed by a flood in January 1996.

The lock, which is just below Great Falls, was designed by the early canal builders to work like a gigantic door that would be slid into place in the canal to divert destructive flood waters from the Potomac River out of the canal and back to the river.

The park already has performed about $20 million in flood control repairs along the canal to try to mitigate damage from the 1996 floods, which washed out several sections of the canal's berm, destroyed bridges and wrought other havoc.

A new tethered, break-away bridge, designed to float free in a flood, has been installed near Cabin John. Causeways over the canal, which tended to increase damage, have been removed. And special subterranean concrete spillways have been poured, to direct excess water out of the canal.

The stop lock is the last major piece of the improvements to be finished, officials said.

The stop lock originally was built with large stone abutments on either side of the canal bank, with a winch house across the top, like a covered bridge, in which were stored the huge timbers that made up the gate, according to Daniel Copenhaver, the park's civil engineer and flood project manager.

The lock was made of about 20 timbers or "members," that, in the event of a flood, were fitted horizontally into slots in the abutments and stacked up in the canal like the wall of a log cabin.

The lock blocked the flow of damaging flood water into the lower part of the canal, and directed it back to the Potomac via a stone levee.

The gracefully carved stone abutments and tree-covered levee survive intact, but the winch house was swept away in a flood in 1889. Timbers are now installed with a crane.

In January 1996, a blizzard followed by a huge snow melt sent a deluge of ice and flood water cascading out of the Potomac and down the canal. Among other things, the flood smashed the existing, but probably not original, timbers of the stop lock, which vary in length from about 20 to 29 feet.

Nine months later, replacement timbers held when Hurricane Fran sent a slightly less potent flood out of the Potomac. But park officials wanted to be sure the lock would not be breached again. So new members, many of an entirely different kind, were ordered.

Copenhaver said that because the greatest water pressure in a flood builds up near the bottom of the canal, the bottom six members are being made entirely of aluminum. The next seven are made of a combination of wood reinforced with slabs of aluminum, like a double-decker sandwich.

The remainder will be pressure-treated pine, he said.

The new gate was supposed to have been ready earlier in the summer, Park Superintendent Douglas Faris said last week.

As Hurricane Floyd approached last week, park officials expressed concern about the "test" Floyd might pose for all the new flood control measures installed since 1996.

Copenhaver said that even though the new members of the lock were not yet ready, the older, temporary wooden timbers were stacked along the canal towpath as a backup in case they were needed.

"We were monitoring the weather quite closely," he said. "Had there been a need to put the stop lock in service, we would have installed the old one."

In the end, the storm kept to the coast, sparing the canal any flooding that could have occurred had its rain been dumped upstream in the mountains. "We did quite well," Faris said this week. "We did not have any significant damage. . . . All things considered, we were very fortunate."

Copenhaver said the new lock members should be ready this week or next, and there would then be a "trial fit" to make sure the new ones are the right size.

He said the bottom seven members would then be installed and left in place, and the remainder would be stored beside the towpath, ready for the next flood.