NAPLES — When the Songo Lock at Sebago Lake State Park in Naples opened for business on June 1, 1830, it catered to 65-foot canal boats laden with lumber, apples and stoves bound from Portland to Harrison. Now, 184 years later, it’s mostly party boats hauling lotion-slathered sunbathers, fishermen and tourists. Songo Lock is the last vestige of the 50-mile Cumberland and Oxford Canal system, which once boasted 28 locks used to level out the 260-foot difference in elevation between Portland Harbor and Long Lake.

But one thing hasn’t changed. The lock’s massive wooden doors and cast-iron sluice gears are still operated by hand.

“It’s fun, being part of how it all works,” Tim Smith said as he leaned his back against the squared timber attached to the creosoted door and pushed with his legs. Smith is a volunteer with the Lakes Environmental Association. He checks boats for invasive plants as they pass through. He doesn’t need to help lock operator and park Ranger Lauren Hillard, but he does anyway.

Hillard greets every boat that enters the lock. Through 12-hour shifts, she directs them and collects $6 if they’re coming downstream — there’s no charge for going up. Once in a while, she admonishes a jet-powered personal watercraft to slow down boaters. “Sorry,” they shout.

She has a gaff pole, which she offers to boaters who can’t quite get into position. She hauls them to the side and tells them to hold onto the ropes along the edge. Some do. Then the wooden gates are closed. Water is either let into the lock if the boats are traveling up the Songo River to Brandy Pond or Long Lake or let out of the lock if the boats are traveling downriver to Sebago Lake.

“We average about 3,800 boats a season,” Sebago Lake State Park Manager Matt McGuire said. The season runs from May to October.

The old Cumberland and Oxford Canal system connected Portland to the interior. Horses and oxen towed the boats through the first leg, 20 miles and 27 locks of hand-dug canal, from Portland to Sebago Lake. Then the long, narrow boats would sail 12 miles up Sebago Lake. From there, the canal men used poles — some 100-feet long — to leverage the boats 4 miles up the Songo River. Once through the final lock, they poled 2 more miles through Brandy Pond to Long Lake. Once again under sail, it was 12 miles up Long Lake to Harrison.

The Cumberland and Oxford Canal’s heyday only lasted about 17 years. It cost $206,000 to build, but it sold for just $40,000 in 1857. Then it went out of business in 1872 because of competition with railroads.

The canal men are gone, but their pleasure-boat echos still ply the river and lake waters on either side of the Songo Lock, which never closed down. The gates still swing wide, water rises and falls, and boats pass upstream and downstream, just like they have for nearly 200 years.