“Now a boat like that one,” he said, gesturing back to the barge again, “would be carrying over a hundred and ten tons of coal down the canal. So the boatmen would call that a ‘loaded boat’, and it would ride low in the water and be hard to start or stop. After unloading its cargo in Georgetown, it would head back upstream as a ‘light boat’. With boats coming and going at any time, locktenders would usually keep their locks set for a loaded boat. That means the upstream gates would be open and the downstream gates closed. So a loaded boat could drift right in and be snubbed to a stop. Then they would close the gates behind it.”

Vin stepped away from the group for a better view of the upstream gates. Each was a thick wooden door that pivoted around a wooden post set into the lock wall. When closed, the gates formed a shallow V-shape, which helped them seal tightly against each other and withstand the water pressure they faced when the lock was empty. The pivot-posts each supported a heavy swing-beam that was a foot wide and a foot thick. These swing-beams converged at the juncture of the gates, and from there angled upward and outward, reaching the height of a man’s waist at their distal ends, a dozen feet outside the lock walls.

Planks had been nailed to the upstream faces of the swing-beams to create a V-shaped walkway across the lock when the gates were closed. At least one set of lock gates had to be closed at all times, Vin realized, so during the canal era there was always a way to cross the lock on foot, even without the footbridges.

The ranger was still talking. “Once a boat was in the lock and all the gates were closed, they needed to drain the water out of the lock to drop the boat. Anyone know how they did that?” The ranger pointed to the only child present, a boy of ten or eleven standing next to his parents. “How did they get the water out of the lock?”

“Through those windows at the bottom of the doors,” the boy said.

“That’s right,” the ranger said. “But you mean the windows at the bottom of those doors,” he said, gesturing toward the downstream gates. “Those windows are called ‘wickets’, and the wooden panels that fit snugly inside the wickets are called ‘paddles’. The paddles can be rotated to open or close the wickets.” Vin studied the bottom of the gates. Each door had two wickets, and the paddles were actually more like wooden spatulas, with iron stems that ascended the back sides of the gates and pierced the swing-beams, extending another foot or so into the air. The stems terminated with butt-ends that he guessed were designed to mate with sockets on the lock-keys. All four lock-keys had been removed.

“When the water in the lock dropped to the level of the canal” the ranger said, “the locktender would push the swing-beams until the downstream gates were flush against the walls of the lock. You can see,” he said, pointing again, “how the walls have indentations to hold the open doors. Remember, getting the boats in and out was a tight squeeze!

“Then the mule driver would untie the snub-line, start the mules moving down the towpath, and pull the boat out of the lock and down the canal. To lock a light boat through upstream, the process was reversed.” He described it, using gestures to mimic the opening of gates and turning of lock-keys. Two people stepped away from the group and Vin sensed the speaker was losing his audience. The ranger raised his voice.

“One more question I forgot to ask you. Who knows the difference between a canal and a river?” No one replied so he sought out the boy again. “Can you tell me the difference between a canal and a river?”

“Sure. A river flows. A canal just sits there. And a river has fish in it.”

“The canal has always had fish in it. Even eels,” the ranger replied.

“Eels, cool!”

“And the canal flows, too. It was designed to flow at two miles per hour. Why do you think they designed it that way?” More people peeled away, but the few remaining were listening again. Nicky nudged Vin with her elbow and rolled her eyes, but he put a hand on her shoulder and focused on the ranger.

“I don’t know,” the boy said.

“To help the mules?” Vin said, making eye contact with the ranger.

The ranger brightened. “That’s right! With the canal set to flow at two miles per hour, it was the same amount of effort for the mules to pull a loaded boat downstream or a light boat upstream.”

“But the canal is flat,” the boy said. “What makes it flow?”

“The same thing that makes a fountain flow,” the ranger said. “Actually, the canal is like a fountain that’s a hundred and eighty-four miles long, and made up of seventy-four connected levels stretched end to end. The water flows into the fountain from the Potomac River, through five separate feeder canals that draw water from the river. The feeder canals have gates like these,” he said, pointing once more to the lock gates, “called guard locks. Once the water is in the canal, it flows continuously across each level at two miles per hour, and then cascades down to the next level using one of two routes: the lock or the flume.

“You already know how the lock transfers a boat and water downstream. But when the lock isn’t being used, it acts like a dam, so water flows through the flume to drop from one level of the canal down to the next. Some flumes are just a straight channel parallel to the lock that leads to a seven-foot waterfall into the lower level. Other flumes are rocky streams that curve around the lockhouse. These flumes take an indirect path to make the descent more gradual.”

“I think we need to take an indirect path out of here,” Nicky whispered, ducking her shoulder out from under Vin’s hand. “This guy could go on forever!” Vin smiled but kept his eyes on the ranger.

“And after the water had flowed down to the lowest level of the fountain, it was used to power the grain mills in Georgetown.”

“So that was part of the business of the canal?” Vin said. “Selling hydro power?” Nicky exhaled loudly and waggled her head in apparent disbelief.

“Sure was,” the ranger said. “Even when other parts of the canal were closed due to flood damage, they tried to keep the Georgetown feeder canal open to provide power to the mills.” The remainder of the group used Vin’s question as a chance to escape, leaving Vin and Nicky alone with the ranger.

“Actually,” the officer continued in a quieter voice, looking at them in turn, “all the feeder canals were important, since the canal was quite thirsty.”

“I’m thirsty, too,” Nicky said, “and I forgot to bring my water bottle. Did I see a water fountain near the Visitor Center?”

“There’s one near the front entrance,” the ranger said, pointing toward the facade.

Vin shot a glance at Nicky. “I’ll meet you over there in a second,” he said as she turned to walk her bike toward the entrance.

“Thirsty?” Vin said. “You mean evaporation, or leaks?”

“Both. Water evaporated from the surface in dry weather. And muskrats would burrow into the banks and cause leaks. Sometimes they’d undermine the bank enough to cause a break. The water would blow through the break and run down into a culvert, or toward the river, and the whole level would drain. Any boats on that level would be stranded.”

Vin nodded. “That sounds like a big problem.”

“Dangerous, too,” the ranger said, “if you were driving mules out on the towpath at night. They’d send a repair crew out in the middle of the night if there was a break. Of course, after a heavy rain, the canal might have too much water.”

“I guess that makes sense,” Vin said, looking back in the direction Nicky had gone.

“When that happened, they’d open the gates to the waste weirs.”

“Waste weirs?”

“Waste weirs are channels that drain excess water from the canal down to the river. They use gates with miniature paddles to shunt the water through culverts under the towpath. There’s one just above this lock.”