A FEW MILES BELOW Okouyi's camp, a major tributary joins the Djidji from the north. The river broadens and then begins stacking up in a series of oxbows. So there were far fewer snags the next day—only the biggest fallen trees could block the river's width—and we started to make up for lost time. I was consigned to a vache with Boyer, and we fell steadily behind the lead boats, then finally stopped worrying about seeing wildlife. Boyer took out his fly rod and made some practice casts under the overhanging boughs along the bank.

On day six I wound up in a green boat with a new partner, Christian Mbina. We hadn't talked much—he was a serious guy who avoided the campfire, instead tucking into his tent after dinner to read the Bible. He wasn't a WCS employee but ran his own NGO, Adventure Sans Frontieres, whose main mission was to take schoolchildren to the beach in Pongara National Park, just outside of Libreville, a prime nesting site for leatherback turtles. He also had an environmentally focused show on Gabonese TV called Ça Se Passe Ici ("It Happens Here"). Still, he confessed, he wasn't altogether thrilled about the way his career was going. There wasn't enough money in it and, worse, not enough opportunity.

"I think most people who think about it are proud of the parks," Mbina said of his fellow Gabonese. "At the same time, they say, ‘They give us the money, the U.S., but what do they do with it?' Look at the WCS project directors in each of their parks—every one of them is a foreigner. Why? Aren't there any Gabonese who can do that job?"

Mbina wasn't the only skeptic. "This is a country where we want one thing but also the other," a Gabonese eco-activist named Marc Ona had told me in Libreville. "The parks are a good project to show to the outside world, but to take logging permits away for something long-term is a huge gamble. It won't pay off for 10 or 20 years, if ever," he said. "And—let's be honest—Gabon is not Kenya. You can go 100 kilometers without seeing anything."

The next afternoon, the pace of the river began to accelerate. We were approaching the edge of the escarpment, where the Djidji tumbled down to meet Gabon's biggest river, the Ogooué. There were more rocks now and long riffles that gave the best canoeing of the trip, then, suddenly, real rapids. We proceeded cautiously, lining the boats through difficult sections and laboriously portaging them around one thundering six-foot waterfall. The thought that ran through all of our heads was, If it's borderline now, at the height of the dry season, what happens when it starts to rain?

By dark, Boyer's GPS told us we were just a few miles, as the crow flies, from Djidji Falls. We camped at a rocky bend where a lone Cape buffalo eyed us warily before bolting. Curran got out the sat phone, waded out into the river for better reception, and called the WCS office in Ivindo National Park. "We should be at the take-out by 11," he said.

But the Djidji wasn't quite ready to let us go. The next morning, our seventh on the river, we funneled into a narrow, rock-strewn channel a mile below our camp without stopping to scout it. The first three canoes made it through. The fourth, a vache piloted by Starkey, flipped and wrapped around a rock, then broke free, pinning Mbina, the bowman, against a thorn-studded pandanus trunk. I ran back up the riverbank to help yank the boat loose, then stupidly decided to float back down to my canoe instead of walking.

The water was only three feet deep, but it was moving fast. I couldn't stop myself or even get over to the bank. A minute later I was swept around a bend, pushed over a small drop, and then driven deep into a hole—shockingly deep, because when I stroked for the surface, I didn't get there. Another desperate stroke and I popped up, then nearly got sucked down again into a submerged tangle of conical pandanus roots—a nightmarish scenario that even now makes me wince. Instead, the current spat me sideways onto a rocky ledge where, badly shaken, I pulled myself out of the river.

Mbina and Starkey's vache was even worse off. After the boat folded around the rock, the aluminum weld running along the transom had partially given way, opening a large gap. We effected a MacGyveresque field repair, inserting a rubber strap in the hole and pounding the metal into place with a rock, and then pushed on.

Half an hour later, with the roar of the fast-approaching falls drumming in our ears, we rounded a left-hand bend and saw a most welcome sight, a crew of half a dozen park workers in coveralls and Wellingtons, smiling and beckoning us to shore. The eight of us staggered out of the canoes, and the trail crew began loading our boats and gear onto a couple of Land Cruisers. A minute later there was an excited shout: A snake was nestled beneath a drybag in the bottom of one of the boats.

It was a short hike down the escarpment to the base of Djidji Falls. There, we lounged on close-cropped green turf and took a final, celebratory swim in the Djidji. I couldn't quite focus on the WCS vision of a nearby eco-lodge; no doubt it made sense, but to me the place was just about perfect—hard-earned, unexpectedly beautiful, unruined.

We drove out to the town of Ivindo on an overgrown logging road that, as soon as we left the park, became as wide and smooth as Libreville's voie express. Empty logging trucks passed us going the other way, kicking up great contrails of red dust, and rounding one corner I glimpsed the rear end of a large, dark ape scrambling into the woods on all fours—the only gorilla sighting of the trip.

Just before Ivindo, we came upon a solitary figure walking down the road, a machete in one hand and a bulging sack made of woven plastic slung over his shoulder. Strangely, for this part of the world, he didn't bother to ask for a ride or even look up as we passed.

"I know that guy," Mbina said after we rolled by. "We stopped him last year when I was up here on patrol. He's a poacher."

"That must be crocodile he's got," Gnoundou said. "Anything else would bleed through the sack."

"Unless it's been smoked," Starkey said. According to Starkey, who'd done his thesis on the bushmeat market, the price would likely be 300 to 400 CFA francs per kilo—30 or 40 cents a pound, affordable for Gabon's non-elite. "When you compare that to chicken or beef, which is anywhere from 1,000 to 2,000 CFA," he said, "you can appreciate what we're up against."

Those days and nights on the Djidji stayed with me for a long time afterwards, and not just because tsetse fly bites itch like nothing else in the world. There were the army ants, Boyer's maps and Gnoundou's fresh fish bouillons, Plage des Éléphants and the waterfall, and not least the bond I'd formed with my vache mates—fond memories all. But the guy with the gunnysack stayed with me, too. In the end, how different was he from us, with our own booty squirreled away in our drybags—our notebooks and film canisters and memory cards? All of us want to have our crocodiles and eat them, too.