IN THE MILES BELOW the Tis Isat falls, the river's current stalled. To make time, we began motoring. The first tribal village we encountered was a Bodi settlement. Petros's command of their language was limited, but he gathered that a stick-fighting contest was about to take place, in which young suitors whack each other silly with poles that have phallus-shaped tips. The winner gets the girl.

Scaturro suggested we take photos from the boats and move on. We were on a tight schedule. At a Mursi village 30 miles downstream, Liz, the photographer, put her foot down. The women were wearing lip plates, and she wanted time to shoot them. Scaturro cut the throttle and steered toward shore. He told Petros to get off and negotiate a deal with the elders: one price, unlimited photos.

"Leave your daypack here so they don't know that it's your bank," Scaturro told him.

Petros scrambled out of the boat, still carrying his rucksack. Two young men arrived by dugout. As lean as gazelles, they were naked but covered themselves with blankets. Their upper arms were dotted with scars, which were self-inflicted to show how many enemies they'd defeated in battle. One of them carried an assault rifle.

Petros gave us the high sign.

"OK, everybody, go ahead," Scaturro said. We piled off with our cameras. The deal was 70 birr, about $9, for as many photos as we wanted. "Don't anybody pull out any money, or it's all over," Scaturro shouted. "Petros will pay."

After five minutes, a commotion developed. Petros was rummaging in his backpack for money while a gray-haired Mursi elder standing beside him peeled off crisp one-birr notes from a stack and handed them out to a knot of clamoring women. They were demanding two birr per photo.

"All right, back in the boats," Scaturro screamed. "Now! Petros, get that fucking backpack on board."

We hurried to the rafts. Zach Gill, our massively strong, six-foot-five assistant boatman, stayed ashore tending the bow line. "Shove off! Shove off!" Scaturro yelled. The women wouldn't have it. They surrounded Gill and held fast to the line, haranguing us.

Suddenly, there was the metallic ker-chank of a round being chambered. One of the angry young men pacing the banks was brandishing a loaded Kalashnikov.

Scaturro turned and calmly said to our security guard, "Baye." Baye understood immediately; he moved to his drybag and started pulling out socks, shirts, and finally his 9mm service pistol, which he cocked and held barrel-down by his thigh.

Another gunman appeared. "John Ricci, get up here," Scaturro barked. "If that guy loads a round, jump him." An Ironman triathlete, Ricci was, at 235 pounds, the bulkiest man on the team. Even if the Mursi were bluffing, I couldn't believe Scaturro would take such a risk. Ricci stepped to the bow and folded his arms. The second gunman backed off.

But money was still an issue. "Petros, get some money up here, right now," Scaturro yelled. Petros remained frozen in place.

We were at a tense standoff. At last, Robel pulled out his own wallet and palavered with the gunmen in Amharic. They settled for 12 birr, the equivalent of $1.50. We were free to leave.