EDITOR’S NOTE: This is one of an occasional series of articles following the Penobscot Riverkeepers Expedition from Quebec to Castine.

RIPOGENUS GORGE — White water boils between Telos Hole and Telos Rock, in a space slightly wider than a rubber raft.

Hit Telos Rock, and you likely will pin the raft against the underwater granite, or flip the boat and leave 10 people to swim the most dangerous 600 yards of white water in the Northeast. Hit Telos Hole and you might not come out.

The Penobscot Riverkeepers Expedition hurtled into that narrowest of margins Tuesday, on its journey from the Quebec border to the sea. The monthlong expedition, which began at a mountainside spring on May 8, is an effort to explore the length of the Penobscot River, and share its discoveries with schoolchildren, community groups and residents along the way.

The three paddlers making the voyage traded in their Old Town canoes Tuesday for a Wilderness Expeditions raft that took them — and six friends — from Ripogenus Dam to Pockwockamus Rapids.

The Appalachian Mountain Club’s “River Guide” introduces this section of the West Branch of the Penobscot with a short message: “This is a world-class whitewater run for expert boaters only. Beware!”

The most challanging stretch of white water is a lengthy Class V rapids known as the Cribworks, and the entrance to the Cribworks lies between Telos Hole and Telos Rock.

Guide Eric Sherman, a laid-back, curly-haired veteran of eight years on Penobscot white water, raised his voice to a yell: “Ahead! DIG, DIG, DIG!” The raft shot between the rock and the dangerous place, and plunged down into a maelstrom.

From rock to falls to churning undertow, the raft flexed and lurched through the rapids. With water flying in all directions, there was a resemblance to bugs riding on a doomed leaf. But when trouble looked imminent, the crew pulled hard and Sherman deftly spun the raft out of an eddy and down yet another chute.

In the glow that follows a run like that, one is tempted to think in the epic terms of man battling nature. But the reality at the Cribworks is much more complex.

The 1,800 cubic feet per second of angry water moving down the rapids that day was not the embodiment of immutable nature, but a gift from Bowater. The paper conglomerate stops the water at Ripogenus Dam, channels it underground, and releases it from McKay Station, where it turns hydroelectric turbines to generate power for paper mills.

The riverbed that distinguishes the Cribworks is the human intervention of another era. Here, massive timbers were built up along either side to prevent the river from its natural inclination to split, briefly, into five streams. Instead, lumber companies wanted a single, strong channel that would accommodate their log drives.

The result of these industrial initiatives is a recreational boon, a stretch of river that draws avid white-water rafters from around the region and nation. In part that is because the river, no matter how it is tweaked by human endeavor, still holds something wild — the bubbling fury, the perverse tendency to speed up over a ledge, then churn in circles at the bottom.

On this stretch of river, that curious juxtaposition of wild and tame is everywhere. Eagles soar overhead, moose munch on the shore, and logging trucks rumble in the background.

Members of the expedition stopped more than once Tuesday to ponder what might have been lost if a still larger dam — the proposed, then abandoned Big A dam — had turned this most powerful portion of the Penobscot into a lake.

Just days before, they had contemplated the question from the other side. After paddling Chesuncook Lake, the product of the 98-foot-high Ripogenus Dam, expedition member Gary Wagner paused to write in the group’s journal, “Is the river lost, or a lake created?”

There was time think about that Tuesday, and there were times when the only thought was the next drop. There were moments that offered no time for thought at all.

In the shadow of McKay Station, just beyond the empty portion of the riverbed known as the Drywork, the expedition met its first white water, a ledge and hole genially named “The Exterminator.”

The raft dropped its nose, shot up, and in the time it takes to grab and miss, one of the bow paddlers was overboard. The raft shot through, but Sherman got a lifeline to the paddler and the group soon had him back in the raft, unharmed.

There were jokes all around, but for a moment, the paddlers were humbled by the force of a tamed river.