Once he’s had a few minutes to calm down, Alves and Bailey usher the beaver into a cloth sack, with only his withers exposed, for the most sensitive step: sexing. Because even males possess internal genitalia, conclusively determining a beaver’s sex can’t be done visually; instead, it requires some serious olfactory skills. Alves presses on the beaver’s belly, feeling for his anal glands—nubbins of flesh whose secretions beavers use to mark their territories and tell friend from foe. The anxious beaver has clenched his tail, making matters more difficult, but at last the glands emerge, like pinkish teats, from the plush underfur. Alves squeezes gently, milking a dollop of viscous, yellowish fluid onto her gloved finger.

Males, they say, are redolent of motor oil. Females smell like old cheese.

Alves wrinkles her nose. “Male,” she confirms.

“It’s kind of musky and urine-y,” Bailey opines later. “At this point it’s all over my waders, and it’s never coming out. Sometimes I’ll be in the car and suddenly smell it and think, oh, where’s the beaver?”

His ordeal mercifully concluded, the beaver is released into one of the raceways, gliding up the narrow pen to huddle against the far wall. A cinderblock hut, floored with woodchips, stands at the raceway’s center. When we return the next morning, we’ll find that he’s settled into the makeshift lodge to await a family reunion. Beavers are kin-oriented creatures, with as many as 10 family members sharing a lodge: the mating adults, their newborn kits, one-year-olds, and two-year-olds. (The latter depart the colony each spring in search of their own territories, like college-bound teenagers.) Relocate a beaver by himself and he’s liable to wander the landscape, searching for companionship, until he’s devoured by a bear or cougar. Move him with his clan, and he’s more likely to stay put and build.

The Tulalip beaver relocation effort, like most projects of its kind, thus strives to capture and release cohesive families. When they catch unattached beavers, they attempt to match them at the hatchery, like a rodent dating service. “If you give them a few nights together, they usually pair up,” Alves says. “They get lonely—they want to be with other beavers.”

When all goes well, relocation can produce spectacular results. Later that week, Bailey and Alves drive me to a stream high in the Mount-Baker Snoqualmie National Forest, the 1.7 million-acre block of Douglas fir and western red cedar that sprawls across the western flank of the Cascade Range. The Tulalip had dropped a family of seven beavers in this creek in the fall of 2015. Nothing happened at first, and Alves and Bailey concluded the site was a bust. When they found signs of dam-building the next spring, they figured another colony had moved in on its own. Grainy camera trap footage, however, revealed that the nocturnal construction workers bore the Tulalip Tribe’s ear tags. The relocation had worked after all.

Now, a lonely beaver outpost has blossomed into an empire. Eleven separate dams have transformed the stream from a string-straight, free-flowing riffle into a patchwork of sun-dappled pools and serpentine side channels. We teeter along felled trunks and bushwhack through thickets of vine maple, exploring a complex that seems only to expand as we press on. The vast maze of channels and ponds would be an impressive feat for an engineer in a backhoe; for a handful of rodents armed only with incisors, it’s practically miraculous.

The largest dams are graceful crescents of latticed wood, buttressing serene ponds with yellow alder leaves swirling across their surface—the platonic ideal of beaver infrastructure. Others are loose stickjams that hold back only a turbid bathtub. “I think they were drunk when they made this one,” suggests Bailey, eyeing a crooked, homely ridge of mud.