From the beginning, the U.S. Army Corps of Engineers' dramatic makeover of the river seemed the obvious culprit for the species' decline. Pallid sturgeon evolved in warm, dynamic rivers that were free to jump their banks and ramble about the floodplain, muddying themselves with the dirt and clay churned up in the roil. Flows swelled every spring with snowmelt, peaked again with early summer rains, then dropped off in early fall. This ebb and flow added organic matter and nutrients to the river and redistributed sediment. For native fish, these changes -- and their associated shifts in temperature and turbidity -- triggered spawning migrations, and enhanced spawning conditions and nursery habitat.

Today's "Big Muddy" -- a nickname the Missouri shares with its cousin, the Mississippi -- is not nearly as big or muddy as it once was. At the start of the 20th century, the Corps began wrestling the river into submission, hoping to create jobs, minimize flood risk, and ensure a reliable thoroughfare for the barge industry. Six dam and reservoir projects went up in the Upper Basin, and the river's southern stretches were straightened and stabilized.

The dam system, which provides water for cities, irrigation, hydropower and recreation, was one of the 20th century's great engineering achievements. But it wreaked ecological havoc, severely altering the river's natural temperature and hydrologic patterns, which play key roles in the movement, growth and reproduction of native fish. It's halted most annual floods, preventing the river from connecting to low-lying lands and reducing key nutrients, forage and habitat for young fish.

Though hard data are lacking (pallids weren't recognized as a separate species from the smaller, darker shovelnose sturgeon until 1905), records suggest pallids may never have been common. Still, according to Braaten's calculations, roughly 1,000 cruised the river between Fort Peck Dam, in eastern Montana, and North Dakota's Garrison Dam when the latter was finished in the early '50s. Fewer than 150 wild fish persist here today.

The U.S. Fish and Wildlife Service listed pallid sturgeon as endangered in 1990 in response to declines throughout its range, which runs from Montana to the Louisiana Gulf -- some 3,000 total river miles. It's considered an indicator species for the Missouri; if it's in trouble, other species probably are, too. Indeed, two native birds -- the piping plover and least tern -- are listed under the Endangered Species Act, due in part to the loss of winter and midstream nesting habitat. And roughly half of the river's 106 native fish species are uncommon, rare or decreasing across all or part of their ranges.

In today's Missouri, the pallid sturgeon's biological quirks aren't doing it any favors, either. Pallids are poky and particular about life and love. They grow slowly, live for decades, don't reach sexual maturity until they are at least 10 years old, and spawn only every few years. And if the spawning set-up isn't perfect -- if the river is too cold, for instance -- the females re-absorb their eggs and wait for conditions to improve.

In the upper river, the dam-locked run where we found Code 117 has been a top priority for recovery teams because it hosts the largest remaining cadre of "heritage" fish, the biggest (50-70 pound), oldest (40-80 year-old) wild fish that hold the genetic key to their species' long-term survival. Scientists believe the Upper Missouri adults are genetically distinct from their Lower Missouri and Mississippi River counterparts, where hybridization with shovelnose sturgeon is thought to be more common.

The first 10 years of pallid recovery here focused on preserving these genes and stocking the river with hatchery fish. "(Stocking) has certainly helped boost population numbers," says George Jordan, who coordinates recovery for the Fish and Wildlife Service. "(But) if we stopped stocking now, pallids won't be any better off." For pallids to make it, they must spawn and their babies must survive.