According to the passage, why were the Elwha and Glines Canyon dams originally built without fish passage structures?

For nearly a century, the Elwha River ran through two dams that had never been built with any way for fish to get past them. The Elwha Dam, finished in 1912, and the Glines Canyon Dam, finished fifteen years later, were built to generate electricity for a growing timber town on Washington's Olympic Peninsula, and neither one included a fish ladder or any other passage structure. The consequence was immediate and lasting: salmon and steelhead trout that had returned to the river's upper reaches for thousands of years lost access to roughly ninety percent of the watershed in a single generation. Ten distinct native runs of salmon and sea-going trout, each adapted to its own stretch of river, went into steep decline.

The dams did more than block fish. They also blocked everything the river would otherwise have carried downstream: gravel, sand, and silt that had been accumulating behind both structures for decades, along with fallen logs and branches that a free-flowing river uses to build the varied pools, riffles, and side channels that young fish need. A river without sediment and wood is not simply an obstacle course with two missing dams removed; it is a river missing the raw material of its own habitat.

When Congress authorized removing both dams, the decision was not treated as a simple act of demolition. Engineers and biologists understood that breaching the dams all at once would release decades of trapped sediment in an uncontrolled surge, burying downstream habitat in a way that could do more immediate harm than the dams themselves had done for a century. The removal instead proceeded gradually, over roughly three years, allowing the river to redistribute its long-withheld sediment load in stages rather than all at once.

Even a carefully staged removal produced real short-term costs. As each stage released more trapped material, suspended sediment levels downstream rose sharply, and monitoring documented a corresponding drop in the density and diversity of the small invertebrates that fish depend on for food. For a river ecosystem already stressed by a century of blocked habitat, this was a genuine setback layered on top of an existing one, not merely an inconvenience.

The recovery that followed did not erase that cost, but it did outpace it. As turbidity settled back toward natural background levels, invertebrate communities rebounded, and the sediment that had caused the short-term disruption began doing the constructive work it had been blocked from doing for a century: rebuilding gravel bars where salmon spawn and replenishing habitat downstream. Within two years of the river's full reconnection, biologists documented summer steelhead upstream of where the former reservoirs had stood, a stretch of river those fish had not been able to reach in living memory.

The Elwha's restoration is now frequently cited by other watershed projects as evidence that a river's capacity to heal itself, once the structural obstacle is removed, can be faster and more complete than the removal process itself might suggest while it is underway.