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Chinook salmon and their habitats in Seattle's Thornton Creek suffered severely due to urban expansion, causing flash floods and biodiversity loss. Despite restoration efforts, the once-abundant salmon all but disappeared. During a conference, biologist Katherine Lynch proposed rebuilding the creek's missing " liver" damaged by urbanization.

Lynch has been studying the hyporheic zone, a layer of wet sediment ( 沉积物) , small stones and tiny creatures beneath the streambed. It facilitates water mixing and oxygen delivery to salmon eggs, earning the nickname " liver of the river. " The disappearance of this zone threatens the health of waterways. Lynch recognized that, however, most restoration efforts in Seattle overlooked it or disconnected it from the surface water.

Teamed with engineer Mike Hrachovec, Lynch redesigned Thornton Creek. They strategically put logs into the water at precise angles to create tiny waterfalls and nearly still water pockets, generating hydraulic pressure to force water down into the hyporheic zone. These accurately placed logs and rocks, known as " hyporheic structures," also create pockets of slow water that provide safe shelters for juvenile fish-all meant to emulate features of a natural stream. Subsequent data analysis confirmed the stream functioned as Lynch's team-and nature-intended.

But was the stream also supporting life? Given that the stone and sand positioned were sterile ( 贫瘠的) territory, Lynch thought that a biological jumpstart might be necessary and that the return of life to restored creeks relied on organisms migrating from healthy upstream habitats. So her team tried another ground-breaking move: inoculating ( 接种) the engineered hyporheic zone with microbes ( 微生物) , which quickly populated the areas. But even though the number of individuals was high, the biodiversity was relatively low. A 2021 study by stream ecologist Sarah Morley noted that while a few of the new species grew rapidly, most were similar to those in unrestored section. Scientists are exploring reasons for the limited survival of introduced species, and because this science is so new, they have not ruled out any potential explanations: differences in the donor stream, size of the restored area, or poor water quality. They might have inoculated the hyporheic too soon, before essential vegetation could establish.

The Thornton Creek restorations have successfully prevented neighbourhood flooding, even during heavy storms, and stabilized the stream's flow. Most notably, Chinook salmon returned to lay eggs in the restored hyporheic zones, touching Lynch deeply. She recalls, " this success suggests that small urban creek restorations can revitalize functioning ecosystems. "