Radical reconstruction in Seattle is bringing nearly dead urban streams back to productive life
Salmon are so elemental to Indigenous peoples who live along North America’s northwestern coast that for generations several nations have called themselves the “Salmon People.” But when settlers came, their forms of agricultural and urban development devastated the mighty fish. The new inhabitants cut down streamside vegetation that once slowed and absorbed rains, causing floods.
For a large river the hyporheic zone can be dozens of feet deep and can extend up to a mile laterally beyond the banks. It keeps the waterway healthy by regulating critical physical, biological and chemical processes, including riverbed aeration, water oxygenation, temperature moderation, pollution cleanup and food creation. Some biologists compare the hyporheic zone to the human gut, complete with a microbiome. Others call it the liver of the river.
As far as Lynch knew, no one had tried to rebuild a missing hyporheic zone in an urban stream. She hoped that restoring the stream’s gut would help Thornton Creek better maintain itself, reducing the need for ongoing, expensive human assistance. She also argued that if the revolutionary approach succeeded, it would set a new standard for urban stream restoration at a time when flooding around the world was routinely costing human lives and billions of dollars in damages.
Scientists have shown how wide and deep a hyporheic zone can reach by mapping aquatic insects and fish embryos found in soil beyond a waterway’s banks. For an urban creek such as Thornton, that lateral reach might extend 30 feet from the stream channel. The depth might be three feet below the streambed.
Hyporheic Rebuild Lynch first learned about the hyporheic zone in 2000 at the University of Washington, but she did not appreciate how extensive the zone is until a 2004 field trip into a forest with geomorphologist and visiting lecturer Tim Abbe. She was amazed when they stopped walking and he pointed out that the ground they were on overlaid a hyporheic zone for a nearby stream.
The restoration was personal for Bakke, who had grown up in the 1960s and 1970s along Thornton Creek, fishing for cutthroat trout and playing with water skeeters. Just before he entered high school, the city issued permits for condos along the creek’s edge, cutting off his access to the water. “These old haunts that I really loved, that were my sort of wilderness ... were suddenly not just blocked but being paved over,” he says. “It was very upsetting.
She also reminded the manager that the monitoring would provide lessons on how to reconstruct urban streams to something closer to their full complexity, making Seattle a leader in this work worldwide. In the end, they negotiated. Lynch was able to keep the Confluence hyporheic restoration intact; the Kingfisher reach was shortened by 25 percent.
If a species is missing from an ecosystem, our instinct is simply to reintroduce it. But ecologists are painfully aware of cautionary tales such as stocking a desirable trout that inadvertently brings pathogens along with it. Even bringing back a native plant can shake up a system that has adjusted to its absence.
With that insight, she got permission from King County to seed four creeks with caddisflies, mayflies, stoneflies and other species. Some of them survived. In 2019 the Thornton Creek team tried another groundbreaking move: inoculating the engineered hyporheic zone with life. In keeping with the human gut analogy, the procedure is somewhat like administering probiotics, or even a fecal transplant, to a person to restore their gut microbiome.
Still, Morley and Rhodes did find that the microbes that began living in the restored stream sections were much more active than those in nearby unrestored sections, indicating they were “getting goosed to do something,” Rhodes says—maybe build biofilms and biomass, clean pollutants or break down organic material. The restored sections had seven times more hyporheic crustaceans, worms and insects, as well as much greater overall species diversity.
The scientists sampled water packets before they entered the stretches of hyporheic and after they emerged and compared them with water flowing downstream above the stretches. The surface flow reduced the concentration of about 17 percent of the chemicals by at least half. The seven-foot stretch of the hyporheic reduced the concentration of 59 percent of the chemicals by at least half, and the 15-foot stretch reduced the concentration of 78 percent of the chemicals by at least half.
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