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16th March 2022
...continued from Part 2

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.

Enter Sarah Morley, a stream ecologist, and Linda Rhodes, a microbiologist, both with the National Oceanic and Atmospheric Administration. They harvested wild microbes and invertebrates in small baskets placed in the healthier Cedar River watershed nearby. They took a few baskets back to the laboratory to document captured species, and they buried the others in Thornton Creek’s restored hyporheic zones.

Invertebrates and microbes quickly colonized the areas. But even though the number of individuals was high, the biodiversity was relatively low. According to the duo’s 2021 paper in Water, a few of the new species proliferated, but most of the other species were similar to those in unrestored sections of the creek.

Morley and Rhodes are considering why more of their introduced species did not make it. Because this science is so new, they have not ruled out any potential explanations. The donor stream may be too different, or the restored area too small, or water quality too poor. They might have inoculated the hyporheic too soon, before small vegetation needed by some critters could grow. And yet in the guts of some trout, Lynch found aquatic insects that had not been seen in Thornton Creek for at least 20 years. “The fish are better at sampling than we are,” she says. The scientists are now conducting another study with more sensitive monitoring.

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 final question about the Thornton Creek restoration was whether it was cleaning pollution that pours in with runoff during storms, from lawn fertilizer to urban wastes. Lynch had to search for three years to find a chemist who would conduct the research. “All of them said it could not be done,” she recalls. They said it was too difficult to track how long water stayed in the hyporheic zone and to measure whether chemicals were removed while the water spent time underground.

Lynch eventually reached Skuyler Herzog, then an engineer at the Colorado School of Mines, who specializes in the hyporheic zone. “He took the next plane out here,” Lynch recounts with glee. After years of studying the hyporheic zone academically, he was thrilled to conduct tests on a real restoration. Lynch recruited University of Washington chemist Edward Kolodziej to help.

The team sent tracer dyes into an engineered plunge pool that pushed water into the hyporheic. They then monitored exit points seven and 15 feet downstream to determine how long a “packet” of water stayed under before rejoining surface flow; water stayed under for 30 minutes to three hours or more. They also collected water samples from the stream and used mass spectrometry to measure different pollutants from storm runoff. They counted nearly 1,900.

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. Because water spent so little time in those short hyporheic stretches, the team thinks the pollutants mostly got stuck on sediments or biofilms rather than being broken down immediately by microbes, although that decomposition is common over longer time periods.

Hrachovec says it is “jaw-dropping” that such short hyporheic spans were able to reduce so much pollution. He adds it was “astounding to contemplate how much good we could do if we had this more available.”


The Thornton Creek findings are encouraging. The neighborhoods around the creek have not flooded since the restorations were finished in 2015, even during large storms. The stream’s temperature and flow are more consistent year-round. The city needs to dredge less often, saving money, and neighbors love spending time in the expanded green space. Yet the work also reveals how complex nature’s systems are and how difficult it can be to restore them once damaged. As cities and agencies increasingly turn to more nature-based solutions, the Thornton Creek lessons can help experts understand which steps work and which need improvement.

Success has helped Lynch convince Seattle Public Utilities and other city decision-makers of the importance of a stream’s gut. Hyporheic restoration has become a formal part of the utilities’ creek projects—not guaranteed but routinely considered. Taylor Creek now has eight planned hyporheic reconstructions along a 1,200-foot stretch. Herzog is testing design improvements to increase water’s “residence time” in the hyporheic and is studying how much that increases cleansing. Plans to restore the north branch of Thornton Creek include hyporheic structures. Because of the zone’s power to reduce pollution, the city will probably include hyporheic structures in a restoration along Longfellow Creek, which contains a chemical from vehicle tire particles that Kolodziej has shown kills salmon; construction could begin by 2026.

Still, small restorations cannot fully compensate for insults to long streams and rivers. “Stormwater runoff, biodiversity, flooding—these are watershed-scale problems,” Bakke says. That is why reconstructions need to be distributed in many places along a stream or river. Abbe, the geomorphologist who inspired Lynch, is now at Natural Systems Design. He has planned and overseen 14 hyporheic restorations in five other Washington State counties. In 2019 Abbe was walking along a project on Poison Creek in Chelan County with Steve Kolk, an engineer with the U.S. Bureau of Reclamation, an agency infamous in ecology circles for building giant dams. As Abbe tells it, Kolk suddenly stopped walking and said, “Ultimately you’re talking about hundreds of thousands of these treatments to restore our watershed.” Abbe said, “Bingo.”

Finding space for more natural water flows in an established city might seem difficult, but buildings are replaced more often than people think, particularly when they flood regularly. Cities can reclaim that land, as Seattle did. Even small projects in key places can make a difference. By restoring the floodplains at Confluence and Kingfisher, Seattle has relieved troublesome flooding along Thornton Creek.

Most exciting for Lynch, the hyporheic innovations won the ultimate stamp of approval in the fall of 2018, when Chinook salmon swam in from Puget Sound and spawned in the creek’s restored hyporheic zones.

“That was just really emotional,” Lynch recalls. “We had done it. You can restore the hyporheic zone. You can restore natural processes to the extent that we are actually attracting salmon to the site to spawn.” If these two small restorations in an urban creek can help restart a functioning ecosystem, she says, “I think there really is hope for the future.”