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18th March 2022
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. They straightened curvy creeks to try to speed floodwater off the land and armored the sides to prevent erosion, but the faster flow gouged the riverbed. Later, urban planners and engineers funneled streams into buried pipes so they could build more city on top, disconnecting waterways from soil, plants and animals. The cumulative impact of these injuries led to flash floods, unstable banks, heavy pollution and waning life. The hallowed salmon all but disappeared.

Across North America and the world, cities have bulldozed their waterways into submission. Seattle was as guilty as any until 1999, when the U.S. Department of the Interior listed Chinook salmon as threatened under the Endangered Species Act. That legally obligated the city to help the salmon when undertaking any new capital project that would affect the fish. Engineers trying to improve Seattle’s ailing streams began to reintroduce some curves, and insert boulders and tree trunks, to create more natural habitat, yet by and large, salmon did not return. Flooding also remained a hazard because rain rushed off the hardened cityscape into the still mostly inflexible channels, which overflowed.

In 2004 biologist Katherine Lynch was sitting through yet another meeting on how to solve these problems—this one held by her employer, Seattle Public Utilities—when she had an epiphany. Maybe restoration projects were failing because they were overlooking a little-known feature damaged by urbanization: the stream’s “gut.”

A stream is a system. It includes not just the water coursing between the banks but the earth, life and water around and under it. Lynch had been tracking discoveries about a layer of wet sediment, small stones and tiny creatures just below the streambed called the hyporheic zone—a term from the Greek hypo, meaning “under,” and rheos, meaning “flow.” Stream water filters down into this dynamic layer, mixing with the groundwater pushing up. Water in the hyporheic zone flows downstream like the surface water above it but orders of magnitude more slowly.

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.

A healthy hyporheic zone is full of life. Crustaceans, worms and aquatic insects constantly move between the zone and surface flow. Nematodes, copepods, rotifers and tardigrades also dig up and down, creating spaces for water to mix underground. Microbes proliferate throughout the zone. Water welling up from below brings oxygen to salmon eggs laid in the riverbed. Lynch realized that few people trying to restore Seattle’s streams were thinking about the hyporheic zone, or that the channelizing of streams scours it away, or that putting streams in pipes disconnects the zone from the stream water above.

The meeting concerned Seattle’s Thornton Creek, which originally wove through rich lowland rain forest, draining an 11.6-square-mile watershed before emptying into Lake Washington. Developers had straightened it and armored it with rocks or concrete, squeezing it into channels only a few feet wide in some places. Its 15-mile course ran along a highway for a while and carved through hundreds of backyards. Some houses were so close to the narrowed stream that their decks overhung the water. Thornton had a reputation as the most degraded creek in the city—and as a dangerous one: it flooded a major road nearly every year, blocking access to schools, a community center, hospitals, businesses and bus routes. At times homes and a high school flanking the creek also flooded.

Talk at the meeting centered on the best practices of the time: reconnecting the stream with some of its floodplains by reclaiming adjacent property, removing armoring and reintroducing native plants along the banks. Lynch boldly told the group the project should go further: rebuild the missing hyporheic zone. That would mean reclaiming space under the stream, filling it with sand and gravel and potentially bringing back the zone’s tiny inhabitants.

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. Cities everywhere had confined and subsumed many thousands of streams, erasing them from public memory. One study found that Philadelphia had buried 73 percent of its streams. Another study counted 66 percent buried in Baltimore. Globally many streams that remained on the surface were sick or dying. Restoring Thornton Creek’s hyporheic zone could create a blueprint for enhancing biodiversity while also reducing urban flooding and drought.

continued in Part 2