Seattle’s Terraformed Landscape#

Seattle has been reshaped more dramatically than almost any American city. Between the 1850s and 1930s, engineers flattened hills, filled tideflats, raised streets, channelized rivers, lowered lakes, created islands, and buried creeks. More than 50 million short tons of earth were moved across roughly 60 separate regrading projects, fundamentally altering the city’s topography to serve urban development. (Regrading in Seattle — Wikipedia)

This guide covers the types of terrain modification, the history behind each major project, environmental consequences, and how the legacy of these changes shapes Seattle today.

Types of terrain modification#

Seattle’s landscape engineering falls into six categories, often interconnected — dirt cut from hills filled tideflats, which became the foundation for new industrial land.

Hill regrades#

Using hydraulic hoses and steam shovels, workers cut down hills to reduce street grades and open land for development. The largest projects removed 85 to 100+ feet of elevation. (HistoryLink: Denny Regrade)

Tideland and waterfront filling#

Dirt from regrades and river dredging was deposited in Elliott Bay and the Duwamish tideflats, creating new land for railroads, wharves, and industry. Today’s Western Avenue, Alaskan Way, SoDo, and Harbor Island all sit on fill. (HistoryLink: Alaskan Way Seawall)

Street raising#

After the 1889 Great Fire, Pioneer Square’s streets were raised 12 to 30 feet on retaining walls to prevent tidal flooding and improve sewer drainage, burying the original ground-floor storefronts and creating the Seattle Underground. (Seattle City Archives: Great Seattle Fire of 1889)

Waterway engineering#

The Lake Washington Ship Canal (completed 1917) connected Puget Sound to Lake Washington via two cuts and the Ballard Locks, while the Duwamish River was straightened from a nine-mile meandering estuary into a five-mile industrial shipping channel. (HistoryLink: Lake Washington Ship Canal)

Lake lowering#

Opening the Ship Canal in 1916 lowered Lake Washington by 8.8 feet to the level of Lake Union, eliminating the Black River, reshaping the regional watershed, and exposing new shoreline land around the entire lake. (HistoryLink: Lake Washington Lowered 9 Feet)

Creek burial#

As the city urbanized, creeks were piped into culverts and integrated into the sewer system. Today, 21 of Seattle’s 65 miles of creek run through underground pipes. (Seattle Public Utilities: Creek Culverts)

History#

1853: First landfill#

Henry Yesler built Seattle’s first sawmill on marshy land at roughly today’s First Avenue and Yesler Way. The western portion of the mill sat on pilings extending into Elliott Bay, and sawmill waste became the city’s first landfill — the beginning of a pattern that would extend the shoreline westward for decades. (HistoryLink: Alaskan Way Seawall)

1889: The Great Fire and street raising#

On June 6, 1889, an overturned glue pot in a carpentry shop started a fire that destroyed 25 blocks of Seattle’s central business district. The city rebuilt in brick and stone, and raised streets in Pioneer Square by 12 to 30 feet on concrete retaining walls to solve chronic tidal flooding and sewer backup problems. Buildings were rebuilt at the original ground level, and when the raised streets were completed, what had been first-floor storefronts became basements. The resulting underground passages, sealed in 1907, became the Seattle Underground tourist attraction that operates today. (Seattle City Archives: Great Seattle Fire of 1889)

1892: R.H. Thomson becomes city engineer#

Reginald Heber Thomson served as Seattle’s city engineer from 1892 to 1911 (and returned for two later terms). He drove the vision to flatten Seattle’s hills, fill its tideflats, straighten the Duwamish River, and build water infrastructure from the Cedar River watershed. As he wrote in his autobiography, with such a hilly landscape he wondered “how will the people in one end of the city be able to do business with those in the other end?” Thomson is credited with doing more than any other individual to change the physical face of Seattle. (HistoryLink: Denny Regrade)

1897-1930: The Denny Regrades#

The removal of Denny Hill was Seattle’s most dramatic regrading project, carried out in five phases over 33 years. The hill, located between downtown and Queen Anne Hill, originally topped out at about 220 feet in elevation and covered 62 city blocks.

• First Regrade (1897-1899): Workers used hydraulic hoses to wash away 1st Avenue between Pike Street and Denny Way, sluicing 110,700 cubic yards of sediment into Elliott Bay.
• Second Regrade (1903-1906): Extended the cuts along 2nd Avenue, supplementing hoses with steam shovels.
• Major Regrade No. 1 (1908-1911): Removed the half of the hill closest to the waterfront — 27 city blocks from Pine to Cedar Streets and 2nd to 5th Avenues. Workers pumped 20 million gallons of water per day from Lake Union through hoses aimed at the hillside, running the slurry through tunnels to Elliott Bay.
• Final Regrade (1928-1930): Removed what remained of the hill east to Westlake Avenue, completing the transformation.

By the end, the hill’s high point had been lowered by more than 100 feet, displacing over 400 homes and businesses. During the project, isolated remnants called “spite mounds” became iconic images — tall buttes of the original hill left standing because owners couldn’t pay for removal or were absent. (HistoryLink: Denny Regrade | Historic Seattle: The Regrade Disasters)

1900-1909: Tideland filling and Harbor Island#

In 1900, the Seattle General Construction Co. obtained a permit to fill the tideflats at the mouth of the Duwamish River. The Puget Sound Bridge and Dredging Co. dumped 24 million cubic yards of soil — from hill regrades and river dredging — to create Harbor Island. When completed in 1909, it was the largest artificial island in the world at approximately 350 acres. The island became Seattle’s primary industrial port, home to shipyards, grain terminals, and later containerized cargo facilities. (HistoryLink: Harbor Island Completed in 1909)

More than 2,000 acres of tideflats south of downtown were converted to solid land during this era, creating the area now occupied by King Street Station, Union Station, Seattle’s sports stadiums, SoDo, and the city’s industrial base. (HistoryLink: Alaskan Way Seawall)

1907-1912: Jackson and Dearborn Street regrades#

The Jackson Street Regrade (1907-1910) removed a ridge connecting First Hill to Beacon Hill that blocked expansion toward the Rainier Valley. The cut reached 85 feet deep at 9th Avenue and Jackson Street, reducing grades from over 15% to around 5%. Workers used hydraulic hoses fed by the Beacon Hill Reservoir and Lake Washington, washing 3.35 million cubic yards of earth into the tideflats below Beacon Hill. The project required demolishing the public South School and the original Holy Names Academy. (HistoryLink: Jackson Street Regrade)

The Dearborn Street Regrade (1909-1912) followed immediately to the south, cutting even deeper — 112 feet at 12th Avenue South and Dearborn Street — and reducing grades from 19% to 3%. The excavated material raised Rainier Avenue South by as much as 14 feet. Together, the Jackson and Dearborn regrades covered 125 acres across 56 city blocks. (HistoryLink: Dearborn Street Regrade)

1909-1917: Lake Washington Ship Canal#

The idea of connecting Puget Sound to Lake Washington via Lake Union was first proposed by Seattle pioneer Thomas Mercer in 1854. Construction began in 1909 under the direction of Army Corps of Engineers officer Hiram M. Chittenden, who designed two cuts (Fremont Cut and Montlake Cut) and a set of locks at the west end of Salmon Bay.

On August 28, 1916, sluice gates opened to begin lowering Lake Washington by 8.8 feet to the level of Lake Union. By late October 1916, the three bodies of water were equalized and boats could travel from Puget Sound to Lake Washington. The official opening was July 4, 1917.

The lowering eliminated the Black River — formerly the lake’s outlet — and reshaped the watershed of the Green-Duwamish River system. It also exposed new shoreline land around the entire lake, creating beaches and waterfront property that had previously been underwater. (HistoryLink: Lake Washington Ship Canal | HistoryLink: Lake Washington Lowered 9 Feet)

1913-1940s: Duwamish River channelization#

In 1909, at the urging of R.H. Thomson, Seattle formed the Duwamish Waterway Commission to straighten and deepen the river for industrial shipping. Work began on October 14, 1913. Workers moved 20 million cubic yards of mud and sand to fill the river’s natural meanders and deepen the main channel.

By the 1940s, the channelization had transformed a nine-mile meandering estuary into a five-mile industrial waterway dredged to 50 feet deep. The project destroyed more than 97% of the river’s wildlife habitat and displaced Native communities — including the Duwamish, Muckleshoot, and Suquamish tribes — who had relied on the river for generations. Boeing established its first plant on the Lower Duwamish in 1916. (HistoryLink: Straightening of Duwamish River | WA Ecology: Lower Duwamish Waterway Site History)

1916-1934: Seawall construction#

The original Alaskan Way Seawall was built between 1916 and 1934 using 20,000 old-growth timber piles. The wall held back the sea from the approximately 250,000 cubic yards of fill placed behind it to create level ground for streets, railroad tracks, and freight operations along the central waterfront. (HistoryLink: Alaskan Way Seawall)

20th century: Creek burial#

As Seattle urbanized, natural waterways were piped underground. Ravenna Creek, which once drained Green Lake into Lake Washington’s Union Bay, was cut off in 1911 when Green Lake was lowered seven feet, then piped into the sewer system in the 1950s and 1960s. Similar fates befell streams across the city. Five main salmon-bearing creeks survive today (Pipers, Thornton, Taylor, Longfellow, and Fauntleroy), but 21 miles of Seattle’s 65 total creek miles remain in pipes. (Seattle Public Utilities: Creek Culverts)

Environmental consequences#

The century of terraforming had lasting environmental costs that Seattle is still addressing:

• Habitat destruction: More than 97% of the Duwamish River’s original wildlife habitat was destroyed by channelization. Filling the tideflats obliterated shellfish beds and acres of marine and shoreline habitat on Elliott Bay. (WA Ecology: Lower Duwamish Waterway Site History)
• Industrial contamination: The channelized Duwamish became a dumping ground for industrial waste. In 2001, the EPA designated the lower five miles of the Duwamish Waterway a Superfund site after finding at least 41 hazardous chemicals including PCBs, dioxins, and arsenic in the riverbed sediments. (WA Ecology: Lower Duwamish Waterway Site History)
• Displacement of Indigenous communities: The Duwamish people, who had lived on the tideflats at the mouth of the Duwamish River for centuries, were displaced by the regrades and channelization. The significance of the river to nearby Native communities did not factor into the planning. (HistoryLink: Straightening of Duwamish River)
• Seismic vulnerability: Land built on fill — including Pioneer Square, SoDo, Interbay, and Harbor Island — is prone to liquefaction during earthquakes. During the 2001 Nisqually earthquake, SoDo experienced pervasive liquefaction including sand boils and ground cracking.
• Watershed disruption: Lowering Lake Washington eliminated the Black River and reshaped the Green-Duwamish watershed. Burying creeks added strain to the sewer system and eliminated fish habitat.

Impact on urbanism and land use#

Seattle’s terraforming didn’t just change the terrain — it determined the city’s land use patterns, neighborhood boundaries, transportation corridors, and equity challenges for the next century.

Downtown expansion and Belltown#

The Denny Regrades created roughly 200 acres of flat, buildable land across 65 blocks north of the original business district. Thomson and boosters expected this land to become a high-value commercial extension of downtown, but for decades the cheap, flat lots attracted marginal uses: flophouses, car dealerships, warehouses, and parking lots. The area — known as the Denny Regrade or Belltown — remained relatively stagnant until the late 20th century, when proximity to downtown made it attractive for apartment towers and mixed-use development. Amazon’s decision to build its headquarters campus on former Regrade land in South Lake Union transformed the area into one of Seattle’s densest employment centers. (HistoryLink: Belltown-Denny Regrade | Historic Seattle: The Regrade Disasters)

Industrial land base#

Filling the Duwamish tideflats and building Harbor Island created Seattle’s industrial core. The approximately 2,000 acres of manufactured land south of downtown were zoned industrial and became home to the Port of Seattle, Boeing’s first plant (1916), rail yards, and shipping terminals. Today, Seattle’s industrial lands account for about 12% of the city’s total land area and support approximately 100,000 jobs. The ongoing debate over rezoning SoDo’s stadium district for housing — with the Port of Seattle filing a legal challenge in 2025 — is a direct descendant of decisions made when this land was created from tideflats a century ago. (Seattle OPCD: Industrial Lands | Cascade PBS: Maritime vs. Real Estate)

Transportation corridors#

The Jackson and Dearborn Street regrades cut through the ridge between First Hill and Beacon Hill, opening the east-west corridors that connect downtown to the Rainier Valley. These corridors became the routes for Jackson Street (a major transit arterial), Interstate 90, and the future Judkins Park light rail station. Without the regrades, the steep grades would have limited connectivity between downtown and southeast Seattle. The Ship Canal similarly defined the city’s major north-south divide, with the Fremont, Ballard, University, and Montlake bridges becoming chokepoints that shape commute patterns to this day. (HistoryLink: Jackson Street Regrade)

Neighborhood boundaries and identity#

Many of Seattle’s neighborhood boundaries trace directly to terraforming decisions. Pioneer Square’s character comes from the post-fire street raising. Belltown’s flat grid reflects the Denny Regrade. SoDo and Georgetown sit on filled tideflats. The Ship Canal separates “north Seattle” from “south Seattle” along a line that did not exist before 1917. Fremont, Wallingford, and the University District developed as distinct neighborhoods partly because the canal and its bridges created natural separations between communities.

Equity and environmental justice#

The communities most affected by terraforming bear lasting consequences. The Duwamish people were displaced from their ancestral tideflats. The industrial land created by filling those tideflats was zoned to attract polluting industries, and the neighborhoods that grew up alongside them — Georgetown, South Park — are now among Seattle’s most environmentally burdened communities with life expectancy 13 years shorter than wealthier neighborhoods. The land-use patterns established by a century of terraforming — where industrial zones sit, which neighborhoods are on fill, which communities were displaced — remain central to Seattle’s environmental justice work today. (HistoryLink: Straightening of Duwamish River)

Modern legacy and ongoing projects#

Seawall replacement (2013-2017)#

The original timber-pile seawall had deteriorated after 75+ years, and the 2001 Nisqually earthquake revealed serious structural vulnerabilities. Between 2013 and 2017, Seattle replaced the seawall between South Washington Street and Virginia Street with a modern structure designed to meet current seismic standards and last over 75 years. The new seawall incorporates habitat features: grooves and nooks that promote algae growth, shallow rock beds for juvenile salmon to hide and forage, and a light-penetrating sidewalk surface to support marine plant growth. University of Washington researchers documented 10,000 juvenile salmon in a single day along the new seawall in May 2018. (Seattle.gov: Seawall Project)

Creek daylighting#

Starting in the 1990s, community groups began advocating to bring buried creeks back to the surface. The Ravenna Creek Daylighting Project (completed 2006) restored approximately 800 feet of Ravenna Creek to an above-ground channel after 15 years of lobbying. Seattle Public Utilities is developing a citywide creek culvert strategy, with a Thornton Creek Culvert Strategy planned for 2025-2026 to prioritize investments and property acquisition. (Seattle Public Utilities: Creek Culverts)

Duwamish Superfund cleanup#

The EPA-led cleanup of the Lower Duwamish Waterway Superfund site covers approximately 450 acres of contaminated riverbed. Washington’s Department of Ecology manages source control across the broader 20,000-acre drainage basin. Responsible parties include Boeing, the Port of Seattle, and King County. The multi-decade cleanup is ongoing. (WA Ecology: Lower Duwamish Waterway Site History)

Waterfront redevelopment#

Following the 2019 demolition of the Alaskan Way Viaduct and the seawall replacement, Seattle is rebuilding the central waterfront with parks, a protected bike lane, and public spaces. In August 2023, the city and Port of Seattle announced Elliott Bay Connections, a $45 million public-private partnership to build a pedestrian and bicycle greenway from the new Waterfront Park to the Olympic Sculpture Park and restore Myrtle Edwards and Centennial Parks. The project is funded by donations from Melinda French Gates, MacKenzie Scott, the Diller-Von Furstenberg Family Foundation, and the Expedia Group, with a target completion of June 2026. (Seattle Mayor’s Office: Elliott Bay Connections | The Urbanist: Waterfront Park Makeover)

Data sources#

Seattle City Archives#

Photographs, maps, and documents from Seattle’s regrading and infrastructure projects, including the Great Fire, regrades, and Ship Canal construction.

Access: Seattle City Archives

HistoryLink.org#

Free online encyclopedia of Washington state history with detailed essays on every major terraforming project, authored by historians including David B. Williams.

Access: HistoryLink.org

Seattle GeoData (GIS)#

Spatial data layers including liquefaction zones, fill areas, creek locations, and historical shoreline positions.

Access: Seattle GeoData

DNR Geologic Hazard Maps#

Liquefaction susceptibility maps and active fault maps showing which parts of Seattle are built on fill.

Access: DNR Geologic Hazard Maps

Key statistics#

Sources: HistoryLink: Denny Regrade | HistoryLink: Harbor Island | HistoryLink: Lake Washington Lowered | Seattle Public Utilities: Creek Culverts

Related resources#

• Earthquake Preparedness & Seismic Risk — Liquefaction zones and seismic hazards on filled land
• Environmental Justice & Equitable Development — Duwamish Valley impacts and displacement history
• Waterfront & Pike Place Market — Viaduct removal, seawall replacement, and waterfront parks
• Historic Preservation & Landmarks — Pioneer Square Preservation District and the Seattle Underground
• Land Use & Planning Glossary — Liquefaction, SEPA, and other land use terms

Last updated: February 2026