Elsewhere, he consulted on projects such as the Rogue River Valley Irrigation Canal, water development for Bellingham, Washington, and power plants in Southeastern Alaska.
The day of his arrival, he met pioneer David Denny at a memorial service for the recently assassinated U.S. president, James Garfield.
Whitworth, Thomson was involved in the initial surveying and dredging of what would, years later, become the Montlake Cut of the Lake Washington Ship Canal.
In 1884 he became the city surveyor, in which capacity he oversaw the building of Seattle's first sewers and the Grant Street bridge across the Duwamish River tideflats.
[2] During the period when Thomson was city surveyor, he and Whitworth also offered their services in the private sector, maintaining an office on Main Street in what is now the Pioneer Square district.
Before returning to become a consulting engineer in Seattle, he spent some time in Spokane, near the state's eastern border, where he was responsible for several railway terminals and two bridges.
He began the process of paving roads, building sidewalks, and adding sewer lines (often through areas that earlier engineers could not work out how to plumb).
[2] From the time of his arrival in Seattle, Thomson had considered the hilly landscape and the extensive mudflats as obstacles to the city's growth.
He consulted on Oregon's Rogue River Valley Irrigation Canal; built hydroelectric plants in Eugene, Oregon and surveyed plant sites in Southeastern Alaska; planned the water supply of Bellingham, Washington and consulted on the system for Wenatchee; briefly, in his seventies, he returned, temporarily, as Seattle city engineer in 1930 to finish the Diablo Dam on the Skagit River[2] after the death of city engineer William D. Barkhuff;[17] consulted to the Inter-County River Improvement Commission for King and Pierce Counties (the counties containing Seattle and Tacoma, respectively), and consulted on the construction of the Lake Washington Floating Bridge (now Lacey V. Murrow Memorial Bridge, carrying Interstate 90 across Lake Washington) and for the foundations of the Tacoma Narrows Bridge.
[2][18] In a sense, Thomson's chief legacy is the physical contours of the city of Seattle as it exists today, including the lay of the land, the transportation system, and the municipal utilities.
Owners who didn't willingly sell were left with their buildings standing uselessly on pinnacles, the remaining land removed around them.
[20] As late as the 1970s, the neighborhood was merely "serviceable but seedy", then increasingly a center of Seattle's bohemian life, while also seeing a growth in condominium and office development.