In the 1860s, new private and public ventures began reshaping the Montlake Portage. They removed land in some places and built it up in others to make it more useful. These ambitious projects changed how people used the landscape and made it an integral part of the larger economy of the city.

Just as Indians had utilized the lakes and rivers as canoe highways for thousands of years, the new settlers in Seattle turned to the waterways to move natural resources for trade. The volume of materials being moved to meet demand required settlers to come up with a new way to transport goods. The glacier-shaped cleft in the hills between Lake Washington and Lake Union offered a relatively easy, low-lying passage into the interior, though it was blocked by a narrow isthmus of land between the lakes. To facilitate the movement of large quantities of natural resources between lakes, efforts to dig a canal across the isthmus began in the 1860s. The difficulty and expense of digging a canal limited progress on the project and it would take several attempts before a narrow ditch for logs opened in 1885.

At the same time, on the southeast side of the portage, at Miller Street, the focus was on filling the land rather than excavation. In a city with little level ground, it became common to dump refuse and soil from regrades into shorelands as an easy solution to waste management and to build up new land. Sawdust, dirt, household waste, and the like slowly built up marshy land into solid ground. Much of the level land along waterways in Seattle was created this way, including a good portion of Pioneer Square, the southern end of Lake Union, and numerous inlets of low land along the lake shores. One of these dumps, at the end of Miller Street at the northeast corner of Washington Park, became a city dump in the 1910s. Loads of garbage dumped into the lake slowly created a lobe of land that grew steadily until the dump closed in 1936 to make way for development of the Washington Park Arboretum.