When the earliest settlers came to the area later known as Seattle in 1851, there was really no place to put a city. Elliott Bay was a wonderful deepwater harbor, but most of the land consisted of steep hills covered in timber plunging right into the sea. Clearly, this couldn't stand.
The early settlers started modifying the land right away, but it wasn't until later in the 19th century that major changes started. First, the massive tidelands south of the downtown area were filled, then the hills started to be modified. Most of the closest hills had some elevation reduced, but the biggest effort was the total eradication of Denny Hill north of downtown. Using high pressure hoses and steam shovels, the engineers and contractors chewed away the hill. It took over twenty years to complete, and even then there was a small hold-out, but the area known as the 'Denny Regrade' was created.
The author discusses the can-do attitude prevalent in the 19th century which led to massive urban renovation, which also occurred in other cities but not to the extent it did in Seattle. The hills could be cut down because they consisted mostly of glacial fill, not rock. The lakes could be connected with the salt water because the land allowed a modest east-west canal. The tidelands could be filled because of all the sawdust and dirt created by the growing city.
No thought was given to the Natives who already lived here, or rather, the thought was to push them out.
As fascinated as the author is with the regrade efforts, he doesn't think most of them were necessary, especially once the automobile superseded the horse. (A large reason people agreed to the regrades was the difficulty of horses climbing a steep grade.) He certainly doesn't think the removal of Denny Hill was necessary. In this day and age, it's hard to imagine Seattle with another hill adjacent to downtown.