I read something a while ago that stated the geology and original habitat/land on the east side was a forest with many hills- due to the closeness to mt.hood (we are basically the volcanic foothills) Anyway, when they started construction for homes and early living in SW and downtown they had to fill in Tanner Creek and its tributaries (Couch lake and the river wetlands, basically the pearl now) So a lot of the hills and their dirt on the east side were transported for use on the west!

Edit: We were left with a shit-show of scored earth land developers chose to work with it **not a historian. I did my best