Map of Floodplains from Lane Council of Governments
There is a reason why West 11th was built where it is -- the current route avoids most of the floodplain of Amazon Creek.
In contrast, the WEP would have maximum impact on the Amazon floodplain.
from the 1990 West Eugene Parkway Final EIS - the WEP route would cut across the maximum area of
Amazon Creek floodplain
(note also that the new "Target" store was built in the floodplain)