Oh, there’s hills right near there … if they had built the trading post up on one of the slopes it wouldn’t have been washed away during a flood. The down side is that they would have had to build an elevated roadbed between the post and the river … this area is impassable overland during winter. That’s why the trading post was built next to the river.
It’s rains there, and it rains a lot … during wet winters the Willamette Valley is one giant swamp … a wagon will sink up to it’s axles.
Much more than you probably want to know about the US equivalent to Britain’s OS maps: http://nationalmap.gov/ustopo/index.html
They have 10 foot elevation contours for the entire country at pretty small scale.
Here is a link to download the (35MB!) pdf map for the area near the OP’s knoll: https://prd-tnm.s3.amazonaws.com/StagedProducts/Maps/USTopo/1/19654/6803032.pdf
Late add: USGS also has historical topo maps going back to the 1880s in some areas. The OP might be interested to look for his knoll on the oldest maps available for that area.
See Historical Topographic Maps - Preserving the Past | U.S. Geological Survey