How far south can I boat on the Willamette from Portland?

Iv'e taken our 18' deck boat (merc outboard) as south as the cove at Ross Island... how much farther can I safely go? I think I heard there are some big boulders just under the surface at some point further south... any info/warnings appreciated!

You can go to Oregon city. The falls at Oregon City are a barrier to further exploration south. Picking up a good chart of the river will help you navigate around those rocks. https://usa.fishermap.org/depth-map/willamette-river/ this is a good chart.

Another good chart is the Navionics site: https://webapp.navionics.com/#boating@12&key=orlsG|vykV

It looks like the usa site is using the Navionics data. I assume they keep it up to date, but I would rather go straight to the source.