Your left join is actually working as expected. My guess is that you're adding the county to a worksheet and seeing something like this.
The problem here is that those county names are not unique to Oregon and we haven't specified a state. Click on the little "Unknown" button on the bottom right, then specify Oregon as the state.
You should then see this:
See attached workbook.
Discharge by County Scrubbed.twbx 128.0 KB
It isn't the join, Wheeler county exist in several different states. Tableau doesn't know which one. I didn't see state in your data but since you mentioned just Oregon create a calculated field call it State. Then just give the correct geographic role. See attached.
Thanks for help - my bad - I hadn't specified 'Oregon' as a fixed option under the maps tab.