Ya gotta love that extra push from the current.
When we were taking our boat down to Portland a couple of weeks ago we were running at 1100rpm's and doing about 12.2kts. We got into the area between Hood River and Bridge of the Gods where Thirteen mile point is. The river narrows from about 3/4 mile down to 1/3 mile right there.
Our speed picked up from the 12.2kts to 16.4kts. Loved it.