Janice,
The Gorge is a tricky place to shoot, with curving shorelines, funny angles, trees that aren't straight because of constant wind... It will drive you nuts. That said, I'm sorry, but that one is definitely off, and it's actually off quite a bit. The problem with the bubble levels is that your camera may not be mounted level. Often the quick releases don't sit perfectly flat. If the level is in the hotshoe, they can be off as well. "Off a lot" in this case is a bit more than 1 degree, which is actually very tiny when it comes to leveling the camera...
Before you can trust it, you need to shoot something you know is totally level, a large carpenters level is one option. The surface of the ocean on the horizon also works well, but it's not nearly as portable.
So, how can I say with authority it's off? Well, the first reason is the rule you stated, trust the trees. The tress on the left are off. Of course you can get distortion at the edge of the frame, so you need to be careful in those area.
However, there's another, even easier way.
Every picture of a reflection in still water has a level built into the photo. You can always use the following technique to level the image.
Find a distinct point in the image, and then find the same point in the reflection. To avoid any distortion issues, work in the center 2/3rds of the photo. Draw a straight line between the two points you selected (a distinct point and its reflection). That line will always be vertical!
Why is that? Simple, a still body of flat water will always be level, since that's what water does. So, since it's level, the reflection will be perpendicular to the water, or straight up and down from the original object.
While you may not know the rule, your brain does, and when it sees that reflection out of line, it senses the picture isn't level, even though you probably can't say why....