JMO: Keep in mind when you are coming up with theories, that the simpler the theory the more plausible.
If in fact she washed her car, why? Think of the reasons for you to wash your car? Trying to impress someone?
If you were going to kill someone would you do it in her car, during the day, at the park, behind offices that are currently full? If you did kill her at this location in her car, would you move the body to another car? The area by the dock is one of the most traveled sections of the park.
If the murder wanted to dump a body into the river he picked one of the worse places in the area. Correct me if I am wrong, but the reason CB exists is the sand bar. The reason the sand bar accumulated in that area is that section of the Columbia has some of the slowest flowing water. While taking the body further down stream in a boat would make sense, it would be suspicious for anyone but a hunter to be on the river during that week.
Put yourself in their position and think out what you would do; that is how you are going to find this murder.