When did I say the river had to be 60 miles long? It was stated that a hill could not be in the line of sight. Well there can be (just not a very big one).
But the original point was why the Rocky Mountains appear to follow the same "Top First" appearance as a ship approaching from the horizon. You posilated the an interposing hill could cause the illusion. I respondes with the existing conditions of why that was an unacceptable explaination.
In order for your suggestion to work, the hill would have to be either far enough from the mountains, or high enough to cause just the peaks to appaer at a great distance. Either the hill would be closer to the mountains and very noticeable, or further away, and now allow the rivers and streams to flow Eastward from the mountains.