It's not just the Rocky Mountains that's the problem. Ever heard the phrase "as flat as Kansas"? Guess what? Even being "flat", if you cross the entire state you end up with about 3500 feet in elevation gain, meaning you have to use energy to pump the water up the entire way.
Pic related the highest point in Kansas. Despite the area being flat, it's still over 3000 feet higher than the other side of the state. Same is true for most of the states between the Great Lakes and the Rocky Mountains.