I learned something here, thanks.That's 100% true, but you have to think about why the borders in the northwestern US are where they are. In the 1850's, everything west of the Continental Divide minus the existing state of Oregon was the Washington Territory and everything east of it was the Nebraska territory. When gold was found in Northern Idaho, Washington cut it loose because it was impossible to administer. The area that's now Eastern Washington stayed in Washington because it had been part of the series of treaties signed by Isaac Stevens to encourage settlement, and white settlers were already popping up east of the Cascades.
Congress created the Idaho Territory out of Nebraska and Washington, basically all the areas that were still uninhabited at the time (present day Montana, Wyoming, and Idaho). Idaho Territory was administered from Lewiston, the only really inhabited area. The next big gold strike was in southwestern Montana, east of the divide. The most accessible settlement to the Montana goldfields was Salt Lake City, followed by coming from Nebraska up the Bozeman Trail, so Lewiston had basically no control over what was nominally Idaho. After the vigilance killings in Bannack, Congress split the territory up again. They had a plan to create three territories called Idaho, Montana, and Wyoming, but no one had any particular idea where the split should be. The most logical was OTL Montana west of the Divide/northern Idaho as one territory, southern ID and Wyoming west of the divide as one territory, and everything east of the divide as one more territory. Idaho rejected that, since the Missoula Valley had a particularly nasty den of outlaws run out of Lewiston and Bannack called the Hell Gate, and asked for the border to be drawn on the Bitteroot mountains instead. The Bitteroots are a more imposing range than the actual continental divide, and purely in terms of geography make more sense. I'm not sure exactly why the Wyoming border is a square instead of Idaho just extending to the Divide and eastern Wyoming being lumped in with Montana (which makes sense because most of it is drained by the Yellowstone and its tributaries), but that's the way Congress drew it.
So yes, the Powell plan and logical watershed borders look good in the abstract, but when you start looking at how territories and settlement evolved, the existing hodgepodge of straight lines and arbitrary divides makes a certain amount of sense.
Let me follow that up, do you know why Oregon came in with so much unsettled land west of the Cascades instead of territorially just the ocean to the Cascades?