This isn’t an idea, or even a promise. Trump thinks that there currently exists a faucet that could divert the Columbia River, a river he does not know exists and would probably think is in Mexico somehow, and that the faucet is purposefully moving water to the ocean as a way to spite the residents of California going through a water crisis. His only promise is that he would turn said faucet to eliminate the water crisis. Why are journalists ascribing so much intelligence to someone who has consistently bragged that he thinks at an 8-year old level?

There’s a flood control gate somewhere around Vancouver, is that what he’s talking about? It’s a bit bigger than any wall though…

Yes, clearly he means demolishing Bonneville Dam, somehow reversing the flow of the Willamette and then digging a trench through Grants Pass, where, if we flood the San Joaquin Valley will provide plenty of water to LA.