Air will eventually start to enter the downflow side since nature abhors a vacuum and gravity does it's thing with the water on that side. So, for a short period of time there will remain a vacuum induced column on the stream side until enough air enters the downflow side and then the whole shiteree fails and the remaining stream side water will return to the stream.
Re-reading your presentation makes me wonder if you were trying to do this with two 300 foot columns or just what the heck you meant. It would turn out the same way as I described using two equal height columns. In no case could you add the resultant static pressures in two 300 foot columns and say there was 260 PSI anywhere within the system unless the downflow side passed the stream side bottom for another 300 feet down. Then, with everything valved off, you'd see about 260 PSI on the downflow side. Open the valve and that goes to zero. Simultaneously measuring the downflow side at the stream level would likely show close to zero or less IMHO. Like machine work, sometimes it's easier to convey and idea with a picture or drawing rather than with words.