I grew up in Portland and the one thing I can attest to is how fast that city - and more appropriately - outlying areas like Beaverton and Tigard are growing.
While I think Portland is ripe for a theme park - Seattle is riper. People from Portland already travel to Seattle for concerts and other events that don't make it to Portland, essentially adding Portland to Seattles demo. I personally believe both cities could house and support their own theme parks, sharing customers. Seattle has the added benefit of proximity to Canada, something that the GNW was well aware of and counting on.
So King County (where Seattle and Bellevue WA are) has 1.7M folks, all of Oregon has about 3.5M. It's a lot easier to see Seattle and outlying areas supporting a park. But, if you add the populations of OR and WA it's almost 9M people that are really no more than 6 hours away whether the park is in Seattle, Portland or both!!
Anyone aware of the turnout at Portlands rose festival? Well it's huge let me tell you. Big enough that they continue to have it even though it costs 10's of thousands of dollars to repair the grass at the waterfront each year.
Portland and Seattle both turned Disney away for theme parks, and that mark still follows the cities I think.
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-Z