Call it a demographic trend, this one led, slightly, by Idaho.

For a long time Idaho, Washington and Oregon have had some population characteristics in common: a large central city (Boise, Seattle, Portland) with the pair of next largest cities spread out a little bit (Pocatello and Idaho Falls; Tacoma and Spokane; Eugene and Salem) and closely matched in population and sometimes exchanging ranking, well below the level of the lead city. These patterns have held for decades.

We’re now seeing some adjustment – new trends.

In the 2000 census we saw it first in Idaho, when the city of Nampa, lesss than20 miles from Boise, boomed and vaulted high over Pocatlelo and Idaho Falls to become the second-largest city in the state. If present trends continue, it could have double the population of either Pocatello or Idaho Falls by 2010. And also by that year, the city in between Boise and Nampa – Meridian – probably also will have surpassed Pocatello and Idaho Falls, which are far afield to the east.

Oregon’s pattern has had Portland in a distant lead among cities, with Eugene and Salem, substantial distances to the south, closely matched for second place. But much of 2005 saw headlines about the expansion efforts of the city of Beaverton, just over the hills west of Portland, to annex some of the large population of unincorporated residents nearby. That effort has aroused much controversy, but at least some of it eventually is likely to happen – many of these people live in effectively urban areas that really should be served by a city. When that does happen, Beaverton is a clear shot to become the second largest city in Oregon. (If growing Gresham, east of Portland, and now fourth-largest in the state, doens’t get there first.)

The development in Washington may be least expected statewide, though it shouldn’t be: Growth in Clark County, across the river from Portland in southwest Washington (and part of the Portland metro area), has for many years been the fastest in Washington. Meanwhile, growth in Spokane and Tacoma has been modest, and even Bellevue – fourth place in the state until Vancouver overtook it in this decade – has fallen short of boom-level.

Spokane and Tacoma each have populations just under 200,000; Vancouver’s now is 154,800 (compared to Bellevue’s 115,500). North of Vancouver lies a big territory of heavily developed unincorporated territory. As is the case with many prospective Beaverton annexees (and southwest of Boise too, among other places) many of the residents want to stay outside of city limits. But that will not last forever.

And maybe not for long. News reports today note that the Vancouver city is moving ahead with a massive annexation plan which would bring in as many as 65,000 new residents, and push Vancouver to second place among Washington cities. A series of informational meetings on the subject have been scheduled for January.

This effort may have many effects. One of the larger, and felt far afield from Clark County, may be the way Washington state re-perceives itsefl. And the way Tacoma and Spokane do.