If America decides to offer the whole sound, the Columbia River is the logical border. The British already control everything of value on their side of the river, and they can push the claim that because David Thompson was the one to successfully navigate it they've got the much stronger claim. All of this is prior to the age of steam in a region that was virtually unexplored, so nobody negotiating knows if there are viable ports on the Columbia or if the Willamette Valley is really great yet. These are all things that will happen decades into the future. In 1818, if the American offer up the sound and the British accept, the Columbia is where the border probably goes as it's the only thing anybody has a good grasp of where it actually is.
Well, shoot, I really liked the idea of a Native Reserve buffer zone between the north bank and the Puget Sound watershed, and clearing the way for the USA to use Portland as the major Pacific port.
But yes, between the European principle of "discovery" and who holds the most assets in the region at the moment, a treaty securing just the south bank on south for the USA probably makes good sense and as pointed out, the Yankees don't have a lot of resources to spare on posting big armed forces back of the beyond like that.
A civil seaport in Portland is perfectly feasible of course, as long as the USA and Britain remain at peace--which has held, with some tense moments (one of them over the Northwest and therefore probably prevented here) from the end of the War of 1812 to this day OTL. But this is less than a decade after the end of that sorry war and I suppose both sides have reason to be wary of each other. Certainly basing USN assets, assuming any can be spared for a Pacific Squadron at all, at Portland itself, or anywhere on the Columbia, is risky. The British can spare more and set up a counter-base across the river, and both sides can match each other setting up shore artillery, again with the advantage going to Britain for the foreseeable future. It is therefore in US interest to limit tensions as much as possible in the region.
Also as pointed out by you, the American settlers are largely dependent on supplies from Fort Vancouver, which IIRC is OTL Vancouver, Washington. Since this is the original center of HBC governance, and now has no particular reason to move north, perhaps the development of the Sound will be slower than I thought. At first anyway; I still think the logjam will break sooner or later and lots of people who went to Australia, Canada, South Africa or Kenya OTL will be attracted to BC. (Using "BC" as loose and not necessarily inevitable description of all British territories in the Pacific Northwest regardless of their evolving legal status--it could almost as well stand for "British Cascadia" as "British Columbia" after all). In addition to them, "Canadians" (again using the term loosely for all the diverse possessions making up British North America east of the Rockies) as OTL, immigrants to the British territories from elsewhere in Europe, Asians as OTL but perhaps more of them.
And yes, I'd think, Yankees. Surely I agree the HBC will take a firm attitude against rival fur trappers and traders backed up by British military and police forces, and I leave it up to experts in the region what effects a different policy (not hunting out the region as you tell me they did OTL) would have down the line--again, a delay in settlement by Europeans and Euro-Americans, but possibly not lasting until the end of the 19th century, quite likely to break before then and surely in the 20th settlement will pick up if delayed that long.
So OTL a lot of US citizens were keen to settle in the Northwest, and not all were content with the Willamette valley. HBC can probably hold the line against any US citizens intruding, surely as long as they discourage British settlers. But OTL their policies were leapfrogged, in part due to minor regional gold rushes. I'd think that where British settlers either force their way in or are grudgingly permitted, a certain number of Yankees will come in with them. It would be sound policy to make sure people from other sources significantly outnumber Yankees, and ideally for particularly loyal British subjects to outnumber all other groups combined. But the more secure British control becomes and the more development there is in the region, the more safe it would be to let some US people in--subject to their swearing allegiance to the Crown of course.
Meanwhile to the south, the British have the highest level of development, so for a long time US settlers will be dependent on them, that is to say practically the HBC, to supply them. This adds to American pragmatic incentives to keep the peace, though it may also be a cause of irritation, but in fact it will be a market-based fact that Britain is not responsible for. On the British side, the American settlers to the south will be a market and even if they start developing alternative commercial sources HBC, based right across the river, will remain a major player even on the US side of the border for a long time.
I've suggested once before, not with any enthusiasm but based on some BC history I have read a little bit of, that HBC and allied colonial authorities may face some serious trouble from disgruntled settlers. The USA would be daft to go to war with Britain in this period, at least not when bogged down in some big European war, but perhaps if the British authorities screw the pooch too badly, an independent Cascadia, one the USA is sworn never to annex, might be an outcome. I'd think that by 1850 or later this would not be at all probable, and do not discount Britain's ability to keep control, especially if the Yankees stay on their side of the line properly.
I also wish to repeat, with a lot more urgency now, the idea that the USA will develop some port for the Navy south of the Columbia mouth. Logistics will suck, compared to Portland--but in terms of big capital goods (cannon and so forth) much must be imported from the Atlantic/Gulf coasts or European markets anyway, so if the harbor is any good it might not be much more difficult to operate from a coastal bay that is practically an island, until trails, roads, and eventually railroads over the coast range to the Willamette valley can be built. Even with RR, I suspect that the main method of supplying the Naval base would be coastwise shipping out of Portland and down the coast, though as I understand it this is a hassle due to prevailing wind and current directions. The idea would be that it would be only moderately costly to supply it by sea in peacetime, but in case of conflict with Britain, with the Columbia mouth essentially closed, it could get by and defend itself on stockpiles and overland communications.
It would be too provocative to build major naval facilities on the Columbia, although Portland being developed to serve as a pretty good naval yard de facto seems likely to me.
....Portland doesn't take off until the 20th century because it's just too far away from anything of note.
A lot depends on whether the USA gets hold of part or all of California of course. Even in a scenario where Mexico (or a British protectorate of California) retains control up to say the mouth of San Francisco and the Sacramento River or some such forms the border in the Central Valley, the northern part of the CV and some coastal areas north of the Bay (notably Sonoma and Napa Counties OTL) would be a huge draw versus Oregon. Even in that scenario with a strong enough Mexico to hold most of the mission coast, where once again the Yankees lose a fine harbor due to it being split with a border, I can see Oregon being relegated to second place. If the USA gets all of the CV and coast below the Golden Gate, even if kept out of the area from Santa Barbara on south, it will be eclipsed for sure.
And yet even then I think that on a regional scale, compared to other US cities in what the USA can get of California, Portland despite the disability of not being a suitable military base due to the desire to keep tensions with Britain low, will grow to be comparable in size to California towns. It must get some of what Washington got OTL, and other than other towns on the Willamette is the only game in town. Certainly if the USA either keeps the peace with Mexico or fails to get the spectacular gains of OTL so far as to gain nothing from California, Oregon is the USA's only outlet on the Pacific, and that was valued enough I think that measures will be taken to safeguard it. There will be a Pacific Squadron, perhaps carefully limited to clearly pose no threat to "BC", but something will be there. Making Portland a major Army base might be provocative but having some camps in an hour's march or so to the south up the Willamette, and a string of small posts along the south bank of the Columbia would seem reasonable, unless prohibited explicitly by treaty.
The original 1820s treaty seems unlikely to go so far as to set severe limits on US freedom to build bases, if only because at the time it would seem preposterous the Americans could afford to divert much there. Perhaps later, in the rounds of treaties relaxing fortifications on both sides of the "Canada" border OTL, some mutual limits will be agreed upon, much to the relief of American central military command, which indeed will not want to tie down resources so far out of communication. Being secure on the British side, perhaps even able to call on British aid in a pinch, Mexico is very very little threat--the threat goes rather in the other direction!
Mexico would have to make quite an effort to secure the north of California. A special effort would have to be made to put some substantial population in the far north of the Central Valley, around OTL Redding, and even then Yankees marching overland from the northern passes or eastern Great Basin would be coming down from the high ground. To be sure, it is high ground that crosses some forbidding terrain to get to; it would take a concerted effort; relying on the sort of manpower that would naturally be present in Oregon would even the fight up considerably. Going down the coast is even more of a stretch; for excellent reasons the stretch of coast between OTL Mendocino County coastal towns (Mendocino itself, and Fort Bragg) is referred to as "The Lost Coast;" no major roads run along the shore even today there, and the alternative inland route the railroads and US 101 took is itself torturous and prone to landslides (which is why the railroad was abandoned generations ago). Going north from San Francisco Bay, there is Bodega Bay, which is puny (though overland one comes into the plains the OTL Sonoma County seat, Santa Rosa, is on, and that is very good land), then Mendocino, then nothing much for ships or boats till one gets to Humboldt Bay in the middle of the county of that name. Beyond there are a few more harbor spots, to Crescent City in Del Norte County at the extreme north of the old Spanish/Mexican claim. For Mexico to defend that would require development of a navy and laborious placement of bases, I suppose at CC and Humboldt Bay, which like the US Naval base in Oregon I suggested would in effect be island colonies, except unlike the Oregonian case, there is not a lot of hinterland to draw from. Given a Mexico much like OTL, even if they benefit from somewhat superior leadership, and start to exploit southern and central California more effectively and can settle the Bay Area with a numerous and loyal population, defending the northern approaches is a daunting exercise. Against that--if the Yankees can indeed overrun the northern Central valley, perhaps Mexico can muster a defense of the approaches to the Bay, and if they do that, there is essentially no outlet for the northern valley. Attempting to develop alternate routes west across the coast range would be a bitter enterprise indeed. I suppose if Americans plan to take California they will reckon up whatever force is needed to force their way to the Golden Gate. If they can do that the tables are turned and it becomes the southern valley that is hard to hold on to. Inland of the Sierras the Mexicans and Spanish before them did very little to secure the region, down to the missions in Arizona anyway.
Given conditions close to OTL, if the Americans can devote a substantial effort, I'd think California at least down to the Tehachapi range which traditionally divides "northern" from "Southern" California will fall; perhaps Mexico can defend and keep control of the coastal settlements farther north on the coast, maybe even all the way to Monterrey Bay, but the Yankees will take the central valley anyway, and San Francisco Bay, and everything north to Oregon on the coast, and inland over the Sierras...pretty much as far east as they like, at least to Nuevo Mexico.
Here I'm decoupling the war from a war in the east based on Texas annexation, assuming an independent Texas remains in existence and shields the northeastern approaches. A unified US-Texan onslaught would probably go much as OTL anyway. Note that OTL the California campaign involved a naval expedition sent around the tip of South America. It also relied rather heavily on the cooperation of Latino settlers in California who were disaffected from Mexican government, which is a variable that might conceivably be butterflied.
Speaking of butterflies...
All of American politics is up in the air because Polk might not win the election, but Tyler may not ever get a shot at the presidency if Harrison doesn't die. Crawford may not have his stroke and win in 1824 too, you can do whatever you want.
Or not. I subscribe to a soft line on butterflies, since it is not reasonable to assert that every small detail must propagate into major ones because of shifting winds, different sperm hitting different eggs and all that. Yes, that would happen in one timeline, but the idea of AH is that there are zillions, and convergence can happen as well as divergence. That is an ATL can actually be derived from a TL that was different than OTL at the time of the POD, and the merely chaotic events happen to zig things in an OTL direction where they are properly random and contingent on small things, rather than zag away. Thus there is no obligation to make all the Presidents different just because different stuff is happening in Oregon. OTOH I understand Polk's victory had something to do with jingoism split between people focused on the Northwest as well as other "Canadian" borders, and those focused on Texas and Mexico beyond. Eliminate the Northwest flashpoint and Polk might have to double down on rhetoric as well as action against Mexico, which might forewarn whoever runs Mexico (per my anti-butterfly stance, Santa Anna) and also screw up Polk's campaign, because I suppose he won with the support of some Northern votes that were preoccupied with Oregon OTL. Can he win without them?
Perhaps designs on California take the place of covetousness against British territory?
BC probably joins Confederation because they'll likely still have the spending program of OTL. Vancouver is bigger because of the lack of a 'proper' American port in the area. I'd give a very hard maybe about Vancouver Island maybe being split off from mega-BC because fears from Quebec and Ontario.
I'll take your word on Canadian politics. Does that last sentence mean that the big eastern provinces (the Canadas proper, Upper and Lower) will fear being eclipsed by a unified BC? But why, if development is held back by HBC policy?
If it becomes a big settler colony as I figured it would, before 1900, then yes, I suppose. But if the development takes a while to get started, will a scary big BC seem to loom that large in the late 1860s?
Other than that not much changes. Britain and America are probably on slightly better terms, and the HBC shares are worth a little less.
Again, why less? ITTL they get a firm anti-Yankee border, which they can enforce, and at the same time these same Yankees are semi-captive customers, while they can set whatever policies they like regarding Puget Sound plus everything else they controlled OTL. Unless we suppose HBC will be overridden in London in favor of settlers, the Company has it all its own way. There would be no need to overhunt the hinterland to create scorched earth firewall to keep the Yanks out, so they can set up a stable arrangement with the Native peoples and generally arrange everything to their liking. Will it be success itself that spoils them?
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I also want to bring up the Alaska issue again. If HBC keeps BC pretty close to deserted then British power is not a whole lot stronger in the region until they decide to go into the business of profiting off of settlers. If BC development is retarded per company policy come the Crimean War (assuming something like that happens) then while the conquest of Alaska by the British is possible, it is also not more likely than OTL. I'd think that as OTL the Russians would rather sell to the USA than Britain, and that British objections would be mild. Then again this was a rather contingent event, so perhaps the choices boil down to an even split between Russia hanging on despite their cost (perhaps eventually discovering gold and reevaluating Alaska's worth), Britain conquering it just because the UK happens to be at war with Russia, or an American purchase, all a roll of the dice.