Bifurcation: The secret giant islands formed when rivers split

The article that follows that one is also very interesting and worth reading. It shows how a coastline that existed 100 million years ago and cut through modern day Alabama is still visible in demographic and election maps. (By a crazy coincidence, I just dug up that article and sent it to someone not even half an hour ago, since we were talking about geology and I remembered reading it. Then I go to HN and see this posted.)

1. https://starkeycomics.com/2021/06/11/how-a-coastline-100-mil...

The parting of the waters in the USA is such an interesting landmark for just how uninteresting it actually is in person. Looks like any other split in a forest river, marked by a sign on a tree that you can only see after a long wilderness hike.

https://en.wikipedia.org/wiki/Parting_of_the_Waters

https://www.anotherlongwalk.com/2022/07/day-119-parting-of-w...

This is not to take away from the fun of investigating and identifying these ‘bifurcation islands’, but on the question of whether they qualify as ‘islands’:

> Like a delta island, it is surrounded entirely by sea and rivers. As an area of land separate from a continent, permanently and entirely surrounded by water, it seems to fit every definition of an island I can find.

I agree that by ‘Air Bud’ rules these qualify as islands (‘ain’t no rule says an island can’t have one end halfway up a mountain and the other at sea level’) but I think there is a reason why we don’t instinctively feel like they qualify as ‘proper islands’.

Specifically, I think the instinctive definition of an island is that it emerges from the water surrounding it at a consistent elevation. It has a shoreline that is broadly level.

The delta islands that he identifies as being basically the same as bifurcation islands are typically surrounded by tidal water on all sides so their shoreline is all at sea level.

Counterpoint, though: Goat Island, which is in the middle of Niagara Falls, and has one end at the elevation of Lake Erie and the other at the lower elevation of Lake Ontario. Does this feel like a ‘proper island’? I know in my mind I sort of struggle to think of Goat Island as being an island, not two islands that happen to be right next to one another… but I can adjust my mental definition of island to account for it; but extending it to the continent-spanning bifurcation islands still feels too much of a stretch.

This is like the ‘dwarf planet’ problem - there wasn’t a good way to make Pluto a planet without making a lot of things that definitely aren’t planets also count as planets.

Is there a good way to make Goat Island an island, without also making the bifurcation islands into islands?

Also… Australia is clearly an island by this definition. But then so is Antarctica.. and the entire Americas. And the Eurasian/African supercontinent…

I live near what used to be several of these - Kaiapoi Island,Coutts Island, McLeans Island, Templar Island, no doubt forgetting some.

Created by the Waimakariri River splitting into 3 branches as it neared the ocean.

They're no longer islands as the two side-branches of the river caused repeated severe floods in colonial days, so large civil engineering efforts went into restraining the river, ultimately leading to the south branch and north branch being blocked by large stopbanks and riprap cassions.

Both branches were then utilised to drain water from high fertility but swampy ground.

Because NZ's braided rivers lend themselves to wandering channels, we've still got some decent sized river islands - Rākaia Island, Rangitata Island.

As a kid I found a bifurcation island in northern Sweden, that may be bigger than Gotland, the conventionally biggest island in Sweden.

It's formed by the Tärendö River which flows from the Torne river to Kalix river.

It's supposedly the second biggest bifurcation in the world: https://en.wikipedia.org/wiki/T%C3%A4rend%C3%B6_River

This is the best image I found: https://www.smhi.se/polopoly_fs/1.30940.1490013619!/image/%C...

Unfortunately, I noticed an error. The Committee’s Punch Bowl Bifurcation Island is much, much smaller than what is reported.

The reason for this, I'm sure you'll be delighted to learn, is about half of that area is in fact another bifurcation island!

See this unnamed lake in northern British Columbia here: 59°03'07.5"N 133°57'08.0"W

This lake has two drainages. The east side drains into Sloko lake and then the Pacific via the Taku river. The west side drains to Atlin Lake, then Tagish Lake, from which flows the Yukon River, traversing all of Alaska to dump out into the far north Pacific just south of Nome.

Thus the one large bifurcation island mentioned, is in fact two much smaller ones.

> The Missouri is the largest tributary of the Mississippi River, which flows into the Gulf of Mexico

He must mean longest. This chart[0] shows that the Ohio is by far the largest tributary of the Mississippi by volumetric flow, and also shows the Atchafalaya bifurcation. However, the wiki articles for the Missouri[1] and Ohio[2] show average discharges into the Mississippi of 87,520 cu ft/s and 281,000 cu ft/s, respectively. Not as great a difference as the first chart [0] shows, but still significantly different. Even more interesting, since the Missouri drainage basin is about 2.5x larger.

[0]https://en.wikipedia.org/wiki/Mississippi_River#/media/File:...

[1]https://en.wikipedia.org/wiki/Missouri_River

[2]https://en.wikipedia.org/wiki/Ohio_River

Most of the population of Bangladesh lives on what is technically an island created by the forking of the Brahmaputra, whose distributaries rejoin 150 miles away (as the crow flies). https://en.wikipedia.org/wiki/Brahmaputra_River#Bangladesh

In Florida a notable non-giant one I remember is because the river is a location which had historically been the legal borderline between different counties.

One county north of the river, the adjacent county existing south of the river.

The island itself, not specifically within either county technically.

Oral history of Rum Island in the Santa Fe river goes back to the time when liquor was prohibited by US law, and regardless of whether local officials knew about it, they could honestly testify that there was no remaining liquor within their jurisdiction.

Very cool, I just learned about these recently. I wonder, at what point does something become not eligible to be an island? Like, that biggest island looks like almost half of North America. Doesn't that make the other half an even bigger island?

I though this about running multiple nvme disks on single PCIE slot. https://www.asus.com/support/faq/1037507/

It’s surprising how common this is.

Other examples: https://en.wikipedia.org/wiki/River_bifurcation#Examples

As always, there's an xkcd for that: https://xkcd.com/2838/

What's the difference between a delta island & a bifurcation island?

Lyapunov is calling and wants his terminology back.

Bifurcation occurs at the Great Lakes, where the Mississippi flows into the gulf and the St. Lawrence empties into the Atlantic. This forms the “great loop”, a circumnavigation course known by many power boaters . This makes about half of the United States and island by your definitions.