How to get your rivers in the right place

[pickaboo]

http://en.wikipedia.org/wiki/Stream_capture
Stream Capture (also known as Stream Piracy) is pretty sweet.

-Rob

Grazy, and lakes can do that as well.

[withlyn]

Lakes are common in some areas, generally those with previous glaciation (Canada, Sweden), tectonic rifting (Africa, Baikal) or an extremely high water table (Florida, Louisiana). Mountain lakes are generally some combination of rift lakes and glacial lakes. Volcanic crater lakes are always fun in an adventure, but are relatively rare within any given region. Elsewhere, most lakes and ponds are formed by manmade dams.

Furthermore, all lakes are subject to eventual siltation. Fast-moving water from the incoming rivers and streams is able to suspend a lot of sediment, but when the water slows down in the lake, that sediment gets dropped, gradually filling up the lake. Depending on the size of the pond/lake relative to the size of the basin it drains (and thus its incoming sediment load), this can take anywhere from a few hundred years (Lake Nasser in Egypt, for instance) to millions of years (Lake Baikal, the African rift lakes). To be truly "realistic" then, a fantasy map should have large numbers of lakes only in regions that are swampy, mountainous, or show signs of geologically recent glaciation.

Although the info about lakes not being found on ridge lines and only draining to one river is generally valid, Isa Lake (actually more of a pond) in Yellowstone National Park is situated on the continental divide at Craig Pass, and actually has two outlets, one which flows to the Pacific Ocean and one which flows to the Atlantic.

Also in Wyoming is Two-Ocean Creek, which splits into two distributaries, one which flows to the Atlantic, and the other to the Pacific.

Just because these things can happen doesn't mean they are normal, or that you would expect them to happen with big lakes or rivers; if you put something like that in a map, make it special!

[Insufferable Fool]

http://en.wikipedia.org/wiki/Stream_capture
Stream Capture (also known as Stream Piracy) is pretty sweet.

-Rob

Extremely relevant example: the artificially-postponed capture of the Mississippi River by the Atchafalaya. http://en.wikipedia.org/wiki/Old_Riv...trol_Structure

Would make for a pretty decent story in a fantasy setting too; a terrible disaster as a major port dries up. Or else an excuse for extensive ruins of that major port, decades/centuries after the fact.

[waldronate]

According to descriptions, Isa Lake only flows in two directions when the water level rises in spring. Two Ocean creek does flow in two directions, but it more seeps out of its meadow on the continental divide than actively splits as we often see in maps here.

Both features are small enough that they wouldn't be visible except on very small local scale maps. The general rules about rivers and lakes aren't affected and budding cartographers should still not be cut any slack for infractions.

[Midgardsormr]

Crosslink to another post with interesting information: http://www.cartographersguild.com/sh...?t=7052&page=3

Juggernaut1981 talks about river behavior, deltas, and Australian sand rivers. Also, Gamerprinter brings up the topic of underground rivers.

[Gamerprinter]

I learned something on a National Geographic show a few weeks ago that the Great Lakes are rising. At first no one was sure why, and a small science post on Lake Ontario proves the truth of the matter, that the Great Lakes have been rising an inch per year for the last 10,000 years.

Apparently, during the last Ice Age, 4 miles (?) of ice sat above what is now the Great Lakes. The tremendous weight of this ice actually pushed the crust downward hundreds of feet lower than lands south of the Great Lakes. Now that the ice is gone, the crust is still gradually "healing" itself and rising to eventually attain the proper elevation. Thus the Great Lakes are rising. Actually some of the Lakes are getting deeper, while the majority of them are getting shallower.

There's evidence of submerged forestland in about 40 feet of water beneath Lake Huron. This is where its getting deeper. There's a tilting effect caused by the crust rising to the north of it.

I hadn't previously heard of this phenomenon. I found it extremely fascinating and since we seem to be talking River Police (and lakes in the above post) issues lately. I thought I'd add this to the conversation.

GP

[Ascension]

That is pretty cool. I watched that History Channel show "How the Earth was Made: The Great Lakes" and never knew about the giant salt basin underneath of the lakes and how it is protected by a giant granite dome.

[kiriona]

Seems about right. It's worth noting that when rivers do split up high in their courses it's most commonly due to stream capture—which takes a very short amount of time, geologically speaking. Probably the most notable example is the Casiquiare which links the Orinoco and Amazon basins. It was originally a separate river, but has currently migrated far enough to capture the upper Orinoco, which will eventually (ie in a few thousand years or so) drain entirely into the Casiquiare. A bifurcating river is possible, just unlikely. A map with lots of bifurcating rivers needs further explanation (eg the area is full of karstic terrain, the bifurcations are artificial etc)

Otherwise yeah... and I must say it's a relief to see a forum where mapmakers actually keep this in mind instead of just filling their maps with circular rivers and rivers flowing from one sea to another etc

[Gidde]

Don't feel bad Ascension ... I live here and I didn't know that until I watched the same show.

@GP: Yep, the shallowing of the Great Lakes is really worrisome to a lot of us Michiganders. I didn't know though that it was due to crust rising until I saw your post. Thanks!

[Steel General]

I wonder if it will have any serious affect on Niagara Falls?