Re: Characteristics of a really gigantic lake on an Earthlike planet

On 10 jaan, 22:03, Bryan Derksen <bryan.derk...@xxxxxxx> wrote:
So I'm getting set up to run a Dungeons and Dragons campaign, and to get the feel for what sort of world my players
would like to play in I run a worldbuilding game called "Dawn of Worlds" with them. It's a little bit like a
paper-and-pencil version of "Civilization" geared toward producing a setting for this kind of use. As a result, some
unexpected things can happen when different players' actions interact.

When I drew up the starting map, basically just coastlines and a few mountains to get things rolling, I included an
inland sea in the northern part of the continent. As worldbuilding progressed a river
wound up being drawn that connected the sea to the ocean. This meant
that it's not really a sea but rather an enormous lake. I estimate it's
got a surface area of about five million square kilometers - sixty times
the area of Lake Superior, twice the area of the Mediterranean Sea.

Could you estimate the area of the watershed of the lake, and of the
river below the lake?

The biggest watershed on Earth is that of Amazon. 7,2 millions of
square km. This is a bit over 40 % of South America (17 millions
square km). The other continents although bigger than South America
are drained by several individually smaller rivers.

The river running from it to the ocean is longer than the Mississippi so
clearly it's got a decent amount of elevation relative to sea level

Not absolutely clear, though. The river might be encountering large
drops, rapids and waterfalls like Niagara Falls. Alternatively, the
river might be deep and placid, like Saint Lawrence between Montreal
and Quebec, or Amazon below Obidos. And Amazon is actually tidal at
sea level in Obidos, about 900 km from mouth.

St. Lawrence is 10 km or so wide at "Lake St. Pierre", but this is
often regarded as part of the river. Lake Pepin on Mississippi and
wide parts of lower Amazon are also often ignored on general maps.

and the flow of water is one-way - the Mediterranean is not a good model for predicting its salinity.

Depends on the depth of the connecting channel. It might be even
deeper than lower Amazon or St. Lawrence. Dardanelles is under 8 km
wide for 100 km or so, and over 70 m deep. And then you have
Sognefjord, 200 km long, under 5 km or so wide throughout, and over
100 m deep - 1300 m at deepest!

I'm not sure of the rainfall, but it's not particularly arid in most places around its shore and the northern border of the
lake edges up close to the arctic ice cap. Personally I expect it's probably going to be fresh water; despite its huge
size it's got a goodly outflow going to flush all that salt out.

Size does not matter; even Black Sea is well flushed over a few tens
of thousands of years. If there is outflow one way, through all
depths, at all times for millennia, then the salt is flushed out.

The most relevant part is the water depth at the shallowest point of
the outflow channel. If it is shallow enough, then the flow speed and
water level above may fluctuate with floods and droughts, but
saltwater never gets in the basin above. If it is deep, then there
will be salt water inflow at depth, and the body of water stays
brackish like Baltic and Black Seas.

But my intuition may not be the best thing to rely on here, this is outside my normal human scope of familiarity.

Try to work from the examples of Baltic and Black Seas.

One other thing I'm pondering: The northern edge of the lake is up near the arctic circle, the southern edge is down in > temperate or subtropical climes. Would such a lake get half-and-half ice coverage in winter, or is it likely to keep too
well mixed for that?

Half and half. The extent of mixing would depend on depth. The Baltic
extends a long way from the southern coast of Germany and Poland to
the northern end of Bothnian Bay in Töre, near the arctic circle. The
southern edge of Black Sea is subtropical; northern edge does freeze.
Ditto about Caspian Sea. I think that an important part would be
salinity distribution as well.

This would have some significant repercussions for a major mining settlement on the north shore.

In the end it probably doesn't matter much how hard the science is, I'll tweak it as needed for the sake of the
campaign

You can keep the science hard and tweak the bottom depth as needed.

and I'm sure it'll be
plausible to my players any which way I go. But my own curiousity (and
desire for realism) has been piqued here. :)

.

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