Question regarding lakes and watersheds

[cyanide_baby]

Hi everyone,

I don't know if this is the right place to post this question, but here goes.

I've been working on a fantasy elevation map, and I'm now working on its river network.

I'm not sure what to do about certain topographical features though. What happens when rivers reach lower elevation sections of the map in the real world: do rivers fill those lower elevations until they become lakes and water spills over to flow towards the even lower elevations? I can't find any clear answer about this online.

See the two screen captures below:

One shows the river network (a work in progress), and the other shows the same exact region (with the same river network) with the addition of two lakes located in some lower elevation parts of the map.
+ Is this correct, as in, realistic? Is this how water would behave in such a region?
+ If correct, does it mean that I'm gonig to have to fill all those lower elevation sections with lakes or bodies of water?

Thanks.

Screenshot 2022-09-15 121411.png

Screenshot 2022-09-15 121349.png

[Redrobes]

This might be worth a read through to see if it clarifies anything in your mind about these things.
https://www.cartographersguild.com/s...ead.php?t=3822

It could be that lower elevations fill with water but it could be that it drains. Water will tend to flow with an amazingly small drop in elevation. Only when its a full basin will it possibly accumulate and then it depends more on weather esp for evaporation and the ground permeability etc. So its not assured what would happen.

[acrosome]

In general, yes, such depressions would fill and then "spill over". Depending upon the landscape it might then quickly erode a canyon or water gap and drain the lake.

[waldronate]

Follow the first rule of hydrology: liquid water flows downhill by the steepest gradient available. If there is only uphill (as happens in a basin), then water will fill the basin until it finds a way to go downhill again. Repeat application of the first rule as required until you've identified all of the downhill, with the edge of your map generally being treated as a really big downhill drop. Areas converted from basins to filled areas will be lakes.

There are, of course, always some hidden ideas in there, namely that water frozen water generally doesn't flow, liquid water won't visibly flow if it's below ground, and water can evaporate into vapor if conditions are correct. Surface flow may also appear intermittent depending on local conditions. Water may flow into a hole and never reappear or just appear out of a hole and go merrily across the landscape (common in karst terrains). Water may start out frozen as a glacier, thaw into a river, and disappear into desert sands (common in interior deserts). Water may flow in fantastic loops and bends across nearly-flat terrain if it has no other way to balance its energy budget (see your favorite meandering river). Rivers may split apart into lots of smaller channels if conditions are right (anastamosing river). But the first rule continues to apply!

Your maps seem to have a recurring "feature" in the form of little basins where rivers join. If water were flowing downstream, those basins would be filled with lakes. Some hydrology sims (e.g. Wilbur) have this misfeature due to how downstream flow is calculated and applied. It can also occur in real-world DEMs that are undersampled because a relatively narrow gorge at an unfortunate angle can be sampled as a saddle next to a basin, which isn't hydrologically accurate. If you've got a set of vector contours, it can be easier to dispose of the little pits than it is to draw lakes everywhere. Some programs (again, Wilbur is an example with which I am familiar) have an option to fill basins or to fill basins as lakes and either of those options will take a lot of the drudgery out of the work.

I marked up some of the places that are likely to be lakes in your example map:
Screenshot-2022-09-15-121349.jpg

[acrosome]

Yes, google "endorheic basins" for exceptions. And there are occasional oddities like the Okavango Delta. But I didn't really think he was wanting to get into that sort of stuff...

[waldronate]

I had already hit my quota of big words with "anastamosing", sadly. I have noticed over the years that lots of folks don't think about how water only goes downhill unless it's provided with sufficient energy not to (e.g. hydraulic jumps and evaporation), so I provided some background. Maybe too much, it's hard to say.