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Various maps of the Continental Divide (the hydrological feature, not the trail) contain a loop that looks like this:

enter image description here

What is the hydrologic meaning of the loop in Wyoming?

Even if it's a basin, it must have at least a theoretical outlet from which a high enough water level would flow.

Or does the definition of the divide treat this as having no outlet, and hence in neither the Atlantic or Pacific watersheds?

Llaves
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  • No water actually flows out. Obviously, if there was enough precipitation to raise the water level high enough, it would flow out at the lowest point. Indeed, if you moved a bit to the west, you could draw a similar no-outflow line around the Great Basin, though parts of it did empty to the sea in late prehistoric times: https://en.wikipedia.org/wiki/Bonneville_flood – jamesqf Oct 12 '19 at 19:01
  • @jamesqf - So this raises the question "should basins with no outflow be considered not to exist in either the Atlantic or Pacific watershed?" You mention the Great Basin. The same would hold for Death Valley, or for that matter any place below sea level, as there could be no outflow to sea level. – Llaves Oct 12 '19 at 22:22
  • Just checked - for what it's worth, the USGS watershed boundaries places the continental divide basin in the Pacfic watershed. – Llaves Oct 12 '19 at 22:42

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No water flows out of it, apparently...

https://www.onlyinyourstate.com/wyoming/continental-divide-basin-wy/

In summary, there used to be a big lake there, now its a basin with no flows into either ocean. Water there seems to be self contained, accumulating from rainfall and removed by evaporation.

Spacedman
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  • self contained is the important part, other basins on the continent are fed from outside the basin. – John Oct 13 '19 at 17:30
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Watershed (catchment) models in GIS, are based on Digital Terrain Model grids of the topography. Errors or uncertainties in the DTM can produce "pools" that will end up being separated from the surrounding catchments. Typically the guidance is to artificially fill these in until they reach the level of the pour point so they will drain to the catchment.

However, endorheic basins (basins with no stream outflow) of various sizes can be actual hydrologic features with physical meaning. Those should be considered as separate watersheds. The reason this one stands out is that it occurs on the major continental divide, not that it is physically any different in terms of surface water flow.

Defining endorheic basins is tricky because some basins can drain in rare wet years. Some may have drained under wetter climatic conditions but now do not. Generally a catchment is considered endorheic if there is no historic record of it draining.

haresfur
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