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In my world, there's a forest that contains several creeks that resemble the Bolton Strid in Yorkshire. They look innocent enough and are only about 7 feet across at most, but they're hundreds of feet deep with underwater cave systems and extremely fast currents, and people that fall in or are pulled in by creatures almost never return. What natural conditions would cause several of these creeks to form in the same general area? If it's relevant, the planet has 30% more gravity than earth, and the forest itself is about half the size of the Amazon.

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    $\begingroup$ The question isn't 'what conditions cause the creeks to exist.' Its 'what causes the underground cave systems that sustain them to exist'. The creeks are the result of water from an extensive and fast flowing underwater cave system breaking though to the surface in the local area. But even then only some, not all of the underground flow reaches the surface. Also by way of clarity you won't get creeks that narrow being hundreds of feet deep their entire length unless your including the depth of the water in the cave systems below them. Fall in? Eventually you get dragged back underground again $\endgroup$
    – Mon
    Feb 18 at 0:15
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    $\begingroup$ @mon is correct, but you already have that solution in your question ("...with underwater cave systems..."). Noting that we're here to help you build an imaginary world and not to make your fiction factual... what kinds of details are you looking for (knowing that you can't have all of them as you're allowed to ask only one, specific, well-focused question)? The water source? The geology of the caves and surface? The nature of the water's exit or collection point(s)? This is a much more broad Q than you might think... do you really need all this detail? What problem are you trying to solve? $\endgroup$
    – JBH
    Feb 18 at 3:22
  • $\begingroup$ You can ask on Earth Science SE, I suspect some folks over there are already stationed on site $\endgroup$
    – user6760
    Feb 18 at 5:57

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What you're basically looking for is an area where a seabed accumulated a thick layer of limestone, and was then raised well above sea level by tectonic uplift. The most likely scenario here is a collision between two continental plates with a shallow sea between them. This region then spends a few million years high above the water table, either because the region is at high elevation, or because it is relatively arid. This allows water to percolate through the limestone, forming large cave networks.

Then, either the region experiences subsidence, perhaps due to those continental plates pulling apart, or the climate becomes significantly wetter, bringing the water table up to surface level, or perhaps a river changes course into the region. Whatever the means, the caves are left submerged. The dissolution of the limestone would cause sinkholes and deceptively deep 'creeks' which are actually full-blown rivers running underground and occasionally surfacing where the water table was high enough, or the ground collapsed. These rivers would also tend to disappear underground into the cave network and emerge elsewhere. Needless to say, someone sucked down in an area where the river ran underground would likely drown.

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I would expect that a fast flow of water would normally carve a V shaped cut into the bed into which it moves, as we were taught in geography lessons since primary school.

The only way around it which I can imagine is that the geology of the area comprises intrusions of very hard rock, something like granite, separated by walls of much more soft rock, like limestone, so that the water can easily carve its passage through the soft rock without appreciably touching the harder rock.

Maybe the area formed after a major impact event, where an meteor hit the ground and its fragments got mixed with the previously present local soft rock.

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Frame challenge.

The Bolton Strid is an unusual formation. Water travels as a river, then drops dramatically through caves, then re-emerges much lower down. There are a few strips like this but they tend to be geologically short-lived. You need the combination of water, rock that is easily shaped by water, and some rather unusual topography that gives you a subterranean waterfall rather than the usual sort.

There is a Tom Scott video on the tides at Saltstraumen, Norway. If you have hard rocks carved by ice into deep fjords, then you can get a tidal race. These maybe be longer lived, and happen all along the coast.

Here, for comparison, is a Tom Scott video on the Bolton Strid

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