On Fri, Aug 14, 2020 at 7:08 AM Paul Allen <pla16...@gmail.com> wrote:
> On Thu, 13 Aug 2020 at 06:42, Mark Wagner <mark+...@carnildo.com> wrote: > >> >> For a larger and far more dramatic example of this sort of situation, >> look at the area to the west of Death Valley Playa. It looks like >> someone stacked hundreds of river deltas on top of one another, but >> forgot to add the water. >> > > As I understand it (possibly not all that well) a sinkhole as the wiki > defines it > is a large hole in the ground which water enters and vanishes without > pooling. > What Ordnance Survey calls "sinks" appears to be more akin to a hole in a > golf > course that water enters and vanishes. What Ordnance Survey calls > "spreads" > is a sand or soil or gravel surface that water vanishes into without > pooling and without there being any noticeable hole. > The WIki picture of a sinkhole happens to be large, but in karst terrain they come in all sizes. https://www.openstreetmap.org/node/5599524737 is a sinkhole of quite a small stream. I couldn't find a good way to tag the rise a short distance to the west. https://www.openstreetmap.org/way/226924460 is a much larger sinkhole. In a wet season there's significant outflow to the east, but in a dry season all of the outflow from the lake runs through the caves, exiting through cracks in the limestone below the cliffs to the east. Many of the small streams thus formed haven't been mapped because there are significant technical challenges to mapping them. GPS coverage at the cliff bases is so poor that one would probably have to resort to alidade and plane table, and the evergreen cover is dense enough that you can't see much that's useful on satellite imagery. I'm not sure if any of those fit what you have and maybe what you have is > more of a network of intermittent streams. > What Mark is showing is usually called an alluvial fan. https://en.wikipedia.org/wiki/Alluvial_fan Some fans have well-defined (perennial or intermittent) distributary streams flowing through them. Often, though, most of the stream channels are ephemeral in nature. Sometimes an individual channel was cut in a matter of hours by a debris flow coming from upriver. In arid climates, it's entirely possible for the entire flow of the stream, except during flash flooding events, to vanish by percolation and evaporation, so that there is no river downstream. There's no well-defined sinkhole, and no well-defined specific point at which it transitions from perennial to intermittent, intermittent to ephemeral, ephemeral to a dry wadi that has seen water only in geologic time, eventually disappearing entirely into a salt flat. It's relatively rare to find a fan that's still actively depositing sediment. One example is that Mòlèqiē Hé (莫勒切河) in Xinjiang forms an enormous and nearly unique one near 37.4° north, 84.3° east. -- 73 de ke9tv/2, Kevin
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