On Fri, 14 Aug 2020 at 14:21, Kevin Kenny <kevin.b.ke...@gmail.com> wrote:
> > 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. > It's been a long time since I've been near (never on) karst. The stuff I saw had no sign of running water or channels at the time but was pocked with many holes (presumably caused by rainfall alone) large enough for a person or a sheep to fall into. The way the wiki has it, I don't think sinkhole is appropriate for small streams vanishing into small holes that even a cat couldn't drop into. > > 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. > Ah-ha. I recently saw a network that could be one of those. Search the map for "Teifi Pools" then zoom out, a lot. I fixed some lake names, fixed which of two tributaries was the main one on a river and a couple of streams, and named a couple of streams. Oh, and added a dam to a reservoir. I didn't do anything about tweaking the waterways that had been crudely mapped, or adding the tangle of waterways shown on OS OpenData StreetView that haven't been mapped. That might be an alluvial fan with intermittent and ephemeral waterways, although the slope countours don't seem to support that idea. Doing a full tidy-up on that was more work than I felt like doing, so I didn't worry about it. I wouldn't even have done that much, but the local newspaper mentioned the area and I corrected a few egregious errors as well as adding a locality so that it would at least turn up on a search. Now I've had another look around the wider area to try to identify if it's an alluvial fan I see more misnamed streams and missing streams. I've just fixed some of them. I want to forget the entire place exists - far too much work to do it all right. -- Paul
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