Properly mapping dry washes

Some of the USGS rendering was similar to sand, and some of it was distinct.

But I don’t think we want to confuse the rendering style with the underlying tags. There might be several different tags that could end up being rendered the same way.

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I’m in favor of dedicated tagging for dry washes. To me an intermittent waterway is wet a significant portion of the time and dry for another significant portion. Washes are dry most of the time and that makes them a different kind of feature in my mind. The same also applies to dry lakes and ponds.


I am generally in favor of experimenting with increasing the specificity/granularity of tagging. Renderers catch up, mappers get more savvy etc. If we pick a few things now, and they are applied reasonably consistently, we can always decide “Well that didn’t work” and do a bulk edit into the more general tagging.

I was joking with another mapper about how I’d be happy with anything that isn’t natural=water, water=no_water.

I have been mapping in the Middle East and encounter this more often than not. One potential tag would be natural=water, water=inshallah_water.

I did notice that wadi has been used (on linear features) in California… That doesn’t help with area features, of course.

Perhaps a surface tag in addition to something that says “this is a wash”?

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If you notice above, USGS doesn’t even bother trying to map the water course through wide dry washes. For the sake of OSM’s data model, I think using waterway=* ways to approximate the water course is worthwhile, especially given the ongoing discussion about mapping virtual waterways.

We might not be having this discussion right now if it weren’t for the effort to analyze the connectedness of waterways in OSM. That depends on having waterway=* ways to make the connections.

That was a combination I thought might work well for the area of the wash. Maybe natural=wash or natural=dry_wash could work to define the area?

Hmmm. Maybe terminology has changed since I was raised in the Sonoran Desert area of Arizona, but “dry wash” seems redundant. Though I guess it might help explain what a “wash” is to people unfamiliar with the term.

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Arroyo and wadi are two other terms for dry waterways I’m familiar with. I’m sure there are many others. Perhaps rather than using any of these regional terms (none of which seem to be British English) we might build off existing tagging for perennial waterways.

Linear way tags


  • waterway=river
  • waterway=stream

Potential tags for (usually) dry equivalents:

  • waterway=dry_river
  • waterway=dry_stream

Area tags


  • natural=water + water=river
  • natural=water + water=stream

Potential tags for (usually) dry equivalents:

  • natural=dry_river
  • natural=dry_stream

A dry_lake would also fit into the scheme that uses dry_river and dry_stream.


There is a dry lake proposal that probably just needs to be finalized and voted on.

But what’s the distinction between natural=dry_river and natural=dry_stream?

I was thinking dry_stream would be for a wash that is waterway=stream sized when water is flowing and dry_river would be for a larger one. Basically an easy transition for small and large washes that are currently tagged as waterway=stream|river + intermittent=yes.

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I’m not super familiar with desert environments, but do washes ever have what would be considered “normal” flow for a non-intermittent stream? Or do they alternate between dry and flood-stage? If the latter, a waterway=dry_stream wouldn’t necessarily be jump-able when water is flowing.

This would be my interpretation. Though I’d probably classify a wash less than maybe 5 meters wide as dry_stream which is a bit wider than I think a stream would be tagged if perennial.

The ones where I was raised basically had two states: Dry and full. Though “full” did vary a bit but usually had water across from bank to bank the variation was how high up the banks the water was when full.

@cactolith said it well in the thread on mapping virtual waterways.

There may be seasonal (or less frequent) events where the flow of water is small, in which case it is likely constrained to the active channel. In the case of a significant flood, it’s basically sheet flow across the entire area of the wash.

I guess there’s some variation based on the local topography and weather. As @n76 says, in some areas when the wash floods, it’s entirely full. But otherwise it’s dry.

(Edit) I should add that I’ve discarded the measure of “jump-ability” to determine whether something is a waterway=stream or waterway=river. For me, the distinction is whether it’s wide enough to be mapped as an area in OSM.


When is it a wash and when is it an intermittent stream? When is it an area and when is it a line? Since the distinction is very vague, I’d suggest the same tag for both. The NHD washes are often just a vaguely wide spot in a gulch.

It’s possible that Colorado is over represented due to the nature of the NHD data or the import.
Currently there are a huge number of dry gulches tagged as waterway=stream and intermittent=yes. It seems like reasonable tagging for the most part, but sometimes I question whether these features are relevant enuf to even be in the data base. Most are not seasonal, but would have flow once or twice a year or once or twice a decade. Most are narrow gullies. Most have no name, but if they have a name it’s Something Gulch. Ephemeral seems like a good tag for many of these dry gulches.

In AZ it’s flatter, the intermittent water flow areas tend to be wider and are usually called washes. They also do not seem to be as thoroughly tagged.

For a taste of how thorough the database is near me (for dry gulches):

We make a clear distinction between the course of a river mapped as a linear way tagged with waterway=river and the broad water area of a river, mapped as an area tagged with water=river. The same distinction applies to the course of water through a wash and the broad area between the well defined banks of the wash. I think it makes sense to distinguish between the linear water course and the broader area of the feature.

I don’t personally have much interest in the detailed tagging that goes on in urban areas, but I’m glad that some people do. OSM is richer for it. There’s room for complex tagging schemes for crosswalks and benches, so there should be room to tag natural features as they are. I think OSM data for natural features should be just as rich as the data for urban features. Getting by with tags that don’t quite work is like fitting square pegs into round holes.


When hiking in the desert, a wash can be a pretty good landmark for navigation.

For urban vs rural tagging, I seldom bother with power lines in a suburban or urban area. There are just too many and provide little information for my suburban/urban map use (mostly car navigation). But in the desert they are a feature that can be visible for miles and good for keeping oriented while hiking so I am likely to map power lines there. I guess it comes down to what you plan on using the map for and how you personally use a map.

That said, almost any slight terrain feature could carry water during a downpour and we probably shouldn’t clutter the map too much with every wrinkle in the landscape being tagged as an ephemeral stream. For me, a wash has to be long enough, maybe 1/2 mile, and/or wide enough, maybe 3 meters, to count.

I look forward to a consensus on how to tag these desert washes so that I can improve my mapping and update the scripts that generate my hiking maps from OSM data.


I’m 100% with this. We need to be able to distinguish a significant wash from a very small gulch, one of many.

This is pretty much what we have some places, every wrinkle. We probably should keep the data that’s there, but the maps don’t need to show all of them. Perhaps there does need to be a distinction between intermittent and ephemeral.

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Sadly, the proposal for ephemeral=* failed and everything has been pushed into intermittent=* and seasonal=*. Those are very broad brushes with which to paint the range from wet to dry.

I recall being disappointed that the ephemeral tag failed to get support.

But I also think a lot more states could be defined for when water is present: Perennial, seasonal (or perhaps list which season, i.e. “summer”), vernal (at least for pools/ponds), intermittent and ephemeral. Rather than adding a bunch of *=yes tags it seems to me that something like water_present=perennial | seasonal | vernal | intermittent | ephemeral would be better than what we have. Though vernal is probably just a subset of seasonal.

That would be for tagging the main waterway centerline. I am beginning to like the water = dry_stream | dry_river suggestion for the area tagging between the banks.