16

If you pull up a river on Google maps it's not clear what's downstream from what.

Like the Colorado River... Hoover Dam is downstream of the Grand Canyon, which is downstream of Lee's Ferry, which is downstream from the Glen Canyon Dam. I know this about the Colorado River not from Google Maps but from wikipedia articles talking about it. But not all rivers or streams or whatever have the number of landmarks with wikipedia articles that the Colorado River does. And even if they did, having to do bespoke research on each river is kinda a PITA.

Is there a way to easily know which direction is downstream or upstream for a given river or stream or whatever?

  • 2
    I had always assumed that this is indicated by the direction the river name is written, but a quick survey of my map stash disproved this. Am I imagining this, or is it just a convention that is not followed everywhere..? – phipsgabler Nov 29 '20 at 8:31
  • 3
    @phipsgabler What do you mean by the 'direction' it is written? Don't most maps orient their text towards the top where possible? – Weckar E. Nov 29 '20 at 9:06
  • But you could agree e.g. that the writing direction matches the flow when the label is "above" the river, and the other way round when it's below. Anyway, this seems not to be the done, I don't know how I came up with the idea. – phipsgabler Nov 29 '20 at 11:32
  • @phipsgabler and if the river is wide enough that the name is in the water (a fairly extreme/estuary example, but close to home)? – Chris H Nov 29 '20 at 18:32
  • @phipsgabler I've never seen a river that runs east-west with its name upside-down on a typical map where north is at the top of the map. – Nuclear Hoagie Nov 30 '20 at 15:08
42

USGS has a web map called Streamer that will let you trace a stream or river in the US. If you click on a point on a river, it will highlight every part of the river that's downstream of that point. Or you can choose the "trace upstream" option and it will highlight everything upstream of that point. For example, here's everything upstream of Hoover Dam:

enter image description here

Aside from a map such as that one, there are some visual clues you can look for on a normal map.

Dams and Reservoirs - if there's a dam, there's usually a reservoir upstream of the dam. So if your map shows a dam and reservoir, you immediately know which way is upstream on that river. Even if the dams aren't marked on the map, sometimes you can tell where they are by the shape of the reservoir edge. In a graphic map, a dam creates an unnaturally straight line with a wide stretch water along one side of it and a narrow strip of water plus bare land on the other side. Of course in satellite view you can see the physical structure of the dam itself.

enter image description here

Unfortunately you may not be able to tell from the shape of the reservoir which end is the outlet and which end(s) is/are the inlet(s). For example, Lake Mead, the reservoir above the Hoover dam, looks like it has a dam on each of its three corners. You would have to zoom in on each corner to see if there's a dam there or not.

enter image description here

Confluences - a confluence is the point where two or more streams flow together. Streams usually flow together at an acute angle. If you draw the acute angle on the map, it makes an arrow that points downstream. This is not 100% reliable, but it's helpful nonetheless.

enter image description here

The "Rule of Vs" - on a contour map, contour lines that cross a river will bend into a V shape, with the V pointing upstream (the opposite of confluence angles).

enter image description here

Image Source

Find one end of the river - trace the river until it either flows into a lake or sea, or until it peters out. When it gets too narrow to see on the map, that's (approximately) where the headwaters are. In the case of major rivers like the Colorado, you have to trace the river for quite a long way.

  • 18
    If you've got a topo map, then you don't need to guess which way is downhill, because you know the elevations, which are marked as numbers on the contour lines. (There can be cases where you can't tell what elevation a particular line is at, but that's uncommon.) – Ben Crowell Nov 28 '20 at 21:30
  • 7
    @BenCrowell, only if you're using something other than Google's terrain view. Google is remarkably reluctant to put numbers on the contour lines. – Mark Nov 29 '20 at 1:50
  • 5
    @BenCrowell The rule of V's makes it easy to tell at a quick glance which way a river is flowing. To tell from the elevation you have to locate the elevation number on two different topolines and trace them until they cross the river. It takes longer, but it's less error-prone. I use the rule of Vs for quick map reading, and use the elevation numbers to double-check if I'm not sure. – csk Nov 29 '20 at 21:36
  • Topo maps also have spot elevations which indicate the elevation at one point. So locating 2 points with known elevations you can tell which way the terrain is sloping and if you know the contour interval (usually noted on the bottom of the map) you can make a quick mental calculation to even tell how higher or lower a river section is. – Techie_Gus Nov 30 '20 at 12:08
  • 1
    @AaronF There may be some few areas with unique geography where the rule of V doesn't apply at all, but it generally holds true throughout the world. It becomes less reliable along rivers that have been heavily modified by humans (usually for flood control or irrigation). – csk Nov 30 '20 at 19:26
13

Look up a topographic map ("topo"), such as the USGS maps, which are free online. These maps show elevations.

As a quick and dirty expedient, you can also just try zooming out and seeing which end of the river drains to the ocean, or which end is up high in mountains or hills. I can imagine that this might not always work, e.g., if the river flows underground in some places, or dies out, or has loops and branches that connect in complicated ways.

  • 3
    How do I zoom out on a paper map? – Mark Nov 29 '20 at 1:50
  • 5
    @mark: the OP specifically mentioned google and other online maps when asking the question. – whatsisname Nov 29 '20 at 7:04
  • 2
    @Mark Paper maps usually have elevation lines and users of paper maps are expected to be able to read them. Then the question is moot. – Nobody Nov 29 '20 at 17:06
  • 1
    @Nobody Not all landscapes are steep enough such that a river may be expected to be crossed by a contour line. For example, Dutch topo maps have contour lines ever 1.25 metre, but on a single 1:25k sheet the river doesn't drop by that much. Unless you have at least two contour lines crossing a river in a single map sheet, simply counting contour lines doesn't work. – gerrit Dec 1 '20 at 12:28
  • 1
    @Mark hold it further away from your face – TylerH Dec 1 '20 at 14:36
12

In addition mostly to @csk's answer, there are maps that give the direction of rivers, e.g. OpenTopomap

Colorado with direction arrow

For most parts of the world, this information is redundant at the first glance since you can see the overall (larger) shape of the valleys in the map and follow the valley until it is clear whether you go upriver or downriver, but there are some interesting exceptions:

Rak/Slovenija(https://opentopomap.org/#map=16/45.79082/14.29744)

Yes, the side creeks point into the correct direction (but not very distinctly). But the elevation lines are ambiguous (I guess the elevation model used to calculate elevation lines is not sufficiently detailed here): elevation close to the caves suggestes that the river flows out of the caves at both ends. I've been there and can confirm that as the arrows say, it flows east to west, though. From one cave into the next (sinkhole).

  • 2
    Open TopoMap is a clearer presentation of OSM data for this than my ideas, +1 and I'll link to your answer in mine – Chris H Nov 29 '20 at 18:25
9

The answer is often to not use Google maps, and this is no exception.

If you want an online source, Openstreetmap.org can help - the cycling layer shows contours so you can use the methods in another answer, but if you're at a PC and select "Edit" the default editor shows arrows pointing downstream. As cbeleites unhappy with SX says in another answer, there are better ways to see this data.

This information is available to apps that use OSM data, but the only such app I've got doesn't show the direction of flow (unnecessary on a bike computer, and I could always refer to contours)

  • Or if you do use Google Maps, use the satellite view, and you'll almost always be able to see the direction of flow from the shape of the land. (And just out of curiousity, why on Earth would I think to use something called OpenSTREETMap to see rivers & landforms? If it does have useful information, it badly needs a name change :-)) – jamesqf Nov 29 '20 at 16:55
  • 1
    @jamesqf there's a hint in my comment to WeatherVane - in the UK the government mapping agency owns the copyright to its output, and is funded by that. It was late to the digital world and is still expensive, though a service called streetmap.co.uk licensed and republished it. OSM was founded to provide a completely free equivalent – Chris H Nov 29 '20 at 17:39
  • 1
    @jamesqf my earlier comment was based on a misunderstanding but OSM started as an open provider of UK street-level data, with other rights of way following soon afterwards. It's now global (which is why I suggested it), and probably the most complete global map. Recently I've been adding public defibrillators that I've passed on my rides, even a few benches from yesterday - the level of detail is amazing. Many open equivalents to proprietary services have changed their names, but others have too much recognition under their current names – Chris H Nov 29 '20 at 17:54
  • 1
    @jamesqf: the missing link here may be that the OSM project collects all kinds of map data, topographic and many others. From these data, various different maps ("layers") are rendered (computed, produced) with different styles. The style deciding which of the available information to render (and how). Opentopomap in that sense is openstreetmap just like the standard rendering or opencyclemap are. Opentopomap renders typical features of a topographic map close to how topographic maps are usually done (in case that varies regionally, it is close to what I'm used to in Germany). – cbeleites unhappy with SX Nov 30 '20 at 10:12
  • 1
    @cbeleites unhappy with SX: Yes, I quite understand all that now, since you've explained it. (For which my thanks.) All I've been saying is that without that explanation I would never have thought to look at it, simply because of the name. – jamesqf Nov 30 '20 at 16:53
1

As other answers have said, you need a map which shows contour lines. From seeing which end of the river is higher, you can work out which way it's flowing.

Google Maps does do this, but it's not very user friendly.

First, turn on "Terrain" from the "layers". This will show you a shaded map giving a rough clue to the shape of the landscape. This may be good enough, but it's not quite what you want.

Zoom in gradually. At some level of zoom, the map changes from just showing shading to also showing contour lines. Zoom in slowly, and the numbers showing height in metres will become visible. You can now work out your river direction as normal.

Zoom in a little too far though, and the contour lines are removed. Like I said, not very user friendly.

Not the answer you're looking for? Browse other questions tagged or ask your own question.