17

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?

Improve this question
5
  • 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, 2020 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, 2020 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, 2020 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, 2020 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, 2020 at 15:08

5 Answers 5

Reset to default
43

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.

Improve this answer
16
  • 22
    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.)
    – user2169
    Nov 28, 2020 at 21:30
  • 8
    @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, 2020 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, 2020 at 21:36
  • 2
    @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, 2020 at 19:26
  • 1
    Ah sorry, now I understand what you mean! Initially I understood what you said to be a feature on some maps: a modification of the contour lines to indicate flow direction; but instead it's a natural effect due to the flow of the river altering the terrain.
    – Aaron F
    Nov 30, 2020 at 22:11
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.

Improve this answer
2
  • 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, 2020 at 12:28
  • Given how weird and wonderful nature is, I am waiting for someone to come up with an example of a river flowing uphill, at least for a bit.
    – Jörg W Mittag
    Dec 1, 2020 at 16:35
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 curvature of the elevation lines are ambiguous (I guess the elevation model used to calculate elevation lines is not sufficiently detailed here): contour lines 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).
In this particular rendering every elevation line is labelled somewhere, so one can also track down the information

Improve this answer
4
  • 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, 2020 at 18:25
  • the contour nearest the east cave is 560 and nearest to west cave is 510, so I feel its fairly clear which direction it is flowing..#
    – Ken
    May 31, 2021 at 13:30
  • @Ken: well, it's been a while since I was there. But IIRC, near the east end the river is quite deep in, at the bottom of the 3 pits (which are several 10s of m deep), and in between it is still underground. The valley is quite a bit steeper than the contour model used by opentopomap suggests and a photo of a local sign carnets-voyages.org/wp-content/uploads/2019/03/… shows the 550 m contour line clearly all the time above the gorge. Unfortunately, the elevation of the bottom of the valley is not entirely clear here, neither. But it's much closer to reality, and...
    – cbeleites unhappy with SX
    May 31, 2021 at 14:15
  • allows correct judgment of the flow direction. Take home message: careful with the elevation model behind maps in fragmented terrain. (I've noticed similar differences also for some topographic map series in Germany, the newer ones relying on digital surface models not being as precise on such terrain as some older map series)
    – cbeleites unhappy with SX
    May 31, 2021 at 14:17
10

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)

Improve this answer
7
  • 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, 2020 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, 2020 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, 2020 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, 2020 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, 2020 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.

Improve this answer

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