Rivers — how this layer is made

What you are looking at

This map shows the world's rivers, coloured by which mouth the water reaches. A whole river system shares one colour, and no two neighbouring systems share theirs, so you can follow one watercourse into another and see where it all ends up. You can switch the colour to show size instead, which Colour describes.

A line grows heavier as the land draining through it grows, under either colouring. That weight follows catchment area in square kilometres, the land whose rain eventually passes the point you are looking at. The Amazon passes about 4,500,000 km² at its mouth. The Thames passes about 13,000 km².

Two sources, and where they hand over

No single dataset draws a river well at every scale, so this map uses two.

At low zoom it draws HydroRIVERS, a modelled river network published by HydroSHEDS, holding about 8,500,000 reaches. A reach is one stretch of river between two junctions, which is how a network is cut up so that each piece can carry its own measurements. Each reach here carries the area of land draining through it, and that is the only thing on the map that can say which of two rivers is the larger.

Its geometry comes from a 15 arc-second grid, roughly 500 m, so every turn it makes is a right angle or a diagonal. We round those corners when the map is built. No point moves more than about 125 m, which is well inside the model's own error.

At high zoom it draws the OpenStreetMap rivers, where more than 2,500,000 rivers and canals have been mapped by people who know them. That is the real course of the water, with no measure of size at all.

The changeover happens at a different zoom for every river. As soon as a river is large enough that its OpenStreetMap course is worth drawing, the HydroRIVERS line steps aside for it. For the Amazon that is zoom 9, and for a river draining 50 km² it is zoom 11. One changeover zoom for the whole map would either draw the big rivers twice or leave the small ones with nothing.

Colour

There are two readings and you can switch between them.

By system is the one you start with. It colours by which mouth the water reaches. Neighbouring systems never share a colour. We work out which catchments touch and give no two adjoining ones the same, which is why the Rhine and the Danube cannot come out alike. Its key shows no numbers, because a colour here is an identity rather than a quantity.

By size colours by catchment area. Its key shows the bands in square kilometres.

Where OpenStreetMap has not been

More than 40% of the HydroRIVERS network has an OpenStreetMap river running along it, and the share depends enormously on where you look. For rivers draining between 10,000 and 100,000 km² it is more than 60%. In Europe and the Middle East it is more than half. In Greenland it is under 5%.

So the map keeps the HydroRIVERS network drawn at high zoom only where nothing has replaced it. In the middle of the Amazon the coarse line stays, because it is the only river there. Over England it gives way.

Canals

We include canals, and we draw them dashed rather than solid. They are about a third of the OpenStreetMap watercourses, and leaving them out empties the Netherlands and much of Hungary. A canal crosses watersheds deliberately, climbing through locks, so the question this map asks of a river may not have an answer for one. We show it in the colour of the catchment it sits in, and the details panel names it as a canal.

Up to date, mostly

We take a new copy of OpenStreetMap every few months, so a river mapped today reaches this map within a few months. The HydroRIVERS network does not change at all, because HydroSHEDS has not revised it since 2019.

Known weaknesses

The HydroRIVERS network is worse above 60 degrees north, where no elevation survey of the same quality exists and a coarser model was substituted. Canada and Siberia are the places to distrust.

Discharge figures are modelled rather than gauged, and are least reliable in dry regions, on glacier-fed rivers and in wetlands.

A river system's outlet is not always the sea. The Okavango ends in an inland sink, and we colour it as a complete system. Which systems count follows HydroSHEDS, which treats the Caspian as a sea, so the Volga does not.

Half of all the OpenStreetMap watercourses carry no name.

Source and licence

River network: HydroSHEDS HydroRIVERS v1.0 and HydroBASINS v1.c, © World Wildlife Fund, Inc. Lehner, B., Grill, G. (2013): Global river hydrography and network routing: baseline data and new approaches to study the world's large river systems. Hydrological Processes 27(15): 2171-2186. hydrosheds.org

HydroSHEDS data are free for commercial and non-commercial use under the HydroSHEDS v1 licence. Watercourses are © OpenStreetMap contributors, licensed under the Open Database Licence, extracted from the worldwide planet file.