Rivers and lakes: the water that shapes borders
Rivers made most of the world's borders and most of its cities, and they are still moving, which is why a surprising number of boundary disputes are really arguments about a channel that shifted. Lakes are stranger still: what counts as one is a legal question as much as a geographic one.
Which river carries the most water to the sea?
The Amazon
The Amazon discharges roughly 209,000 cubic metres per second, about 20 percent of all river water reaching the oceans, and more than the next seven rivers combined. The Nile is longer by some measures but carries under 3,000.
Source: Discharge figures, US Geological Survey
How many countries does the Danube flow through?
Ten
The Danube passes through Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine. That is more countries than any other river in the world.
Source: International Commission for the Protection of the Danube River
Is the Caspian Sea a sea or a lake?
The largest lake in the world
It has no outlet to any ocean, which makes it a lake: the largest, at about 371,000 km2. Its legal status was disputed for decades because seas and lakes are divided between neighbouring states under different rules.
Source: 2018 Convention on the Legal Status of the Caspian Sea
What happened to the Aral Sea, once the fourth-largest lake on Earth?
It shrank drastically
Soviet irrigation projects diverted the Amu Darya and Syr Darya from the 1960s. The Aral lost more than 90 percent of its volume, leaving fishing boats stranded tens of kilometres from water.
Source: UNEP and NASA Earth Observatory time series
Which of these major rivers flows north?
The Nile
The Nile runs north from equatorial Africa to the Mediterranean. Water flows downhill, and northern Africa is lower, so a north-flowing river is unremarkable rather than strange.
Source: Standard hydrology; Nile basin topography
How many bridges cross the main stem of the Amazon river?
None
Not one bridge crosses the Amazon's main channel. The river is enormous, the banks are largely rainforest with few roads to connect, and ferries do the job instead.
Source: Brazilian transport ministry records
Which is the deepest river in the world?
The Congo
The Congo reaches depths over 220 m in places, deep enough that its lower reaches hold fish species that have evolved in near-total darkness and isolation.
Source: American Museum of Natural History Congo surveys
What did researchers report flowing far beneath the Amazon in 2011?
A slow flow of groundwater
The Hamza flow moves through porous rock at metres per year rather than metres per second, so calling it a river is contested, but it broadly parallels the Amazon at about 4 km depth.
Source: Brazilian National Observatory, 2011 presentations
Which country has the most lakes?
Canada
Canada holds more lake area than the rest of the world combined, with over two million lakes carved by retreating ice sheets. Finland is the leader by lakes per person.
Source: Natural Resources Canada
Finland is nicknamed the land of a thousand lakes. How many does it actually have?
About 188,000
Finland has roughly 188,000 lakes larger than 500 square metres, one for every 30 people. That is the highest ratio in the world.
Source: Finnish Environment Institute
Roughly how many countries have no permanent rivers at all?
About twenty
Around twenty, mostly desert states and small islands including Saudi Arabia, Qatar, Kuwait, Libya, Oman, the Maldives and Malta. Some have wadis that flow only after rain.
Source: FAO Aquastat water resources data
Where does Malta get most of its drinking water?
Desalinated seawater
Malta has no rivers or lakes. Reverse-osmosis plants supply well over half its drinking water, with the rest from limited groundwater.
Source: Malta Water Services Corporation
Roughly how wide is the mouth of the Amazon?
About 200 km
Including Marajo Island, which is roughly the size of Switzerland, the mouth spans around 200 km. Fresh water is detectable in the Atlantic over 100 km offshore.
Source: Brazilian Institute of Geography and Statistics
Roughly what share of Egypt's population lives along the Nile and its delta?
About 95 percent
Almost the entire population lives on roughly 5 percent of the land. Herodotus called Egypt the gift of the Nile, and satellite night images show the river as a bright ribbon in blackness.
Source: Egyptian Central Agency for Public Mobilization and Statistics