Rivers run. But can they drown?
Yes. Along many coasts, the sea has climbed over old river valleys and stayed. The result has a name: a ria. Watch one drown.
20,000 years ago, the sea sat about 120 metres lower.
Ice sheets were at their biggest. They held so much water that the oceans shrank.
Then the ice melted. The water ran back to the sea. Coastlines moved, almost everywhere.
Estimates run from 118 to 130 metres. A 2018 study in Nature used drowned coral reefs off Australia and put the low at about 125 to 130 metres below today. At the Great Barrier Reef, the sea then rose about 3.5 millimetres a year for around 4,000 years.
Next: rivers kept running while the sea was far away. Where did they go?
The sea climbed about 120 metres, then stopped
The path between the two points is a rough guide. Sea level rose fast at times and slowly at others. Sources: Yokoyama et al., Nature, 2018; Britannica.
Water cannot drown. A valley can.
When the sea was low, rivers cut deep valleys into land that is now under water. Then the sea rose and flooded the lower end of each valley.
The river's mouth went under. The valley became an arm of the sea. Geographers call it a ria, from a Galician word that comes from río, river.
A ria has branches, because the old river had branches. The coast looks ragged, like fingers.
Fjords look similar, but glaciers carved them. Today geologists use the word ria only for river valleys that ice never carved. In England, the Kingsbridge Estuary is a large ria with almost no river in it, only small streams.
Next: can you stand beside a ria today? Yes. A very big one.
Same sea. Two different carvers.
Cross-sections are simplified drawings. Source: Wikipedia, "Ria"; Scottish Geology Trust.
Chesapeake Bay is the drowned mouth of the Susquehanna River.
Chesapeake Bay, on the Atlantic coast of the United States, is one of the world's largest rias. It is the largest estuary in the country.
Starting about 10,000 years ago, the rising sea flooded the Susquehanna's valley. The bay is nearly 200 miles (300 km) long.
You can still find the old river. Deep holes in the bay floor, more than 170 feet (52 m) in places, are thought to be pieces of its channel.
Sources give different ages. Some say the bay began about 10,000 years ago. Others say it reached its present shape only about 3,000 years ago. Both can be true: flooding was slow, and shorelines kept changing.
Next: the Susquehanna still feeds the bay. But how much water do rivers really hold?
One river still gives the bay about half its fresh water
Share of fresh water entering the bay. Source: Chesapeake Bay Program.
All the world's rivers hold about 0.0002% of Earth's water.
Oceans hold about 96.5 percent of all water. Glaciers and ice hold about 2 percent. The small amount of fresh water on the surface is mostly locked up in lakes.
Pour Earth's water into 400 squares. The oceans take 386. Ice takes 7. Water underground takes 7. Rivers would need about one thousandth of a square.
By our math from USGS figures, the oceans hold about 630,000 times more water than rivers. Lakes hold about 83 times more.
Some textbooks round the ocean share to 97 percent. USGS gives 96.5. Rivers are 0.49 percent of surface fresh water. The ratios are our calculation from USGS volumes, not a published figure.
Next: numbers like these can fool you. Look at what grew beside those thin ribbons.
Rivers are too small to draw
One square is 0.25% of Earth's water. Source: USGS, "Where is Earth's water?"
Yet the first big societies grew beside rivers.
Early Mesopotamia grew around the Tigris and Euphrates. Egypt grew around the Nile. The earliest Chinese societies grew around the Huang Ho, also called the Yellow River.
Rivers gave four things.
- Fresh water for people and animals
- Water for crops, once farming began
- Fish to eat
- Fast, easy travel
Start dates are rough, and sources disagree. The Nile flooded on a steady schedule each autumn and left rich silt. The Tigris and Euphrates flooded less predictably, so people had to build canals and flood defences. The Indus valley is another early river civilization.
Next: back to the Chesapeake. Did the drowned river stop working?
Four rivers, four head starts
Hatched parts show where sources give different start dates. Soft bars are single rough estimates. Sources: Tennessee State University course notes; Vedantu.
The drowned river still serves 18 million people.
The land that drains into Chesapeake Bay is home to more than 18 million people. The bay holds more than 18 trillion gallons of water.
Trade and fishing earn money on it. Two big road links cross it: a 4-mile bridge near Annapolis and a 17-mile bridge-tunnel in Virginia.
The sea can cover a river's valley. It cannot undo what the river gave us.
The Bay Program counts people in the whole watershed, which stretches about 524 miles from Cooperstown, New York to Norfolk, Virginia. Not all of them live on the shore.
The whole story in five steps
- 1Ice locks up water. The sea falls about 120 m.
- 2Rivers cut deep valleys on dry land.
- 3The ice melts. The sea rises.
- 4The valleys drown and become rias.
- 5People gather where rivers meet the sea.
Step 5 is a summary of the sources above, not a measured finding.
Four numbers to keep
How far the sea fell at the peak of the last ice age, roughly.
Britannica; Nature, 2018Years ago, about, that Chesapeake Bay began to form.
Chesapeake Bay ProgramShare of all Earth's water that sits in rivers.
USGSPeople living in the bay's watershed.
Chesapeake Bay Program