1. How Much Salt Do Rivers and the Ocean Contain?
When we compare the dissolved substances in ocean water and river water, the difference is huge. According to the USGS, ocean water contains about 35 grams of salt per kilogram, which is about 3.5% [2]. In contrast, a 1963 paper estimated that the average river contains only about 0.12 grams per liter, or 0.012% [1]. This means ocean water is about 300 times saltier than river water. River water has almost no salty taste because it is so dilute, but it is not zero. The same paper estimates that rivers carry about 3.9 billion tons of dissolved material to the ocean each year [1]. The source of these river components is land rocks. Rainwater dissolves a little carbon dioxide from the air, becoming weakly acidic. This acid slowly breaks down rocks, releasing minerals into the water, which then flow into rivers and the sea [2][3]. Additionally, hot water vents and undersea volcanoes add components directly to the ocean [3][2]. Rivers keep adding water to the ocean, yet the ocean does not overflow. Where does the water go?
2. How Evaporation Returns Ocean Water to the Sky and Leaves Salt Behind
Data from Japan's Ministry of Land, Infrastructure, Transport and Tourism shows that global annual precipitation is about 577,000 cubic kilometers [4]. About 119,000 cubic kilometers falls on land. Of this, about 74,000 cubic kilometers evaporates, and the remaining 45,000 cubic kilometers flows into the ocean as rivers or groundwater [4]. Rain falling directly on the ocean is about 458,000 cubic kilometers, though another source rounds this to 460,000 [5]. If inputs and outputs balance, the ocean returns about 500,000 cubic kilometers of water to the sky each year. This is an estimate based on adding the data, not a direct figure from the source. River water is only about 10% of this amount. The ocean does not overflow because evaporation sends the water back to the atmosphere. Only water evaporates; salt stays behind. Lakes with no outlet, like the Dead Sea, become salty for the same reason. The Dead Sea has no outflowing river, and its salinity is about 34% (as of 2011), which is about 10 times that of the ocean [6]. In 1715, Edmond Halley explained the salinity of oceans and such lakes using this same logic [7].
3. Halley's Idea and Joly's Calculation of the Ocean's Age From River Salt
In 1715, astronomer Edmond Halley, famous for his comet, wrote a paper for the Royal Society. He proposed that rivers carry salt particles to the ocean, but only salt-free water evaporates. Therefore, ocean salt increases over time. By comparing past and present salinity, one could find the rate of increase and calculate the age of the world [7][8]. Halley did not have data on past measurements or the amount of salt rivers carried. About 180 years later, Irish geologist John Joly actually performed the calculation. In the late 1890s, he measured the sodium carried by rivers each year and divided the total sodium in the ocean by that amount. He assumed the ocean started as fresh water and only accumulated sodium from rivers [10]. His answer was about 80 to 100 million years [9][10]. Today, the Earth's age is estimated at about 4.54 billion years [11]. Joly's answer was about 1/50th of the correct age. It was later pointed out in 1910 that Halley had proposed this method nearly 200 years earlier [9]. Where was the error? The calculation relied on two assumptions: Did the ocean start as fresh water? And does river salt only accumulate in the ocean? Let us look at the origin of the ocean first. Note: This calculation assumed the ocean was originally fresh water and only accumulated sodium from rivers [10].
4. Rubey's Doubt: Was the Ocean Fresh Water at the Start?
Joly's calculation assumed the ocean was originally fresh water and only accumulated salt from rivers [10]. American geologist William W. Rubey doubted this. In 1951, during his presidential address to the Geological Society of America, he reconsidered the history of ocean water using mass balance [21]. Rubey counted substances that become gases, like water, chlorine, and sulfur. He added the amounts in today's oceans and air to those trapped in ancient rocks, then subtracted what weathering could produce. For chlorine, out of a total of about 306 units, weathering could explain only about 5, or 2% (units are 10^20 grams). Sulfur was about 80% unexplained, and water was almost entirely unexplained [21]. These figures are estimates from 1951 data and may differ from current research. The ratio of the missing amounts resembled the composition of gases from volcanoes and hot springs. Rubey concluded that gases from Earth's interior were the source of ocean and air materials [21]. Volcanic gases mainly contain water vapor, sulfur dioxide, hydrogen sulfide, and carbon dioxide [26]. They also contain hydrogen chloride, which dissolves well in water and causes acid rain [25]. Rubey suggested that sodium could be explained by rock weathering, but chlorine and others needed an internal Earth source. The USGS explains that ocean salt comes from land rocks but also notes that undersea volcanoes supply salt [2]. Rubey categorized scientists into "quick soaks" (who thought everything was there from the start) and "slow soaks" (who thought it accumulated gradually). He leaned toward the slow soak view [21]. This breaks one of Joly's assumptions: the ocean was not formed only by accumulated river salt. Note: Joly's division relied on the premise that the ocean was originally fresh water and only accumulated salt from rivers [10]. The ocean is not water formed only by accumulated river salt.
5. The Acidic Early Ocean Idea and What Experts Think of It
A common explanation for the ocean's origin is that volcanic gases like hydrogen chloride dissolved in rain, making the first ocean acidic. This acidic water then neutralized by dissolving rocks, releasing sodium and calcium, resulting in today's saltwater [27]. "Neutralization" means the acid reacts with rock and decreases, while sodium and calcium dissolve into the water. Chlorine remains in the water. However, experts disagree with the "strongly acidic ocean" part. Rubey calculated that if all chlorine dissolved in the first ocean, the pH would be about 0.3, very acidic. But neutralizing such an ocean would require more rock weathering than occurred in Earth's history, and large amounts of limestone are not found in early strata. So, he thought such an ocean was unlikely [21]. A 2010 review states there was no ocean of hydrochloric and sulfuric acid, and by the time life appeared, the pH was nearly neutral like today [22]. This is an objection to the strong acid ocean theory. On the other hand, calculations suggest the ocean might have been weakly acidic due to high carbon dioxide. A 2025 study says the pH right after ocean formation was about 5, rising to neutral by about 4 billion years ago due to rapid rock weathering [29]. Studies from 2017 and 2018 estimated the pH around 4 billion years ago at about 6.5–7.0 and 6.6 (range 6.2–7.2), attributing lower values to high atmospheric carbon dioxide [23][24]. Estimates of past ocean pH vary from strongly acidic to alkaline, and the value right after formation is still debated [24]. In summary, the story that a "strong acid ocean was neutralized into saltwater" is disputed. The story that a "slightly acidic ocean due to carbon dioxide became neutral through weathering" appears in recent calculations. Regarding chlorine, Rubey thought weathering alone could not explain it, requiring supply from Earth's interior via volcanic gases. Does the salt then continue to accumulate in the ocean? Let us examine Joly's second assumption.
6. Why Joly's Calculation Was Wrong: Salt Also Leaves the Ocean
Joly's other assumption was that all salt carried by rivers stays in the ocean and increases. But in reality, sodium and other elements leave the ocean by becoming minerals that sink or turning into sea spray [10]. The USGS also explains that some components are removed by ocean life [2]. When inputs and outputs balance, the average time an element stays in the ocean is called "residence time." Oceanography textbooks state sodium stays for about 55 million years, chloride ions for about 87 million years, and water molecules for about 3,100 years [12]. In the same ocean, water turns over in thousands of years, while sodium stays for tens of millions. The difference is more than 10,000 times. While figures vary by source, Joly's answer was close to sodium's residence time [10]. The number he intended as Earth's age was actually close to how long sodium stays in the ocean. Calculating with current figures gives about 50 million years (approximate, using values from cited sources). The composition of river and ocean water is also very different. River water is about half bicarbonate ions, followed by calcium [1]. In seawater, chloride and sodium make up about 85% [3], while bicarbonate is less than 1% [13]. Calcium precipitates easily and is taken up by life, leaving the ocean relatively quickly. Sodium and chlorine have fewer removal mechanisms and stay in the ocean [13][2]. Seawater is not just concentrated river water. Calcium, which leaves easily, is low, while sodium and chlorine, which stay, are high. Moreover, chlorine, according to Rubey, needs supply from Earth's interior, not just weathering [21]. Salt also leaves via thick rock salt layers formed from dried ancient seawater, showing large amounts of sodium leave the ocean [10]. Studies estimating past salinity from chlorine suggest ancient oceans were saltier than today, and salt was removed to form new oceans [28]. A review suggests the early ocean might have been 1.5–2 times saltier than today [22]. Figures vary, but this does not fit the idea that river salt accumulated from zero. What happens when these two different waters meet?
7. How River Water and Seawater Meet and Mix at River Mouths
Technical standards from Japan's Ministry of Land, Infrastructure, Transport and Tourism define the transition from river to sea. The area where river water and seawater mix is called the "brackish water zone," with salinity between 0.5 and 30 parts per thousand [15]. The section where tides change the water level is the "tidal influence zone," and the section where salt water moves upstream is the "salt water intrusion zone" [15]. The National Institute for Land and Infrastructure Management explains that at high tide, salt water moves upstream and changes the flow, returning to normal at low tide [16]. The boundary between river and sea is not a line but a moving band with the tides. River water and seawater have different weights. In rivers with weak tidal force and high flow, seawater wedges in underneath, with river water flowing above, creating a clear boundary. This is a "salt wedge" (weak mixing type). The standards note this is common in large-flow rivers on the Sea of Japan side, where tidal variation is small. The Pacific side of eastern Japan often has a "loose mixing type" with a slanted boundary. Rivers flowing into Pacific inner seas have large tidal variation and are often "strong mixing types" with thorough vertical mixing [15]. The brackish zone is also a passage for river and sea fish. Stories about the goby, which lives in dirty water but tastes refined, are found elsewhere.
8. How Chum Salmon Live in Both Rivers and the Sea
According to the Fisheries Research and Education Agency, chum salmon (commonly called "salmon") come to northern Japan's coast from September to December and swim upstream. Hatched juveniles go down the river to the sea the following March to May. The main returning group is 4-year-old fish [17]. Juveniles that enter the sea grow in coastal areas for about 1–2 months before heading to distant seas [18]. River and ocean water differ greatly. Salmon, like other teleost fish, keep their body fluid concentration at about one-third of seawater. This is much higher than river water but lower than seawater [14]. In rivers, the outside is more dilute, so water enters the body. They produce large amounts of dilute urine and use energy to take in salt through their gills. In the sea, water is drawn out of the body, so they drink seawater and expel excess salt through chloride cells in their gills [14]. Fish that can move between river and sea can switch between these two methods. The nasal glands of albatrosses and the eye glands of sea turtles also expel salt using similar mechanisms [14]. Salmon physically pass through the brackish zone between river and sea.
9. A Home Activity: Make a Small Ocean on a Plate and Watch Water Levels
You can see evaporation leaving salt and two waters overlapping at home. Do this with an adult. Dissolve 1/4 teaspoon of table salt in 3 tablespoons of water (about 3%, like seawater). Put it in a shallow dish in a sunny indoor spot. In a few days, the water will decrease, and white crystals will remain. Do not drink it. Keep it away from pets and small children. In a thin cup, dissolve 10 grams of salt in 100 milliliters of water and add food coloring. Gently pour colorless water over the back of a spoon. Because the salt water is denser than the colorless water, if done well, it will form layers. The colored heavy salt water stays at the bottom, and the water sits on top. This is a small version of a salt wedge. Instructions for making rainbow-colored water are in teaching materials from Chuden [19]. On the Ministry of Land, Infrastructure, Transport and Tourism's "River Disaster Information" page, compare the water level graphs of the observation station closest to the river mouth and an upstream station for one day. In the tidal influence zone near the river mouth, water levels change with the tide [15]. Compare if the graph shapes differ (some rivers may not show tidal effects depending on the station location or weirs) [20]. For safety, do not enter rivers or the sea to check this. When visiting river mouths or banks, go with an adult and view from bridges or levees. Do not approach after rain or during high water. Do not drink salt water. Note: Do experiments with an adult. Only water goes into the air; salt remains. Do not enter rivers or the sea to check. View river mouths from bridges or levees with an adult.