We just had visitors (in Monterey, California) from Montevideo, Uruguay.

Not being very familiar with Uruguay, after having taken them to Big Sur, I asked them how far they lived from the ocean.

They said it was hours away, and that they lived on a river.

I later looked at a map and, AFAICT, they do live right on the Atlantic Ocean.

If they view the river that flows into the Atlantic to the west of them to be the river they live on, that seems very odd to me - from my point of view, once the river dumps into the ocean, that's ocean, and the river stops where they meet.

The body of water gets very much wider at that spot, so it certainly "looks" more like the ocean on the map than a continuation of the river. But it leads to the question: when a river dumps into an ocean, exactly where does it transition from being the river to being the ocean?

  • $\begingroup$ They say they live right on the river. My question is why do they call a river what to me seems to be the ocean? $\endgroup$ Commented May 16, 2017 at 19:16
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    $\begingroup$ Similar situation at the Saint Lawrence River. I would imagine geophysicists might use salinity to define the edge, but let's see if someone who actually knows will answer :) $\endgroup$
    – gerrit
    Commented May 16, 2017 at 19:24
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    $\begingroup$ After reading aretxabaletas answer, I realise there may also be a communication issue as to the difference between sea and ocean. Looking at the satellite photo and map, I can imagine why someone would say Montevideo is located right at the sea, but still quite far from the ocean. $\endgroup$
    – gerrit
    Commented May 16, 2017 at 21:23
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    $\begingroup$ @gerrit, it is tricky. When you look at the mouth of the Amazon for instance and you see low salinities extending into areas that are clearly not the "river". That is the reason we have terms like estuary, delta and so on. $\endgroup$
    – arkaia
    Commented May 16, 2017 at 21:29
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    $\begingroup$ B. Clay Shannon -- Is the San Francisco Bay "ocean"? The Rio de la Plata drains an area over 16 times larger than does the San Francisco Bay. The San Francisco Bay drains mostly semi-desert while the Rio de la Plata drains the Andes and rain forests. This makes the Rio de la Plata considerably less saline than much of the San Francisco Bay. $\endgroup$ Commented May 17, 2017 at 1:50

2 Answers 2


In the case of Río de la Plata, part of it is history and politics, and part of it is oceanography.

Most of Argentina and Uruguay considers Río de la Plata as a river (thus, the name, río) and as such it is the widest river in the world (maximum width >200km). Río de la Plata is formed as the confluence of the Paraná and Uruguay rivers and results in a pretty wide estuary with Montevideo (Uruguay) in the northern shore.

As can be seen in the following satellite image, the combined discharge from the Paraná and Uruguay rivers creates a current that flows into the Atlantic Ocean and that is visible under calm wind conditions and high discharge periods. Satellite Rio de la Plata

Source: https://eoimages.gsfc.nasa.gov/

Spanish and Portuguese explorers in the XVI century described Rio de la Plata as a "Mar Dulce" (freshwater sea). The Italian explorer Sebastiano Caboto explored the area and traded silver with the local Guaraní tribes and thus gave it the name of "River of Silver".

In practice, the system is best described as an estuary: neither river nor ocean, but an area of freshwater and saltwater mixing. In the area of Montevideo, the salinity in spring is around 10 g/kg (way lower than ocean salinities ~30-35g/kg), but it can be higher in periods of low discharge. In the area of Buenos Aires, salinities below 5 g/kg are commonly found and thus, it makes sense to call it a river. Most oceanographers agree that the system is a salt-wedge estuary, with freshwater flowing near the surface toward the ocean and saltwater intruding landward near the bottom of the estuary.

Rio de la Plata salinity Salinity of the Rio de la Plata: Source: Guerrero, R.A., Lasta, C., Acha, E.M., Mianzan, H., Framiñan, M., 1997. Hydrographic Atlas of the Río de la Plata. CARP-INIDEP, Buenos Aires, Montevideo.


Typically, the river delta is considered the end of a river where it flows into either an ocean or lake.enter image description here

From the image shown below, the end of the river can be defined by either at the edge of the active delta plain. or the edge of the submerged delta plain

Active boundary will vary due to tidal level, river level and the rate of subsidence of the of the delta plain or fan.

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    $\begingroup$ The Rio de la Plata isn't a delta. The delta (deltas, actually) is upstream of the Rio de la Plata. The geography and the huge freshwater discharge rate make the Rio de la Plata somewhat unique. $\endgroup$ Commented May 17, 2017 at 1:55

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