Posted by: Rob Viens | July 23, 2012

Entering the Río de la Plata

On the 23rd of August the Beagle had rounded the coastline, leaving the Atlantic behind and entering the great estuary of the Río de la Plata (the “River of Silver”).  Darwin writes:

“All day we have been beating up the river, & now at night we are come to an anchor.— We were generally at the distance of four or five miles from the Northernshore.— Thus seen, it presents a most uniform appearance.— a long straight line of sandy beach was surmounted by a sloping bank of green turf.— On this viewed through a glass were large herds of cattle feeding.— Not a tree broke the continuity of outline: & I only observed one hut, near to which was the Corral or enclosure of stakes, so frequently mentioned by all travellers in the Pampas.— I am afraid we shall not even tomorrow reach M. Video.” (July 23)

The going was slow, as the ship was moving “upstream” against the outflow of 20,000+ cubic meters of water moving out to the ocean every second.  If the wind was not favorable for the Beagle, it would be a long haul up he estuary.

Now to be fair, when I looked at maps of South America I had always assumed that the large inlet that divided Uruguay and Argentina was a simple ocean inlet. I suppose that in some senses it is, however, the freshwater river dominates here, making this large body of water either brackish (mixed fresh and salt water) or mostly freshwater (in the western part of the inlet).  So really, it is more accurate to call it an estuary that forms at the (very large) mouth of a river. In fact, at about 140 miles wide at the “mouth”, it is considered the widest river in the world.

Map of the Río de la Plata and the Paraná River Basin (from Wikipedia Commons):

Map of the Rio de la Plata and the Parana River

The Río de la Plata is the name of the final stretch of a very long river system. Most of the drainage basin that feeds the river is made up of the Paraná River (which is nearly 5000 km long) and the Uruguay River.  And a large basin it is – the Río de la Plata drains an area of over 4 million square kilometers (about 1.6 million square miles).  On average, its discharge (the amount of water that flows out the mouth of the river ever second) is 22,000 m3/s (about 777,000 cfs). This is pretty close to the Mississippi River which has an average discharge of about  17,000 m3/s, but it is far less than the Amazon River which discharges about 10 times more water (about 210,000 m3/s)! Together, the Río de la Plata and the Amazon drain about 60% of the South American continent.

NASA image of the Río de la Plata (including cities Darwin will soon visit)

Satellite image of the Rio de la Plata

As noted earlier, this inlet is an estuary – a place where freshwater and saltwater meet. To some degree the waters in an estuary mix forming brackish water. However, due to the difference in density between fresh and salt water (freshwater has a lower density) there are some places where the fresh water flows over the saltwater forming layers of different salinity.  This leads to interesting dynamics – for example, when the tide goes in, the deeper (saltwater) layer can flow “upstream” while the top (freshwater) still flows out to the ocean. In any case, estuaries are interesting places to find some of life’s interesting little adaptations, since plants and animals have to be able to live in both fresh and salt water.

15,000 years ago when the world was in the grips of an “ice age”, the Río de la Plata was a regular old river valley.  At the time, enough ocean water was frozen into the massive ice sheets that sea level was about 100 meters (330 feet) lower than today. As you can see from the bathymetric map below (from sciencedirect.com), the coastline would have been roughly 100 km offshore.  So it was the melting glaciers we have to thank for turning the Río de la Plata valley into a body of water Darwin could sail upon.

Bathymetric Map of the Rio de la Plata

It is also interesting to see how shallow the inlet is – especially in the vicinity of Buenos Aires.

For now, we’ll leave the Beagle as it slowing sails upstream – unaware that revolution is brewing… (RJV)

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