[Tagging] Rio de la Plata edit war

[Christoph Hormann]

On Friday 31 July 2020, Andy Townsend wrote:
>
> For what it's worth, neither extreme position looks the best answer
> to me - looking at the salinity change between river to ocean at
> https://www.sciencedirect.com/science/article/pii/S0307904X07000716
> (see
> https://www.sciencedirect.com/science/article/pii/S0307904X07000716
> for the key picture) and looking at
> https://de.wikipedia.org/wiki/Datei:Rio_de_la_Plata_BA_2.JPG suggests
> a location some way between the two.  Despite the NASA photo it looks
> like there isn't a "step change" in salinity - and of course values
> will fluctuate based on winds and tides etc.

Surface salinity is not a good universal measure for the transit between 
the riverine and the maritime domain because

a) depending on the threshold you would exclude large maritime areas 
like the Baltic Sea, Hudson Bay or the Sea of Azov.
b) at the mouth of a river salinity often varies significantly between 
the surface layer and deeper water because saltwater is heavier.

Suspended particles are also often not a good measure because we are 
usually talking about very fine particles that stay suspended for a 
long time and in shallow water currents can re-suspend silt from the 
bottom as well.  The presence of suspended particles is therefore an 
indication of a lack of large volume dilution of the water in the area, 
not of the dominance of river water over sea water in general.  See for 
example

http://maps.imagico.de/#map=7/32.361/122.212&lang=en&l=sat&ui=10

where strongly visible turbidity reaches up to more than 50km from the 
shore into the open sea.

As i wrote in my old proposal on the transit placement looking at the 
cross section of the river and the resulting average water flow 
velocity due to discharge gives you a relatively good idea about the 
situation.  In case of the Rio de la Plata you have an average 
discharge of 22000m^3/s.  At the claimed baseline you have an average 
water depth of about 20m and a width of more than 200km that is an 
average waterflow velocity of 6mm/s.  At Montevideo with a width of 
about 100km and a depth of about 8m you get an average velocity of 
3cm/s.  That is still smaller than typical coastal currents induced by 
tides and wind (which the paper you cited confirms).  But you are not 
that far off any more and around where the average water depth is about 
5m you will have reached the lower limit my proposal suggests.

I still think the people best qualified to make the assessment where 
exactly the transit is best placed are those with local knowledge, who 
have first hand knowledge of the effects of waves, tides and currents 
on the shore over the course of the year as long as their perspective 
is not dominated by political considerations (i.e. they are able to 
look at this purely from a physical geography perspective).

-- 
Christoph Hormann
http://www.imagico.de/