Rio de Plata
As far as I know, A river's mouth is simply called the 'river mouth'!
Yes, it's an estuary but only connects Uruguay with Argentina. The one that goes to the north from the "de la Plata" River is the Uruguay River.
The Uruguay River and the estuary "Río de la Plata" (Silver River)
it is the mouth of the river; where it meets the sea
The Uruguay River and the estuary "Río de la Plata".
The estuary is Río de la Plata (River Plate).
The medways mouth is at The Thrust Estuary
"Río Uruguay" (Uruguay river) in the east. It also limit with Argentina in the south east with the "Río de la Plata" (Silver River) estuary.
Yes
The Humber Estuary.
The Uruguay River in the west and the estuary "Río de la Plata" (River Plate) in the south.
The 'mouth' of the river - or an 'estuary' - or a 'delta'.