The Rio de la Plata is the second largest river system in the world. It is one of the three main outlets from the interior of South America to the sea, and carries almost twice as much water as the Mississippi. At its mouth the river is one hundred and eighty miles across from Cape San Antonio, Uruguay, to Cape Santa Maria, in Argentina. A little further inland, which some consider as the real mouth, the distance is one hundred and forty miles. Opposite Montevideo the width has narrowed down to sixty-five miles, and at Buenos Aires it is about twenty-eight miles from shore to shore. Just above Buenos Aires the river is divided into a number of forks, which form an extensive delta through which the great branches run and a number of islands have been created. The principal branches of this river in Argentina are the Paraná, Uruguay and Paraguay. The Uruguay River rises in Brazil, less than one hundred miles from the Atlantic Ocean, and has a length of one thousand miles. The Paraguay and Paraná Rivers also have their sources in Brazil, near the centre of the continent, and the former has a length of seventeen hundred miles before its waters mingle with the latter. It has two tributaries, the Pilcomayo and Bermejo, which are navigable for small craft. Each of these rivers is more than five hundred miles long, but they are exceedingly tortuous, so that navigation is rather difficult and uncertain. The Paraná River reaches way up into Brazil. It has its source only a few miles from one of the principal tributaries of the Amazon, over a stretch of swampy ground of which a part of the water flows into one river and part into the other. All of these rivers carry down immense quantities of mud. In places the deposit on the river bottom is from thirteen to twenty-five feet deep, and it has many banks and shoals. The problem of keeping channels open to Buenos Aires is a big one, and many dredges are kept constantly at work. It is generally believed that the interior of Argentina was at one time a vast inland sea, and that the flat plains have been formed by the soil which has been deposited by these rivers during the prehistoric geological ages. The waters of the Atlantic are coloured by this mud long before the mouth of the river is reached. The water in the bath-tub looks almost like thin pea soup. [Illustration: ON THE UPPER PARANÁ RIVER]