Plate XXII.

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The study of the estuary may be taken separately from that of the river. It consists of three parts, unequal in size, which open with increasing breadth toward the Atlantic. The upper Rio de la Plata, above Colonia and Puntá Lara, has a width of about thirty-five miles. The middle Plata, twice as wide, extends to the latitude of Montevideo and Puntá de las Piedras. Then the outer harbour opens between Maldonado and Puntá Rasa. The water is still fresh in the middle estuary up to eighty miles below Buenos Aires.

The bottom is alluvial except in the channels between Martin Garcia and Colonia.[132] Differently from up the river, where the channels have sandy bottoms, while the banks are of fine clay, the channels of the estuary have bottoms of mud and clay. In the outer harbour the pilots recognize the approach of banks by the sand which is brought up by the sounding-lead. The action of the waves, which is not found in the river, accumulates stuff of comparatively large size and weight on the banks.

In spite of the conclusions embodied in the nautical instructions, which describe the estuary as a theatre of rapid changes "occasioned by the continual deposits of sand brought down by the Paraná and the Uruguay,"[133] the estuary is, as a matter of fact, in a remarkable state of equilibrium, and there is no trace of a gradual accumulation of alluvia, or of important changes of channel. The shore of the delta north of the Paraná de las Palmas, covered with rushes which protect it from the attack of the waves, shows neither advance nor retreat. The broad lines of the hydrography of the Rio de la Plata are plainly indicated on Woodbine Parish's map. The English Navy map of 1869 (on the basis of observations in 1833, 1844 and 1856) only differs in detail from the present map. The stability of the channels is surprisingly different from the changes in the bed of the river in the flood-zone. The permanence of the bottom, in spite of the loose deposits of the estuary, is explained by the regularity of the currents. These currents, which determine the submarine topography of the Rio de la Plata and the distribution of the banks, are not of river origin. They are tidal currents.

There are two groups of shoals in the estuary. The first, the Playa Honda, occupies the whole western part of it up to a line drawn from Buenos Aires to Colonia. These banks leave a narrow passage in the north, opposite the Uruguayan shore, and this is followed by ships going to Uruguay and the Paraná Guazu. The second group of shoals is the Ortiz Bank, triangular in shape, which rests in the north on the Uruguay coast below Colonia, while its point extends south-eastward to eighteen miles north of the Puntá de las Piedras. It keeps the zone of deepest water in the middle estuary to the south, near the Argentine shore. In the latitude of the point of the Ortiz Bank, on a line from Montevideo to Puntá de las Piedras, the middle estuary is separated from the outer harbour by a bar (barra del Indio) with thirty-eight feet of water, caused by the transverse currents which circulate from point to point inside the English Bank.

The tide in the estuary is very irregular. The south-east winds increase the flow and retard the ebb. When they are blowing, it often happens that the level of the water in the upper estuary keeps up from one tide to the next, sometimes for several days. The tide, which is slight at Montevideo, is greater at the bottom of the harbour on the Barra del Indio, sometimes rising nearly forty inches there. From there it advances with difficulty northward, over the Ortiz Bank, along the Uruguayan shore, whereas it passes freely into the deeper zone on the Argentine side.[134] At Buenos Aires it still has a depth of thirty inches. From there it advances northward by the Martin Garcia channels beyond the Playa Honda. The channel of the Pozos del Barca Grande, which crosses the Playa Honda bank from north to south, parallel to the edge of the delta, is oriented in conformity with the tidal currents and maintained by them. It is not attached to the river, and it is separated from the mouths of the Paraná de las Palmas or the Paraná Mini by shallows which are navigable only to small boats. The Rias of the Uruguay, where the tide raises the water twelve inches, forms a sort of reservoir which, at the ebb, feeds a strong current round Martin Garcia and sweeps the channels there.

The work done for the improvement of the estuary includes the deepening to thirty feet of the Barra del Indio and the dredging of a straight channel from that point to Buenos Aires. Steamers of large tonnage going up the Paraná leave this channel twenty-six miles east of Buenos Aires, and turn north in order to pass east of Martin Garcia, and enter the river by the Paraná Guazu or the Paraná Bravo. Since 1901 the Argentine Government has considered a plan of opening a direct route from Buenos Aires to the Paraná de las Palmas, either by cutting an artificial canal at the foot of the cliffs, across the Tigre archipelago, or by using the channel of the Pozos del Barca Grande and cutting the narrow bar which closes the Paraná de las Palmas below. If this were done, the ports of the Paraná de las Palmas would have direct access to the sea. Moreover, the new route from the Paraná to the Atlantic would be entirely within Argentine territory, out of range of the Uruguayan shore, and Buenos Aires would become a necessary port of call both on departure and return.