Above the estuary, the work for the improvement of the Parana began in 1904 and 1905. Since 1910 the material dredged from the bed of the river has risen to 3,500,000 cubic metres a year on the average. The experience gained in the course of this work has enabled the Argentine hydrographic service to adjust its methods to the incomparable force of the river. It is impossible to maintain a general rectification of the bed and the banks, as is possible with European rivers. The only thing to do is to submit quietly to the plan which the river sketches for itself, and be content to deepen the difficult passages on the line of the main arm. Suction dredges, which work easily in the sand, attack each ridge or _paso_ from below, making a channel into which the waters flow, so that it tends to enlarge itself up stream. The dredges are shifted from bank to bank according as the soundings tell of the formation of fresh obstacles to navigation. They were at first concentrated below Rosario, where the Argentine Government had to carry out certain engagements contracted with the Port Company; then they were scattered as far up as Santa Fe. The actual equipment suffices to carry out the programme that had been drawn up--to maintain a depth of twenty-one feet as far as Rosario and of nineteen feet as far as Santa Fe. [Illustration: MAP VII.--ESTUARY OF THE RIO DE LA PLATA. _The channels of the estuary are parallel to the direction of the tidal currents, which account for their depth. Those which cross the shoals of the Playa Honda, at the bottom of the estuary, finish as no-thoroughfares, and do not give access to the southern arms of the river. From Buenos Aires ships going up the river have to go round by Martin Garcia and the mouths of the Parana Guazu._ To face p. 254.]