From La Paz to Paraná the main course is outlined by the Entre Rios cliffs. There is no further meandering. The cliffs of hard rock offer far more resistance than the soft alluvia over which the river wanders freely. The permanence of the bed in front of the cliffs leads to a depth of as much as eighty feet. Only here and there a fringe of alluvial stuff separates the channel for a time from the cliff. These curves seem, as a rule, to coincide with the confluence of rivers, which bring a heavy load of clay from the tableland; as, does, for instance, the San Feliciano, north of Hernandarias. They are marked by shallows, in strong contrast to the great depths of the straight sections. The San Feliciano paso, which is twelve feet broad to-day, was only six feet broad in 1908. It appeared on Sullivan's map in 1847.[127]
Below Paraná, as far as the estuary, the careful observations that have been made since 1903 on the movement of the river have enabled us to learn some of its laws.[128] We can distinguish four sections of unequal length. From Paraná to Diamante the river remains in touch with the cliffs of the left bank. It is not straight; it describes a series of linked crescents of equal radius, which seem to be traces of so many meanders. Only one in two of the windings of the cliff is followed by the channel. The wandering of the river is confined within limits as in a fixed mould. The Paracao shallow, which for a long time prevented ships from reaching Santa Fé (gradually deepened by dredging from eight to nineteen feet between 1907 and 1911) is at the angle where two of these curves meet. On the right bank the secondary arms continue to follow the river (Paraná viejo, Riacho de Coronda).[129]
Below Diamante the river leaves the cliff on the left bank and slants across the alluvial plain to the cliff on the right bank, which it reaches at San Lorenzo. Over the whole of its thirty miles width it resumes the freedom and regularity of features which it had above La Paz. A comparison of the successive maps of the river shows that the scheme of its movements, which one would be tempted to draw up with a regular migration of the islands and loops down river, would not be accurate. The changes of the bed of the river are essentially due to variations in the volume of the different arms, which are constantly changing their size and adapting their shape to the body of water that flows in them. The radius of the curve of each arm is proportional to its volume. A long island is formed between two arms of equal size which both describe symmetrical curves. If the volume of one of them is reduced, its original curve is replaced by sinuosities of smaller radius, and these nibble the edges of the island and give it an irregular shape. If the volume increases again, the winding bed is abandoned and becomes a dead bed, and a larger meander begins. The track followed by the ships then breaks up into a series of meanders over a course of about eight miles and a half, and this means the concentration in a single channel of the greater part of the water of the river, and in narrower bends in the sections where the current is divided between several arms.
From San Lorenzo to San Pedro the river flows by the cliff of the right bank. It is remarkably regular, and has only one slight bend: an exceptionally good site, on which the town of Rosario is built. At almost equal intervals, differing by only about ten to thirteen miles, the river leaves the cliff, and is separated from it by an alluvial strand, or by an insular zone a few miles in width.[130] Below this bend the current again touches the cliff and landing is easy. The small, older ports of the Paraná—Constitución, San Nicolas, Puerto Obligado and San Pedro—are built on similar sites. It does not seem that the islands at the foot of the cliff tend to extend downward in front of these ports; the points where the river reaches the cliff are fixed. The depth is often considerable at the foot of the cliff (138 feet opposite Puerto Obligado). The shoals are distributed irregularly at the bends, where the channel moves away from the cliff. They all have to-day a minimum depth of twenty-one feet.[131] On the left bank the secondary arms sprawl over the alluvial plain for thirty-five miles north of the river.
The delta begins at San Pedro. The Paraná Guazu, or main arm, leaves the cliff on the right bank and passes to the Uruguayan bank opposite Carmelo. The Paraná de las Palmas, which branches off from it to the south and passes before Campana and Zarate at the foot of the tableland, is deep and easy to navigate, but it is closed at the bottom of the estuary by a six-foot bar, which makes it a sort of blind alley opened only above. The arms of the zone of the delta differ from those of the river-zone proper in the irregularity of their course. Flowing between long islands, they sometimes lie in straight stretches and at other times in meanders or almost perfect buckles. The channels of the southern part of the delta, near Buenos Aires, are called caracoles (snails) on account of their winding shape. The weakness of the current, which is held up by the tide, is seen also in the distribution of the greater depths; they are no longer uniformly found along the concave edge of the bends, but are scattered irregularly. On the Paraná Guazu a depth of 130 feet has been ascertained. Its minimum depth is twenty-two feet.
THE PARANÁ ABOVE THE ESTUARY.
Right bank. The river has moved away from the barranca, leaving at its foot an alluvial plain of imperfect spiral form.