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 Parana--Constitucion, 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. [130] As between La Paz and Parana, it seems possible to show some relation between these alluvial stretches at the foot of the cliff and the confluence of the small valleys of the Pampean plain.