But the different scenes of the river region are most of all due to different conditions of erosion and formation. Above Rosario the configuration is due to floods. Each succeeding flood alters it and leaves some trace of itself in the topography. The beds of sand that it lays down are fixed by rushes and floating weeds, then by willows (_Salix humboldtiana_). This screen of vegetation encourages accretion, and the edges tend to rise higher. In the middle of the island are low, marshy lands. The irregularity of the alluvial deposits causes marked undulations in the whole region of the river, and everywhere gives rise to alternate beds of clay and sand. Below Rosario the river gradually loses its power. The islands become more stable and flatter. Clumps of willow and spiny _ceibos_ (_Erythrina cristagalli_) still cover the edges of them, and sometimes spread over the interior. But as the climate is now less humid, the vegetation fixes the soil less firmly, and the wind becomes the chief sculptor of the landscape. It heaps up the sand during the low-water season, and makes dunes which rise above the level of the greatest floods. These dunes form an unbroken line along the land in the southern part of Entre Rios, in the north of the main arm, with ridges at right angles, advancing toward the south, which rest upon the river clay; like the one which the Ibicuy railway follows across the floodable area. The cattle of the district take refuge on the dunes during floods. During periods of drought, on the other hand, they retain a quantity of water, and this is drawn from surface-wells at their base.