252 THE RIVER-ROUTES 



changes of channel. The shore of the delta north of 

 the Parana 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 hydro- 

 graphy 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 Parana 

 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 Punta 

 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 Punta 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 



