A STABLE CANYON EXCAVATED IN THE BOTTOM OF A LARGE RIVER OF SOUTH AMERICA

The Uruguay is one of the large rivers in South America. It links along 1,838 km the tropical Brazilian meset with the temperate lowlands in Argentina and Uruguay. An exceptional feature in the river channel is a deep and narrow canyon or trench excavated in the bottom of the channel. This feature has been followed by the authors from the Cretaceous basaltic upper basin up to the mouth located in Quaternary littoral deposits. The cannyon is permanently submerged, except in two short segments (where the trench is visible in low waters).

In the upper basin, the canyon is 3 to 7 times deeper than the river channel and the wide varies between 1/3 to 1/8 of the total channel width. The section is well defined with sub-vertical flanks and flat bottom incided in basalt. The trench forms irregular curves which are shorter than the present fluvial meanders. In a 3 km long the canyon is 12 m deep an emerges forming the well known Moconá falls. In the middle basin (between 500 to 800 km from the sea), the river crosses a hilly landscape of Cenozoic sediments and rocks. The canyon there is 30 m wide and 10-15 m deep in a channel 1 km wide and 1-3 m deep.

The lower segment of the river (150 km long) is subject to the littoral dynamics. The fluvial water level is between 1 to 2 m.a.s.l. The river is extremely wide (5 up to 12 km) and shallow (less than 3 m deep) along the 100 km upstream its mouth. The canyon is sharply incided in Quaternary sediments at the middle of the stream. The depth is 10-18 m with extreme values up to 25 m and the wide reaches 200–800 m. That is 1/20 of the total width. Discontinuous trenches up to 20 m depth and 4-6 km long appears at the left of the continuous canyon.

The key point of this issue is that the canyon is not an inactive fossil feature, but it is an active part of the present fluvial dynamics. That is explained by the fact that the bed load reaches 5 million Tn/yr and that volume is transported to the ocean without infilling the narrow trench. We estimate that the canyon was originally generated during a dry climate in the Lower Quaternary.