LAUSR

Abstract The Colorado River has significantly diminished its drainage area and annual discharge. Today it flows across several provinces of Argentina (Neuquen, Mendoza, La Pampa, Rio Negro and Buenos Aires) although during the Upper Pleistocene this watershed area was more extended. Its discharge also diminished significantly during documented times although floods occurred episodically during anomalous ENSO years. The delta plain is composed of several lobes of different ages. Four distributary arms were operating during the Mid-Holocene highstand. At the northernmost distributary arm, Tagelus plebeius remains of 350 ± 50 14C years BP were collected living in position on the supratidal flat. Close to this modern channel several foredune ridges were recognised. A blocking of the river occurred in 1906, and originated a conic lobe located 36 km from the present shoreline. Due to a flood caused by a sudden scabland of the Cari Lauquen Lake in 1914, the blockage opened. Another blockage was achieved between 1924 and 1925, but another flood in 1931 originated a new arm: the Colorado Nuevo. The ENSO-triggered flood of 1983 signified the last connection of the Desaguadero-Curaco River system. Man-made alterations are responsible for the increase in salinity along the lower floodplain and increasing the risk of onion crops. These particular processes of desertification with floods should be monitored in relation to climate-change previsions.