Modern evolution of the Colorado delta, Northern Patagonia

ISLA FEDERICO; ESPINOSA MARCELA A.; FAYÓ ROCÍO; BEDMAR JOSE M.

Ushuaia

Congreso; IV Congreso Argentino del Cuaternario y Geomorfología.; 2015

CADIC, CADINCUA

The Colorado River has significantly diminished its drainage area. Today it flows across Mendoza, Neuquén, La Pampa, Rio Negro and Buenos Aires provinces, although in the past its drainage area also covered San Juan, La Rioja and San Luis. Several lobes corresponding to Pleistocene highstands were distinguished interfingering within the plain (Spalletti and Isla 2003). Four distributary channels were defined growing during the Middle Holocene to historic intervals. Across the Villalonga Channel, the Holocene progradation of the delta was recorded (Weiler 1983). Diatom assemblages from a 172 cm long core drilled close to the current active channel were studied. From this sequence, a freshwater environment dominated by planktonic taxa (Aulacoseira granulata and Stephanodiscus hantzschii) was defined at 4132±35 14C years BP ago. Towards the top of the core, a shallower and more vegetated environment occurred about 142±36 14C years BP represented by freshwater epiphytes and aerophilous diatoms (Ulnaria delicatissima var. angustissima, Rhopalodia gibba, Epithemia adnata and Hantzschia amphyoxis). To the top of the core, similar taxa of the base dominate with the appearance of the tychoplanktonics taxa Staurosirella pinnata and Cyclotella ocellata.At the northernmost distributary, the presence of living-positioned shells of Tagelus plebeius suggest a subtidal flat 350±50 14C years BP. Close to this northern distributary channel several fore-dune ridges were sampled at distances of 3.4, 6.4, 10.9 and 12.4 km from the present shoreline. During the Spanish Viceroyalty administration, the delta was prograding into the Anegada Bay, according to the nautical chart drawn by José de la Peña in 1795. The blocking of the river was performed in 1906 at the site called ?El Tapón?, and originated a conic lobe with a distal width of 3 km spanning to 36 km at the present coast. Due to a flood caused by the sudden discharge of Cari Lauquen Lake in 1914, the blockage opened. Several channels were dug since the arrival with the railway of the first colonies (1921). Another blockage was achieved between 1924 and 1925, but another flood in 1931 originated a new arm: the Colorado Nuevo. The ENSO-triggered 1983 flood signified the last connection of the Curacó River system. It is assumed that man-made alterations (Casa de Piedra Dam, or the water pumping at Pichi Mahuida) are responsible for the slight increase in salinity at the lower plain. This modern situation should be considered, as one of the purposes of this study is to compare present salinity fluctuations to the Holocene changes recorded by the composition content of diatom assemblages in core sediments.