Changes of channel patterns, avulsion and flooding: efects of base level rise in the river net of eastern plains of Tucumán Province, Argentina.

GEORGIEFF, S.M., G.E. BOSSI Y L.M. IBAÑEZ

Miramar – Córdoba

Congreso; Third Joint Meeting of ICSU Dark Nature; 2005

IGCP 490

The fluvial net of the western plains of Tucumán Province (NW -Argentina) surrounded by high mountains (about 4,500 - 5,000) on the West side shows important changes due to episodes of peak flows and avulsions during the last 30 years. Main factors involved in these processes are: 1- intensive tree fall along rivers and at the piedmont (deforestation produced an enlargement of water and sediments into fluvial channels), 2- increase in mean annual precipitation (an increment from 119 to 185 mm per year was recorded in different measurement areas from the 1970) and 3- base level arises (this elevation is produced by a dam, built on the SE end of the province). The dam produced a base level elevation around 30 m, an aggradation of the river thalweg and rising water tables. The main effects started closer to the dam on unstable margins, lateral migration, avulsion and finally flooding. At the present day the influence area is extended more than 40 km around the reservoir coast. Five catastrophic avulsions have been recorded, only in one river, in an area of 25 km around the dam from 1985. Most rivers have decreased the wavelengths of its bends on the last 20 km before to enter into the dam and increased its sinuosities. Sediment supply increased rapidly after 1980 as is observed on satellite images by the growing of the delta heads on the lake. A 2D simulation run from field data showed a progressive lateral migration toward Southeast and predicted that new avulsions are coming in the next few years. We have determined that the base level is the unique factor that it is possible to change in a short time in order to recover the fluvial equilibrium.