Sediment transport of Pilcomayo River: qualitative and quantitative variations at Misión La Paz (Salta, Argentina)

ORFEO, O.; SARTIRANA, M,; SUÁREZ, P,

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

Sediment transport of Pilcomayo River: qualitative and quantitative variations at Misión La Paz (Salta, Argentina) Among the large Argentinean Rivers, the Pilcomayo River presents one of the largest sediment transport rates, generating important changes in its fluvial morphology and sedimentology. This fact needs to be attended at an appropriate scale to understand the dynamics of this vast fluvial system. Towards this aim, the sediment transport at the cross section located between Misión La Paz (Argentina) and Pozo Hondo (Paraguay) (22º 22’ 35” S - 62º 31’ 18” O) were analyzed. During the measurement period, the local hydrometric level varied between 2.3 and 6.6 m (approximately between 8 m3 s-1 and 300 m3 s-1 in terms of discharge) from December 2006 to December 2007. The obtained results showed clear differences of the considered variables related with the hydrometric level. Local levels higher than 3 m were related with electric conductivity values around 600 μS cm-1, total  salt concentration around 370 mg L-1 (in average); pH equal to 7.7 and water temperature higher than 30°C. Under conditions less than 3 m, the electric conductivity and total salt concentration were approximately double; pH was significantly higher approaching values of 8.2 and the water temperature diminished around 10°C. In average terms, the wash load of the whole section varied between 1357 mg L-1 and 7749 mg L-1 maintaining a direct relation with the discharge. As with other parameters, these values can be empirically associated in two conditions related with the water level. With more than 3 m of water level, the suspended sediment concentration was approximately equal to 7000 mg L-1, whilst in conditions less than 3 m the concentration was around 1500 mg L-1. Vertical sediment distribution showed a typical stratification with highest values near the river bed during all the studied period.The size distribution of the wash load revealed the presence of sand, silt and clay. The silt fraction was more abundant in suspension (around 50% of the total sediment frequency) showing a nearly homogeneous distribution in flux. Suspended sand concentration maintained a direct relation with the discharge but the clay fraction revealed the opposite trend. Suspended sediment transport varied between 1200 tons day-1 during the low water period and 155000 tons day-1 during the flood showing a direct relation with the discharge. Bed material was basically sandy (sand fraction frequency ranged from 88% to 99%) with less proportion of silt (between 2% and 6 %). The clay abundance at the bottom was always less than 1%. The granulometric composition of wash load and bed material load was practically the same during the complete period of analysis. The textural homogeneity of the sediment input in the plain stretch of Pilcomayo River explains the regularity of the particle size despite the discharge variation.