The degradation of plant material in the centre of two plain streams (the Parana Medio River main channel − 0.6 m.s−1 - and the Colorado River − 0.03 m.s.−1) was studied in order to obtain a degradation coefficient for the species Panicum prionitis, Paspalum repens and Eichhornia crassipes, representative of the vegetation of the area of the future man-made lake of the Parana Medio, Dam. The experimental sites were chosen at the limit (minimum and maximum) velocities of the two streams in the valley to be flooded.