In the last fourteen years (1986--1999), fluidizations of confined bed--sediments due to underground springs in basin BII of lake Banyoles (Spain) have been detected. As a result of periods of high precipitation, sediment confined at the bottom of karstic basins migrates from 45 m depth to approximately 25 m depth. Fluidization processes are associated to monthly rainfall 1.5 to 4.5 times larger than the long-term mean (1970--1999). As a result of a fluidization event, the water column of the lake maintains the particle volume concentrations of suspended sediment (pVC) as high as 44 ¿l/l, which is ~8--29 times larger than the usual values of pVC.