Fluvial sediment budget of a Mediterranean river: the lower Tordera (Catalan Coastal Ranges, NE Spain)

The lower Tordera River is a representative case of water and sediment dynamics and uses in a large river in the Western Mediterranean region. Along the study reach, the fluvial regime of the river changes, shifting from a perennial water and sediment circulation in the upper parts to an ephemeral flow pattern in the lower sections. The fluvial sediment budget of the lower Tordera was estimated based on field measurements of suspended and bedload and a survey of cross-sections. Field data were obtained between January 1997 and June 1999, a period that was dryer than the average. Most sediment is transported as bedload (more than 3/4 of the total load), both at the entrance section of the river reach and at the outlet section, close to the Mediterranean Sea. A total of 156,700 tonnes of sediment (62,680 tonnes/year) entered the system, mostly during discharges below bankfull, while 107,000 tonnes (42,800 tonnes/year) were exported to the sea, almost exclusively during floods. The net accumulation of sediment was 14,600 tonnes per year (as bedload), especially during the dry years of 1998 and 1999, when small floods circulated through the upper sections but did not reach the lowermost part of the river. As a consequence, a mean aggradation of the riverbed of around 6.8 mm/year has occurred, a figure that has been corroborated after the analysis of a series of cross-sections. Residence time of sediment in the perennial reach, calculated at a long-term, ranges from 6 years for sediments in the active base flow channel to 164 years for sediment placed above bankfull level, which is only entrained during high flows. Residence time of sediment in the ephemeral reach, calculated for the period 1997-1999, is only slightly higher than that of the perennial part of the river. This fact can be related to the current adjustment of the river system to historical mining. The riverbed is slowly adjusting to the new conditions, and important sedimentation is taking place in many upstream sections, while erosion still occurs in the lowermost reaches, thus causing the relatively low residence time of sediment.