Suspended sediment dynamics in a large regulated river over a 10-year period (the lower Ebro, NE Iberian Peninsula)

Abstract

This paper presents a comprehensive assessment of the suspended sediment load for the period (1998–2008) in the lower river Ebro (NE Iberian Peninsula), one of the most regulated large rivers in Europe. Sediment load estimates are based on continuous discharge and turbidity records, calibrated with direct suspended sediment samples covering a wide range of river's hydraulic conditions. The temporal variability of the sediment load and the role of the effective discharge at different temporal scales are specifically examined. At the log-term, the study period can be considered as dry. Mean annual load is estimated at around 0.092 × 10 6 t, a value that represents less than the 1% of what was transported at the beginning of the twentieth century in the absence of dams and under different land uses. In general, sediment concentrations observed between 1998 and 2008 are low (mean was 9 mg l − 1 and maximum was 240 mg l − 1 ). Specific sediment yield was in the order of 1 t km 2 y − 1 , also much lower than values reported in other western European rivers of the same scale and impact. At the annual scale, large floods were responsible for most of the transported suspended sediment (up to 80%). Nevertheless, when the effective discharge is calculated for the whole study period, floods of moderate magnitude and high frequency are responsible for the largest amount of sediment transport. The temporal scale used to calculate the effective discharge thus determines the final results. Load estimates derived from continuous suspended sediment and discharge data and from the flow duration method yielded similar values. This suggests that the latter method is also appropriate when sediment samples cover a large proportion of the observed discharges and statistical significant relations can be obtained and applied to long periods of record. This study provides further insights into the sedimentary status of one of the largest rivers in Europe. Results corroborate previous studies highlighting sediment deficit downstream from large dams. This type of information constitutes a key element to inform and improve current management and restoration actions.