A. Tena

Geomorphic status of regulated rivers in the Iberian Peninsula

River regulation by dams modifies flow regimes, interrupts the transfer of sediment through channel networks, and alters downstream bed dynamics, altogether affecting channel form and processes. So far, most studies on the geomorphic impacts of dams are restricted to single rivers, or even single river stretches. In this paper we analyse the geomorphic status of 74 river sites distributed across four large basins in the Iberian Peninsula (i.e. 47 sites located downstream of dams). For this purpose, we combine field data with hydrological data available from water agencies, and analyse historical (1970) and current aerial photographs. In particular, we have developed a Geomorphic Status (GS) index that allows us to assess the physical structure of a given channel reach and its change through time. The GS encompasses a determination of changes in sedimentary units, sediment availability, bar stability and channel flow capacity. Sites are statistically grouped in four clusters based on contrasted physical and climate characteristics. Results emphasise that regulation changes rivers flow regime with a generalized reduction of the magnitude and frequency of floods (thus flow competence). This, in addition to the decrease downstream sediment supply, results in the loss of active bars as they are encroached by vegetation, to the point that only reaches with little or no regulation maintain exposed sedimentary deposits. The GS of regulated river reaches is negatively correlated with magnitude of the impoundment (regulation). Heavily impacted reaches present channel stabilization and, in contrast to the hydrological response, the distance and number of tributaries do not reverse the geomorphic impact of the dams. Stabilization limits river dynamics and may contribute to the environmental degradation of the fluvial ecosystem. Overall, results describe the degree of geomorphological alteration experienced by representative Iberian rivers mostly because of regulation, challenging the successful long-term implementation of river basin management programmes.

Reach-scale suspended sediment balance downstream from dams in a large Mediterranean river

Abstract This paper presents a reach-scale sediment balance of a large impounded Mediterranean river (the lower Ebro, 1998–2008). Multi-temporal sediment storage and the influence of floods and tributaries on the sediment load were examined using continuous discharge and turbidity records. The mean annual suspended sediment load at the reach outlet (Xerta) is 0.12 × 106 t, corroborating previous results. Suspended sediment concentrations were low (SSCmean = 13 mg L-1), attaining a maximum of 274 mg L-1. Erosion processes (channel-scour, bank erosion) are dominant, and net export of sediment occurs over the long term. Unexpectedly, ephemeral tributaries were found to contribute significantly: sediment delivered during torrential events attained 5% of the Ebro annual load, and was even larger than that in flushing flows. Overall, most of the suspended sediment load is transported by floods (up to 65% in some years). The results constitute basic information to underpin current management actions aiming to achieve the sustainability of the riverine and deltaic system. Editor D. Koutsoyiannis; Associate editor D. Hughes Citation Tena, A., Batalla, R.J. and Vericat, D., 2012. Reach-scale suspended sediment balance downstream from dams in a large Mediterranean river. Hydrological Sciences Journal, 57 (5), 831–849.

Suspended sediment dynamics during flushing flows in a large impounded river (the lower River Ebro)

PurposeThe aim of this work was to improve the understanding of the spatial and temporal dynamics of suspended sediment transport during flushing flows in a large regulated river, the lower River Ebro (NE Spain).Materials and methodsRelationships between sediment and discharge (i.e. discharge (Q)–suspended sediment concentrations (SSC)) were examined during six flushing flows using continuous discharge and turbidity records obtained at six monitoring sections distributed along the lower Ebro River for the 2008–2011 period.Results and discussionAnalyses revealed marked spatial and temporal patterns. At the spatial scale, the Q–SSC relationships were mostly influenced by the different routing velocity of discharge and sediment waves. At the upstream sections, the sediment peak usually preceded peak discharge (i.e. clockwise loop); however, flow routing through the 85-km channel length tends to increase the lag between them, modifying the hysteresis towards counter-clockwise patterns in the downstream direction. At the temporal scale, the season when the artificial releases were performed strongly influenced the sediment availability, with similar-magnitude flushing flows generating higher sediment peaks in autumn than in spring.ConclusionsThese results are of great interest in order to reinforce the flushing flows programme in the lower Ebro River, so as to help achieve the sustainability of the riverine and deltaic ecosystems.

Sediment transport in two mediterranean regulated rivers

Mediterranean climate is characterized by highly irregular rainfall patterns with marked differences between wet and dry seasons which lead to highly variable hydrological fluvial regimes. As a result, and in order to ensure water availability and reduce its temporal variability, a high number of large dams were built during the 20th century (more than 3500 located in Mediterranean rivers). Dams modify the flow regime but also interrupt the continuity of sediment transfer along the river network, thereby changing its functioning as an ecosystem. Within this context, the present paper aims to assess the suspended sediment loads and dynamics of two climatically contrasting Mediterranean regulated rivers (i.e. the Ésera and Siurana) during a 2-yr period. Key findings indicate that floods were responsible for 92% of the total suspended sediment load in the River Siurana, while this percentage falls to 70% for the Ésera, indicating the importance of baseflows on sediment transport in this river. This fact is related to the high sediment availability, with the Ésera acting as a non-supply-limited catchment due to the high productivity of the sources (i.e. badlands). In contrast, the Siurana can be considered a supply-limited system due to its low geomorphic activity and reduced sediment availability, with suspended sediment concentration remaining low even for high magnitude flood events. Reservoirs in both rivers reduce sediment load up to 90%, although total runoff is only reduced in the case of the River Ésera. A remarkable fact is the change of the hydrological character of the River Ésera downstream for the dam, shifting from a humid mountainous river regime to a quasi-invariable pattern, whereas the Siurana experiences the opposite effect, changing from a flashy Mediterranean river to a more constant flow regime below the dam.

Spatial and temporal dynamics of macrophyte cover in a large regulated river

The River Ebro basin is extensively dammed. Dams alter the geomorphological functioning of the river by altering its flow regime (e.g. reducing mean and maximum discharges), increasing bed stability (armouring) and decreasing turbidity (water clarity). These effects, together with an increase in nutrient concentrations and water temperature, have generated optimal conditions for the proliferation of aquatic macrophytes. In this paper, we analyse the temporal and spatial changes of macrophyte cover in the lowermost Ebro through a series of field campaigns carried out between 2009 and 2010. Special attention was paid to the spatial distribution of macrophytes in relation to flow hydraulics, channel geometry and bed sedimentology. Temporal changes in macrophyte cover were analysed in relation to the frequency and magnitude of both natural floods and flushing flows (artificial flow releases from dams with generally a magnitude that equates around a2-year flood in the river). Spatially, the proportion of macrophytes along the reaches showed a variable pattern, with a succession of areas with both high and low plant density, coinciding with the alternation of riffles and pools in the channel. The highest values of plant cover (>65%) occurred in riffles and in transition to riffle areas, while the lowest densities (1% or almost negligible) were observed in pools and transition to pool areas. Water depth and the grain-size distribution of the riverbed materials (i.e. D84), are found to be the main factors controlling the degree of plant cover in the lower Ebro. Temporally, the macrophyte proportion varied during the hydrological year, with a clear increment from late spring to early autumn (i.e. vegetation cover reached 40%, on average, of the channel surface). Macrophyte coverage decreases immediately following a flushing flow but in the long term, vegetation re-occupied the area again, even slightly increasing in some sections; overall, the mean percentage of macrophyte cover was 19% higher at the end of the study period, despite the numerous flow events occurred on the meantime. This increase enhanced riverbed stability, which in turn reduced the possibility for bed-material entrainment. This study empirically confirms the necessity of improving the management options applied in the lower Ebro with complementary measures to help maximise the efficiency of flow releases (for instance, subject the macrophytes to a severe hydrological stress by decreasing discharge before a given flushing flow, undertake localise mechanical removal of plants in areas where density is high, and increase the frequency of floods in winter time when macrophyte stands are weaker).