PhD research in REFORM - Vegetation changes and flow regulation in gravel bed rivers (Upper Esla Basin, NW Spain)

About "PhD research in REFORM"

In the newsletter items dedicated to PhD research in REFORM, PhD students introduce the topic and the initial results of their research.


Rivers in Spain are highly altered due to the existence of more than 1200 large dams. There are few detailed studies on the cumulative effect that this intense flow regulation has over time and space. Today, it is known that the presence of dams not only modifies flow regime but greatly modifies sediment flows causing different responses in channel morphology, habitat quality and dynamics of riparian vegetation. Particularly in Spain, the diversity of bio-geographic gradients and river types, together with the different geological contexts and substrates result in a multiplicity of responses to modifications. Therefore, specific case studies are needed to consider the interaction between river types and pressures (e.g. land use of the watershed).


The general objective of this thesis is to improve the knowledge on bio-geomorphic responses of rivers to human pressures and to contribute to their management based on the Water Framework Directive. Particularly, this work analyses the homogenisation of channel morphology and significant changes in the riparian corridor that have been observed in the Upper Esla Basin in the last 50 years. This fluvial system is affected by intensive flow regulation for irrigation purposes that, in Mediterranean regions, entails an increase in low summer flows (in comparison to natural conditions). Current knowledge indicates that flow regulation initiates riparian vegetation changes through flood reduction that could decrease successful recruitment of native riparian pioneer species. Our objectives were to analyse changes in riparian vegetation patterns in time and to verify if these changes could be associated with flow regulation.

Figure 1: Curueño River, Summer 2013 (photo: Marta González del Tánago)


River reaches where natural vegetation remains were selected for the research sites. The initial analysis was conducted on regulated reaches of the Esla and the Porma rivers and one un-regulated reach of the Curueño River (Figure 1). Mean daily discharge data were analysed considering pre- and post- dam periods. A set of aerial photographs (1956 - 2011) were analysed in each studied reach and changes in active channel and vegetation were quantified. Fieldwork was done to analyse current landforms and woody vegetation structure, the results being the identification of seven different landform types and the collection of data on presence and size of woody vegetation along 3 transects in each reach.

Preliminary Results

Active channel area has decreased steadily over the studied period. In 1956, the active channel area was largest in all cases. In the successive pictures, channel narrowing and loss of the initial active channel area together with vegetation encroachment and poplar plantations is observed in all rivers (Figure 2).

Figure 2:  Example of a part of the Porma river reach that exhibited a pre-dam multithread channel with wide banks in 1956 (left) and a single channel in 2011 (center) with human pressures (right) (figure: Vanesa Martínez-Fernández).

The most active landforms, bare gravels with frequent recruitment of pioneer species, were absent in the regulated reaches (Esla, Porma). Multivariate analysis revealed that seedlings of pioneer (Salicacea) species commonly related with active bars were only found in the Curueño River. However, larger individuals of these pioneer species are found in old channel banks of the Porma and Esla rivers, where flood disturbance has disappeared after flow regulation. Since the operation of the dams, the maximum annual discharge has decreased (about 70%) and minimum annual discharge has risen. In both regulated rivers, no flood events exceeding the 2-years return period (incidence based on natural flow regime characteristics) have occurred since the operation of the dams. In the Curueño River, flood disturbance has continued to occur but with smaller magnitude. On the other hand, an increase of the minimum flows during summer months has occurred in the regulated rivers due to irrigation purposes. This contrasts with the natural Mediterranean seasonality presented by the Curueño River, where low flows still occur in the summer months.

Our preliminary results show narrowing processes in the studied rivers associated to a decrease of flood disturbance and highlight the risk that native riparian forest will not rejuvenate but will age and disappear in due course under regulated flow regimes.

Future work will include more reaches and other type of catchments and landscape variables linking the evolution and trends of the fluvial system to human pressures that have modified natural conditions.

Author:  Vanesa Martínez-Fernández, UPM (Polytechnic University of Madrid)

For further information: 

Vanesa Martínez-Fernández