Vegetation encroachment ratios in regulated and non-regulated Mediterranean rivers (Spain): An exploratory overview

Vegetation encroachment is an expected response of rivers to a decrease in their flow variability and flood magnitude and frequency. It frequently occurs together with channel narrowing and a decrease in braiding, both of which are likely associated with different human alterations but most frequently with river damming. Vegetation encroachment ratios display a wide range of values according to local hydrologic and geomorphic contexts and dam operation management, and thus, they are very site specific. As vegetation interacts with fluvial processes, understanding the mechanisms that promote encroachment is important for predicting river trends and designing proper management and restoration measures. Our research aimed to explore the causal relationship between flow regulation and vegetation encroachment in Mediterranean rivers in Spain. We quantified vegetation encroachment and channel changes over the last 50 years in 13 river reaches, 5 of which corresponded to non-regulated rivers and 8 of which were located in rivers regulated by large dams. We analysed aerial orthophotographs from 1956, 1977 and 2011 or later up to 2014, according to image availability. We considered 1956 conditions as a reference and quantified ratios of changes in riparian vegetation cover, active channel width and braiding index over two periods, 1956-1977 and 1978-2011. We quantified the changes in the flow regime and clustered rivers according to their hydrological pattern, looking for potential hydrological drivers of the observed changes over time. Vegetation encroachment, channel narrowing and braiding decreases occurred in all the studied rivers. Significant differences in annual ratios between the studied periods were found for both vegetation encroachment and channel narrowing, but no statistically significant differences were found between the regulated and non-regulated rivers or between the hydrological river groups previously clustered by flow variables. After discussing existing literature, we suggest a multi-scale approach to understanding river changes over time, encompassing larger historical perspectives. We hypothesize a generalized hydrologic decline since the beginning of the 20th century affecting rivers at the broad-regional scale would be overlapping the effects of land cover changes (i.e., renaturalization) at the catchment scale in reducing flood intensity and sediment supply. River damming would reinforce these patterns at river segment scale, in turn overlapping the influence of local geomorphic and ecological features. This hypothesis, which should be tested by further research, may explain the constancy of encroachment and channel narrowing regionally reported but with a wide variety of change ratios, determining converging trajectories of Iberian rivers that have also been found in other regions.