GEOMORPHOLOGICAL CONTROLS ON VEGETATION RESPONSES TO FLOW ALTERATIONS IN A MEDITERRANEAN STREAM (CENTRAL‐WESTERN SPAIN)

The expected recovery of the natural conditions of large regulated rivers over the distance downstream from a dam is limited by relative tributary size according to the Serial Discontinuity Concept; however, geomorphology may also influence the recovery process. We examined the woody vegetation of the riparian zone in seven river segments distributed along the regulated reach of the Tiétar River in central‐western Spain, which flows through two distinct geomorphic templates. Whereas the annual runoff has decreased by 30% on average along the entire studied reach following the construction of the Rosarito Dam and the initiation of field irrigation in the region, the magnitude and frequency of the peak flows decreased by 30% immediately downstream from the dam but recovered the natural values with the distance downstream. We evaluated the recovery patterns toward the natural riparian conditions by comparing woody species composition, diversity and distribution of vegetation patches established prior to and after dam completion. Our results did not indicate a recovery gradient of any of the analysed vegetation attributes downstream from the dam. Instead, we found that the difference in the slope of the stream channel and banks, the width of the valley and the size of substratum particles among the surveyed patches were factors that significantly mediated dam and tributary effects on vegetation and influenced the degree of vegetation recovery. Hence, the maintenance of the intensity of the flow alteration scheme by the numerous water withdrawals and the low tributary contributions, coupled with differential geomorphological characteristics along the reach, overwhelmed the natural tendency for the river to restore its natural conditions with distance downstream. Improving water management and, particularly, restoring endangered riparian ecosystems require a detailed understanding of existing and potential woody species behaviour across the geomorphological settings of rivers. Copyright © 2012 John Wiley & Sons, Ltd.