Discerning climatic and environmental change signals on palaeoflood records
G. Benito1, V.R..,Thorndycraft2, M.J Machado1, M. Rico3, Y. Sánchez-Moya4, and A.
Sopeña4
1
Museo Nacional de Ciencias Naturales, CSIC, Serrano 115 bis, 28006 Madrid, Spain.
2
Dept. Geography, Royal Holloway, Egham, Surrey, TW20 0EX, U.K.
3
Instituto Pirenaico de Ecología, CSIC, Zaragoza, Spain
4
Instituto de Geología Económica, CSIC-UCM, 28040 Madrid, Spain
Interpretation of palaeohydrological changes from alluvial sequences is a complex task
because of problems in the identification of their causation (climate vs. land-use
change) in a region with a long history of human intervention. During the last decade
there has been increasing interest in applying palaeoflood hydrology for the record of
individual floods (magnitude and frequency) during the Holocene. This presentation
will review the results of these analyses and evaluate changes in the spatial and
temporal distribution of floods in the Spain over the last 2000 years on the bases of
radiocarbon dating and environmental (documentary) history. In the Iberian Peninsula,
increased flood frequency and magnitude were identified in the sedimentary record at
AD 1000-1150, 1430-1700 and 1730-1800; though chronological resolution using
radiocarbon dating during the last 300 years is relatively poor. The largest historical
flood peak discharges since AD 1500 occurred when the North Atlantic Oscillation
during the winter months was in its negative phase. In Iberian Mediterranean
catchments, documentary sources indicate highest flood severity during AD 1580-1620
and AD 1840-1870. The lack of slackwater deposits between AD 1200 and 1400
highlights as a period of particularly low frequency of large floods, and is also evident
in the documentary record. However, this period (1150-1450 cal. AD) comprises one of
the main phases of historical floodplain aggradation of Iberian rivers that has been
interpreted as a result of land-use changes. This phase of accelerated alluviation during
the late medieval period is ubiquitous in Spanish rivers, and commonly resulted in the
burial of irrigation structures and dams. At site-specific records (e.g. Guadalentin
River), palaeoflood records indicate an anomalous increase in the frequency of large
magnitude floods between AD1830-1900, which was attributed to climatic variability
accentuated by intensive deforestation and land-use practices during the first decades of
the 19th century. However, this late Agricultural Revolution was not synchronous all
over Spain, and periods of high sediment production associated to floods may started
earlier in other Mediterranean rivers (e.g. Rambla de la Viuda in Castellón).