Bouziotas et al. presented a paper at the EGU a few weeks ago (PDF) and concluded:
Analysis of trends and of aggregated time series on climatic (30-year) scale does not indicate consistent trends worldwide. Despite common perception, in general, the detected trends are more negative (less intense floods in most recent years) than positive. Similarly, Svensson et al. (2005) and Di Baldassarre et al. (2010) did not find systematical change neither in flood increasing or decreasing numbers nor change in flood magnitudes in their analysis.This finding is largely consistent with Kundzewicz et al. (2005) who find:
Out of more than a thousand long time series made available by the Global Runoff Data Centre (GRDC) in Koblenz, Germany, a worldwide data set consisting of 195 long series of daily mean flow records was selected, based on such criteria as length of series, currency, lack of gaps and missing values, adequate geographical distribution, and priority to smaller catchments. The analysis of annual maximum flows does not support the hypothesis of ubiquitous growth of high flows. Although 27 cases of strong, statistically significant increase were identified by the Mann-Kendall test, there are 31 decreases as well, and most (137) time series do not show any significant changes (at the 10% level). Caution is advised in interpreting these results as flooding is a complex phenomenon, caused by a number of factors that can be associated with local, regional, and hemispheric climatic processes. Moreover, river flow has strong natural variability and exhibits long-term persistence which can confound the results of trend and significance tests.They conclude (emphasis added):
Destructive floods observed in the last decade all over the world have led to record high material damage. The conventional belief is that the increasing cost of floods is associated with increasing human development on flood plains (Pielke & Downton, 2000). However, the question remains as to whether or not the frequency and/or magnitude of flooding is also increasing and, if so, whether it is in response to climate variability and change.
Several scenarios of future climate indicate a likelihood of increased intense precipitation and flood hazard. However, observations to date provide no conclusive and general proof as to how climate change affects flood behaviour.
Bouziotas, D., G. Deskos, N. Mastrantonas, D. Tsaknias, G. Vangelidis, S.M. Papalexiou, and D. Koutsoyiannis, Long-term properties of annual maximum daily river discharge worldwide, European Geosciences Union General Assembly 2011, Geophysical Research Abstracts, Vol. 13, Vienna, EGU2011-1439, European Geosciences Union, 2011.
Kundzewicz, Z.W., D. Graczyk, T. Maurer, I. Przymusińska, M. Radziejewski, C. Svensson and M. Szwed, 2005(a):Trend detection in river flow time-series: 1. annual maximum flow. Hydrol. Sci. J., 50(5): 797-810.