Pekarova et al. (2003) analyzed the annual discharge rates of selected large rivers of the world for recurring cycles of wet and dry periods.  For those rivers with sufficiently long and accurate data series (primarily European rivers), they also derived long-term discharge rate trends.  Their analysis, in their words, "does not show any significant trend change in long-term discharge series (1810-1990) in representative European rivers," including the Goeta, Rhine, Neman, Loire, Wesaer, Danube, Elbe, Oder, Vistule, Rhone and Po, even though this 180-year time period saw a climatic transition that propelled the planet from the depth of the Little Ice Age to what climate alarmists claim is the warmest period of the past two millennia, which finding does little to support their contention of CO2-induced global warming raising havoc with earth's hydrologic cycle.  In fact, it pretty much contradicts it.

Focusing on a somewhat longer time span, Pederson et al. (2001) utilized tree-ring chronologies from northeastern Mongolia to reconstruct annual precipitation and streamflow histories of that region for the period 1651-1995.  Their analyses of standard deviations and 5-year intervals of extreme wet and dry periods revealed, as they put it, that "variations over the recent period of instrumental data are not unusual relative to the prior record."

In a similar study, Hidalgo et al. (2000) also utilized tree-ring chronologies, in their case to reconstruct a streamflow history for the Upper Colorado River Basin of the United States.  The results of their investigation showed evidence of what they describe as "a near-centennial return period of extreme drought events in this region," which stretched all the way back to the early 1500s and once again suggested nothing unusual about the 20th century in this regard.

Moving to a millennial time-scale context, Campbell (2002) analyzed the grain sizes of sediment cores obtained from Pine Lake, Alberta, Canada (52°N, 113.5°W) to provide a non-vegetation-based high-resolution record of climate variability for this part of North America over the past 4000 years.  This effort revealed periods of both increasing and decreasing grain size (moisture availability) throughout the 4000-year record that recurred at decadal, centennial and millennial time intervals.  The most predominant departures included several-centuries-long epochs that corresponded to the Little Ice Age (about AD 1500-1900), the Medieval Warm Period (about AD 700-1300), the Dark Ages Cold Period (about BC 100 to AD 700) and the Roman Warm Period (about BC 900-100).  In addition, a standardized median grain-size history revealed that the highest rates of stream discharge during the past 4000 years occurred during the Little Ice Age at approximately 300-350 years ago.  During this time, grain sizes were about 2.5 standard deviations above the 4000-year mean.  In contrast, the lowest rates of streamflow were observed around AD 1100, when median grain sizes were nearly 2 standard deviations below the 4000-year mean.  Most recently, grain size over the past 150 years was found to have generally remained above average.  This sediment record demonstrates the reality of the non-CO2-induced millennial-scale climatic oscillation that alternately brings several-century periods of relative dryness and wetness to the southern Alberta region of North America during concomitant periods of relative hemispheric warmth and coolness, respectively.  It also demonstrates there is nothing unusual or unexpected about the region's current moisture status.

Documenting periods of extreme floods was the objective of Brown et al. (1999), who analyzed proxy measures of the flow of the Mississippi River over the past 5300 years.  Their analysis revealed the occurrence of large megafloods at about 4700, 3500, 3000, 2500, 2000, 1200 and 300 years ago that were, in their words, "almost certainly larger than historical floods in the Mississippi watershed."  Furthermore, they concluded these fluvial events were likely "episodes of multidecadal duration," spawned by an export of extremely moist air into mid-continental North America that was driven by natural oscillations in Gulf of Mexico ocean currents.

In conclusion, as ever more data are analyzed, there is ever more evidence that nothing unduly dramatic has happened to streamflow rates around the world as the air's CO2 content has continued to rise in concert with the natural warming that at long last broke the Little Ice Age's chilly grip on the planet's climate.  What has happened, in fact, is rather mundane, amounting to little or nothing. And even if there were to be some significant changes in streamflow characteristics, such as those found to accompany prior appearances of the warming phase of the millennial-scale oscillation of climate that has reverberated throughout the Holocene, there would still be no reason to attribute those changes to anthropogenic CO2 emissions, as it is much more reasonable to associate them with the natural warming of the planet that experience teaches us should be occurring at the present time in light of past climate periodicity.

References
Brown, P., Kennett, J.P. and Ingram B.L.  1999.  Marine evidence for episodic Holocene megafloods in North America and the northern Gulf of Mexico.  Paleoceanography 14: 498-510.

Campbell, C.  2002.  Late Holocene lake sedimentology and climate change in southern Alberta, Canada.  Quaternary Research 49: 96-101.

Hidalgo, H.G., Piechota, T.C. and Dracup, J.A.  2000.  Alternative principal components regression procedures for dendrohydrologic reconstructions.  Water Resources Research 36: 3241-3249.

Pederson, N., Jacoby, G.C., D'Arrigo, R.D., Cook, E.R. and Buckley, B.M.  2001.  Hydrometeorological reconstructions for northeastern Mongolia derived from tree rings: 1651-1995.  Journal of Climate 14: 872-881.

Pekarova, P., Miklanek, P. and Pekar, J.  2003.  Spatial and temporal runoff oscillation analysis of the main rivers of the world during the 19th-20th centuries.  Journal of Hydrology 274: 62-79.