In recent decades, summer floods in China's Yangtze River basin have become more frequent and severer, threatening the economic development and ecosystem environment in the basin. While great efforts and a large amount of resources have been devoted to minimizing damages and mitigating consequences of floods, several scientific questions are raised:what may have attributed to the increase of the floods, and what may be anticipated of future floods? These questions are addressed in this note from a perspective of natural variability of the region's climate. Major results show that a centennial scale variation of the region's climate has shifted from a dry epoch, from 1910 to the late 1960s, to a wet epoch after the 1960s. Associate with this change are 1) a trend of increase in summer rainfall at an average rate of 30mm per ten years in the eastern portion of the basin, 2) a trend of decrease in summer season surface evaporation at an average rate of 10mm per ten years in the same region, and 3) a combined result of rising surface runoff and flood potential. These trends are speculated to continue because the current wet epoch in the centennial variation will persist in the next few decades.