The middle and lower reaches of the Changjiang (Yangtze) River are the heavy disastrous region of flood and waterlogged damages in China. The flood and waterlogged damages are resulted from the conjugate evolution between the natural environment systems and the human social systems. The change of climate system is the direct leading factor to cause flood and waterlogged damages. The factor evolution of environment geology is the background condition to form the flood and waterlogged damages. The ENSO event and monsoon anomaly are the trigger mechanism of flood and waterlogged damages, and the precipitation course and river hydrological features in the flood season are the main signs of flood and waterlogged damages.

The change of climate system is the leading factor of flood and waterlogged damages in the middle and lower reaches of the Changejiang River, and this change is seriously influenced from the external forcing. In the middle and lower reaches of the Changjiang River, the external forcing is displayed as ENSO events, accumulated snow in the Qinghai-Xizang plateau, the mid-latitude air-sea interaction, and hydrological cycle of land-atmosphere system. The direction of climate change is influenced by the intensity of external forcing. It is confirmed as follows as you know:

1. the ENSO events are connected with the variability of the Summer Monsoon over Asia, the summer rainfall in China, and summer flood in the middle and lower reaches of the Changjiang River,
2. the accumulated snow in the Qinghai-Xizang Plateau is correlated with the run-off main area for Changjiang River,
3. the atmospheric circulation is responded to the SSTA. Under the external forcing, some changes of the internal course of atmospheric dynamics will take places such as the low-frequency oscillation (LFO), the monsoon anomaly in Asia, and the anomaly of atmospheric circulation in East Asia. The disastrous rainstorm is followed the changes.

The regional environmental, hydrologic and geological conditions are the background factors to form flood and waterlogged damages. The flood course and features are effected by the features of river system and riverbed, as well as soil moisture. And then the regional disastrous condition is also determined by those factors in some case. With the consideration of high rational level, it maybe is one of basic factors to result on the change of climate system that human activities change the lower surface of atmosphere. The condition of environmental destruction and the validity of flood regulation will result on the loss condition of the disasters directly.

In this paper, the relationship of conjugate evolution between the climate change and the flood disasters in the middle and lower reaches of the Changjiang River are analyzed in the case of 1998 flood disasters in the Changjiang River Valley.