(1) By analyzing the information in the map, it can be seen that the industries, technologies, talents and information along the Yangtze River radiate from the north to the south through numerous tributaries and railway trunk lines, which makes China's north-south economy integrated. The west can enter the international market with the help of river-sea combined transport.

(2) The middle and lower reaches of the Yangtze River are prone to floods in summer due to the numerous tributaries of the Yangtze River, abundant precipitation and fewer natural lakes. The control measures include building reservoirs, returning farmland to lakes and controlling soil erosion.

(3) The Hanjiang River flows into the mainstream of the Yangtze River in Wuhan, and Shanghai is located at the mouth of the Yangtze River.

(4) According to the analysis in Figure 2, compared with the 1950s, the forest coverage and lake area in the Yangtze River basin decreased in 1990s, while the area of soil erosion increased. The main man-made reasons for this phenomenon are destroying vegetation, reclaiming land from lakes and planting on steep slopes.

So the answer is: (1) tributary; Railway trunk line; International; (2) There are many tributaries, abundant precipitation, and the area of natural lakes is reduced; Building reservoirs, returning farmland to lakes and controlling soil erosion;

(3)

(4) reduce; Increase; Destroying vegetation, reclaiming land from lakes, and reclaiming land on steep slopes.