Problem 12, (3)B Natural causes: loose soil, heavy rain, easy to cause soil erosion, less vegetation on the surface, difficult to maintain soil and water 13, (3) favorable: flat terrain. Disadvantages: there are many swamps along the way, and it is necessary to cross many rivers, which is difficult to construct and global warming. . 13, (2) distributed along the west coast and along the river in the east. Reason: Flat terrain and convenient water supply. Question 13, (1) Features: Branches are in the east and northeast. Reasons: ① It is rainy all year round and the surface water is abundant. ② There are many rivers and dense water networks. (3) Northeast China is a plain with low terrain and poor drainage. Question 12, (2) The process that the flow decreases, the velocity slows down, the sediment concentration decreases, and the ice age increases 12, (1) impacts the fan plain: ① The water in the Loess Plateau flows out from the Taihang Mountain mouth and enters the North China Plain, and the terrain suddenly becomes flat; (2) The water flow slows down, and the materials transported by the river gradually accumulate at the foot of the mountain, forming a fan-shaped landform.