(2) The middle reaches of the Yellow River cross the Loess Plateau with loose soil and sparse vegetation. Under the scouring of heavy rain in summer, soil erosion in the Loess Plateau is serious, and nearly 90% of the sediment entering the Yellow River comes from the middle reaches.
(3) The sediment concentration at Lijin Hydrological Station is less than that at Shanxian Hydrological Station. This phenomenon is caused by the widening of the lower reaches of the Yellow River, slow water flow and a large amount of sediment deposition.
(4) The loess plateau has serious soil erosion and the surface morphology is broken. Ten measures to prevent and control soil erosion in the Loess Plateau include engineering measures, agricultural technical measures and biological measures. Specific measures are vigorously planting trees and grass, increasing vegetation coverage, rationally arranging production activities, returning farmland to forests and grasslands, transforming terraces and building dams.
Therefore, the answers are: (4) Estuary; Peach blossom rain; (2) the middle reaches; Loess Plateau; (3) small; The lower reaches of the Yellow River are widened, the water flow is slow, and a large amount of sediment is deposited. (4) Planting trees and grass, returning farmland to forests and grasslands, etc.