It took $80 million, five years, and enough trees to circle the world five times. But Lanzhou, a gritty industrial city on the Yellow River in northern China, says it is stopping erosion that is clogging one of the country's main waterways with silt and making it more prone to deadly floods."Planting a tree in Lanzhou takes more effort than raising a child," Zong Xiaoxu, director of the city's chief forestation project, said. "After completion of the project, we feel that the erosion problem has been greatly alleviated," Zong told reporters on a hill overlooking the Yellow River.