本案例中为何要实地调查河流的流向:
①地势起伏很小 ② 河流密集 ③ 湖泊众多 ④ 地下管线多 ⑤ 河流季节流向变化 ⑥ 辫状水系
Why is it necessary to investigate the flow direction of the river in this case?
①small terrain undulations ② many rivers ③many lakes ④many underground pipelines ⑤seasonal change of the flow direction ⑥braided rivers