(1)河道概况永定河自官廳以下,奔流于八达嶺乱山间,河槽深窄,多急灘乱石。出三家店到北天堂附近,屬山麓段,河道放寬,河床多为卵石夾沙。北天堂以下流经平原地帶,为沙底河道,河床高仰于堤外地面以上。 (1) Overview of the river Yongding River from the Guanting Hall, running in the Badaling Mountains, deep river narrow, more urgent beach rocks. Out of the three stores to the vicinity of the North Heaven, is a foothills, river relaxation, the riverbed is mostly pebble sand. Below the northern paradise through the plains, as the bottom of the river, the river Yangyang embankment outside the ground above.