长辛店地区位于北京城之西南,北距卢沟桥二公里。该区平均年雨量633毫米,雨季集中于七、八月份。原小清河流经该区东部,东河、李家菜园水厂皆傍河诱导取水。1959年该区小清河河谷上游横截土坝蓄水,即现在的大宁水库(图1)。河谷下游之河床 Chang Xindian area is located in the southwest of Beijing, two kilometers north of the Marco Polo Bridge. The average annual rainfall of 633 millimeters in the area, the rainy season concentrated in July and August. The original Xiaoqing River flows through the eastern part of the district, Donghe, Lijiayuan waterworks are all induced by water near the river. In 1959, the upper reaches of the Xiaoqinghe River valley intercepted earth dam water storage, which is now the Daning Reservoir (Figure 1). Riverbed downstream of the valley