The mystery of sailing on the Dan River

Dan River is a north-south river in Jincheng area. Originated in Danzhuling in the north of Gaoping County, it flows out of the province to the south via the east of Jincheng and flows into Qinhe River, with a total length of1.62km.. North of Jincheng is the Loess Basin, which is a seasonal river. South of Jincheng is a limestone area with spring water supply. It is a Qingshui River with water all year round. The basin covers an area of 3 150 square kilometers, with an annual total flow of 340 million cubic meters. Since the Tang Dynasty, the Danhe River has been open to navigation, and the grain in Jincheng is transported through the Danhe River, which is connected with the Danwei Canal in Henan Province, and the water transport can go directly to Beijing (Xiao Dan River in Ming Dynasty).

As the Danhe River is replenished by underground karst water, the more downstream water flows, the spring flow reaches 6 ~ 7 cubic meters per second to Sangu Spring, the provincial boundary, and the average annual flow of Henan into Danhe River can reach 10 cubic meters per second.

Today, after the Dan River entered the limestone area, the bedrock in the depth of the river turned into a steep canyon. The valley is wide 100 meter, with many rapids, dangerous beaches, pools, waterfalls and boulders, and there are no navigation conditions at all. If it is to be navigable, the river must rise another 7-8 meters to avoid boulders 5-6 meters high in the river. Did ancient rivers have such a large flow?

The annual flow above Sanguquan in Danhe River is only 3 ~ 4 m3/s, which is paved on the waterway with an average width of 20 ~ 30m. Based on the flow rate of 1 m per second, the average water depth of the river is only 0.1~ 0.2m.. Imagine, in the rainy season, the flood surges and the boulders are submerged, but at this time, the speed of the Dan River will reach 4 ~ 5 meters per second, so the risk of sailing is far greater than such a tortuous and multi-bay waterway. Therefore, unless the rainfall in the Ming Dynasty is more than twice that of today (reaching 1800 mm), the Danhe River is navigable.

Hexagonal prism with transverse seam line