Qin dynasty dates back to 221 BC.
Mountain river is a river that runs usually in mountains , in narrow , deep valley with steep banks , rocky stream bed , and accumulated rock debris .. Mountain rivers are characterized by high slope and flow velocity , insignificant depth , frequent rapids and waterfalls , dominated washed out processes .. The gradient of mountain river is calculated at 60 -- 80 m / km in upper stream and 5 -- 10 m / km in lower .. gradient. gradient. More precisely it is greater than or equal to 0.002 m / m along most of its stream length .. Speed of stream is measured at 1 to 4.5 m / s and higher .. Mountain rivers have significant hydro energy producing potential and in arid conditions often used for irrigation .. hydro energy. Hydro energy conversion system. arid. arid