The lowest ecological water demand in the 119 km reducing reach of Yalong river downstream of Jinping waterpower station stageⅡ (Jinping Ⅱ) as an example is studied by the method of eco hydraulics. The results indicate that the lowest ecological discharge in the reducing reach in the dry season is 45 m3/s in order to meet the subsistence of fish. In the condition of 45 m3/s river flow, the 95% river channels can satisfy the hydraulic factors for the subsistence of fish. These hydraulic factors include water depth, velocity, water surface width, wetted perimeter ratio, cross section area, and water surface area. The change of water temperature will not affect fish spawning in 45 m3/s. The place of hydraulic form, which includes torrent, subcritical flow, riffle, deep pool will change, but their amount remains above the same in 45 m3/s. Generally speaking, as the reduction of river discharge, the biomass will reduce. However, the amount of animal community will remain as long as the river discharge exceeds 45 m3/s. This example severs as an exploring work in calculating ecological based on flow in river channels.
