ABSTRACT: The Upper Yangtze (Changjiang) aquatic ecosystem supports some of the world’s highest fish diversity. Three international critically endangered fishes and 67 endemic fish species, are reported to inhabit this area. It is also a rich hydropower resource, with 20 dams constructed or planned, which is transforming the reaches into a dam/reservoir system. A scientific assessment of the ecology of a remaining free-flowing stretch encompassing the Upper Yangtze National Reserve is critically important. A physical environment survey was conducted in a free-flowing reach (411.5 km river length, 93.1% of the length is included in a national nature reserve) of the Upper Yangtze main stream from Chongqing to the Xiangjiaba Dam using a 600 kHz Acoustic Doppler Current Profiler. A total of 20,558 effective survey points along the river channel thalweg indicated that: the water depth range was 3.40−60.73 m (mean 12.51±8.03 m), and the commonest depth range within seven of the nine defined sub-reaches was 5.0−10.0 m; the range in depth-averaged velocity (DAV) was 17.48−397.69 cm/s (mean 159.34±61.26 cm/s), with the most common velocity range in seven of the sub-reaches being 100.0−150.0 cm/s; the mean Froude number (Fr) was 0.171±0.095 (range 0.010−0.513) with 0.10−0.15 the most common Fr range within six of the sub-reaches. Habitat classification methods based on 1) the Fr and 2) the Fr combined with water depth and DAV revealed a saturation index of 0.79−0.91 and 0.61−0.83, respectively. Pools are the rarest habitat type and need to be protected. We strongly urge a moratorium on dam construction in this area to maintain it as a free-flowing river to conserve the habitat and its fish diversity.

Hui Zhang, Qiwei Wei*, Liyuan Sun. Environmental assessment of a free-flowing high-biodiversity stretch of the Upper Yangtze River: implications for river management. Proceedings of 35th IAHR World Congress.