三峡库区支流生境因子对库区蓄水的响应

三峡水库的运行调度,使库区支流形成了截然不同的3种河段类型:完全受水库蓄水影响的145m回水段(完全河段),既受蓄水影响又受自然洪汛影响的145—175m回水段(双重河段)以及不受蓄水影响的大于175m的自然河流段(自然河段)。为明确库区蓄水对河流不同河段生境因子的影响程度及差异,对三峡库区36条重要支流的254个河段进行河流生境调查,进行不同河段下生境指标的因子分析,并进一步分析水文情势自然性与不同河段各生境因子的相关关系。结果表明,植被状况对3种不同河段来说均为重要生境因子,但受三峡水库蓄水影响,完全河段植被结构不完整;受库区蓄水影响,完全河段与双重河段及自然河段相比,流速流态状况、表层覆盖物状况、河岸带宽度、湿润率等生境因子有明显改变;水文情势自然性对不同河段生境因子的影响不同。

Response of habitat factors to reservoir impoundment in tributaries in the Three Gorges Reservoir Region

Water level management strategies specify that the impoundment of the Three Gorges Reservoir (TGR) starts from 145m asl in September to 175m asl in November. The water level is maintained for three months and then returned to 145m asl again in May. Consequently, three types of water-level-fluctuation zones appear along the Three Gorges Reservoir tributaries banks:(1) The area from the estuary to the 145-m backwater (completely-flooding reach), where the former natural riparian zone is submerged permanently since its elevation is below 145m asl; (2) the 145-m backwater area to the 175-m backwater area (double-flooding reach), which is affected by summer flooding from July to September as well as by impoundment from September to May; (3) the area above the 175-m backwater area (natural reach), which is not affected by reservoir water storage. In order to distinguish the influence of reservoir water storage on habitat factors in the different river reaches, we investigated the river habitats of 254 reaches in 36 major tributaries in the Three Gorges Reservoir area. Based on the results of these investigations, we used factor analysis to assess the 18 indicators on river hydrology, river morphology, and the water-level fluctuation zone habitat of different river habitats, and analyzed the relationship between the degree of flow regime naturalness and habitat factors in different reaches. The results showed that vegetation conditions are important habitat factors in three different types of sections. Affected by the impoundment of the Three Gorges reservoir, most riparian vegetation is herbaceous and flooding-tolerant, with nearly no trees or shrubs. Compared with the double reach and natural reach zones, the completely reach zone was subject to significant changes in the flow pattern, surface coverage, width of the buffer zone, and wetting rate. The effects of flow regime conditions on habitat factors in different river reaches differed.