Impacts Of Hydropower Cascade Development In Southwest Mountainous Area On River Ecological Factors

In the current river basin management, the contradiction between River cascade hydropower development and river ecological environment has become an urgent major issue to be solved. Cascade hydropower development project led to a series of negative impact on the environment directly or indirectly, such as water environmental change, damage of river continuity, changes of biological habitat and so on, which made cascade hydropower development much more controversial. At present, relatively less ecological impact researches of river hydropower development, esp. on medium and large scale have been carried out in southwest mountainous area, compared with other geomorphic units in China, and nor is there a rich of knowledge of its response mechanism of river organisms and ecological processes to environmental change. Taking cascade hydropower stations (Qiaoqi, Baoxing, Xiaoguanzi, Tongtou and Yucheng power station) in Baoxing river valley as the research object, focusing on the individual to the system level, this paper carried on research of ecological effect of river hydropower development in the medium and large scale from different aspects, namely water environment, aquatic organisms, plant litter decomposition, and Effects on landscape scale. The main results were summarized as follows:1. Effects of cascade hydropower station construction on water environment(1) After the operation of the cascade hydropower stations, the flow type of the influenced river section changed from jet type into artificial regulated library flow, and the variation range of water level became larger than that of original natural river channel. For Qiaoqi hydropower station, the variation of reservoir water level is with stronger regularity within a year, and the minimum and maximum water level changed between the dead water level 2060 m and the normal storage level 2140 m, with 71.6 m of water level year average change; for the other four stations, the year average change is relatively small, respectively 1.3 m,0.9 m,2.0 m and 1.7 m.(2) The operation of cascade hydropower stations induced water reducing effect on the downstream river water, thus water reducing river section was formed between the dam and the workshop in downstream. The water reducing effect was different between different cascade hydropower stations.1) Qiaoqi downstream reducing river was divided into three parts. The 586m section of river from the dam to the mouth of ecological discharge tube was dehydrated, and there was no guarantee for the ecological flow. And from the mouth of ecological discharge tube to the first branch only 1.7 m3/s ecological flow could be guaranteed. In the third water reducing section from the first branch to the tail water outlet, the river had been effectively hydrated, getting a certain ease for water reducing effect.2) Baoxing downstream water reducing river was divided into two segments. From the dam down to the first branch, it could only guarantee 3-4m3/s ecological flow through the ecological discharge orifice discharge, and from the first branch to the export of tail water, the effects of water reducing was relatively low.3) For Xiaoguanzi and Tongtou power station, ecological flow discharge was not considered, leading to frequent water reducing in dam downstream section. According to this study, the two power stations were recommended to set ecological flow 8.91 m3/s and 9.86m3/s respectively.4) Because Yucheng power station is runoff hydropower station, there is no problem of downstream river water reducing.(3) According to actual measured results of water temperature in Qiaoqi and Tongtou station, there was no obvious stratification of water temperature in Qiaoqi, and for Tongtou, no stratification existed with completely mixed structure of water temperature. In Qiaoqi station, the water temperature in plant downstream tail gate was about 3.0℃ higher than that of the natural channel, with temperature raising interval between 3.6℃ and 5.2℃. Given that the monitoring time was in winter and spring, combined with the limited data quantity, long-term monitoring should be taken to understand the influence on water temperature.(4) In Qiaoqi reservoir area, CODMn, total nitrogen and total phosphorus scored basicly between 30 and 50, showing that water is in the state of medium eutrophication state, with no high eutrophication phenomenon happens. In addition to the ammonia nitrogen and total nitrogen levels, the rest parameters of all met the standard of class I water quality in Baoxing reservoir. The exceeding of water quality of ammonia nitrogen and total nitrogen were not affected by the project, but mainly by river sewage and agricultural non-point source pollution along Baoxing River. For Xiaoguanzi and Tongtou station,20 year after plant construction, no industrial and mining enterprises were built and no new sewage was added, so the water quality was not affected. Although the pollutant carrying capacity fell slightly, no great change happened for river water quality in the two stations, meeting the requirements of class Ⅱ standard of water quality. In general, the overall quality of the reservoir water of cascade hydropower stations can basically meet the requirements of water quality standard, with no deteriorating in water quality. The distribution of pollution sources and hydrating situation from river tributaries had great influence on the water quality. Due to the low pollution load and good tributary filling condition in downstream water reducing river section, no deteriorating of water quality took place.(5) All the cascade hydropower stations need the discharging of ecological flow, except Yucheng station, for it is runoff hydropower station, with no problem of water reducing in downstream. Based on Tennant method, Ecological hydraulic method and wetted perimeter method, and considering the geographical location and environmental situation, we recommended that the discharging ecological flow could be set 2.39 m3/s、3.89 m3/s、8.91 m3/s and 9.86 m3/s for Qiaoqi, Baoxing, Xiaoguanzi and Tongtou respectively.2. Aquatic biological community characteristics and their response to cascade hydropower development(1) Cascade hydropower development activities bring a certain change to the floristic composition, density and biomass of the plankton and zoobenthos. Changes of phytoplankton: overall number of phytoplankton species was significantly reduced, phytoplankton structure was simplified, but it was still dominant in bacillariophyta, and cyanophyta and chlorophyta were in larger proportion. Effects on zooplankton:floristic composition had experienced an obvious reduction in zooplankton species number, with simple composition and less in quantity, dominated by native animals. Zooplankton density increased in the investigated river sections. Effects on benthic animals:in all the investigated river section, benthic animal species significantly decreased in number, esp. in Qiaoqi station and Minzhi station, with ephemeroptera predominating all the sections. The physical and chemical properties, including dissolved oxygen, water conductivity, pH, water depth and salinity, water temperature, altitude and REDOX potential salinity, take certain effects on the change of types, quantities and distribution of Plankton and zoobenthos.(2) Cascade hydropower development activities make the fish population further simplification, larger fluctuation exists in different fish populations. The population of the original inhabitation species in the reservoir area, like schizothorax, Sisoridae and Beaufortia fell sharply respectively, and Oreias dabryi, Paracobitis and Triplophysa gradually transformed into advantage species distributed in the reservoir area and the water reducing section of the river. Affected by the construction of cascade hydropower stations, the spawning grounds of some fishes under the dam of each station Shrunk or disappeared. The effects of power plant construction on wintering and feeding grounds is relatively small. At present, Qiaoqi reservoir area is the largest fish wintering grounds, and then is Baoxing, Xiaoguanzi, Tongtou and Yucheng reservoir in turn.3. Response of litter decomposition to water level fluctuationTaking dominant species alder and cryptomeria in littoral zone as experimental materials, the effects of different water depth (0m, 1m,5m,10m) on litter decomposition rate, nutrient dynamics and the dynamic changes of the microbial community were compared using methods of decomposition bags and in-situ experiments in this paper. Experimental results show that:1) no significant difference (P>0.05) exists between the decomposition rates of the two species under four kinds of processing.2) species, water depth and time all exerted a significant impact on the release of N and P. In general, N, P release rate of Alder was greater than the cryptomeria. Eventually alder characterized by net release, but for cryptomeria, element accumulation emerged in some of the processing. In addition, in processing of different water depths, the relative importance of bacteria and fungi at different decomposition stages is different. With the increase of water depth (5 m,10 m) and the progress of decomposition, the number of alder fungi and bacteria gradually reduced, but fungal biomass rose in the early decomposition and then took on a reducing process, then a rising trend again in the late decomposition. That is related to the fact that low oxygen content in the relatively deep layer (5 m and 10 m) is lower than that in the shallow layer (0 m and 1 m), and oxygen environment inhibiting bacterial decomposition, but also conducive to the degradation function of some special fungi on lignin, cellulose. Obviously, the change trend of bacteria and fungi is not only affected by the decomposition material characteristics and the quality of the decomposition time, but also associated with the change of water depth. The results of the study would provide reference to effect study of the reservoir water level fluctuation on litter decomposition.As an important function of ecological system, decomposition is not only related to the quality characteristics of the resource and the biological species of the disintegrator (bacteria, fungi and invertebrates), but also controlled by the physical and chemical conditions of decomposition. River hydrology situation changes induced by power station reservoir will affect the biological community diversity, breathing and biomass of the disintegrator, and then further affect some important ecological processes of the related river and the structure and function of its ecosystem. At present, few researches have been carried out on the response of decomposition to the hydrological environment changes induced by the construction of cascade hydropower stations. This study can provide reference on the explanation of ecological impact of the construction of cascade hydropower stations.4. Effects of cascade hydropower station construction on landscape scale(1) Moving window based fragmentation index calculation combined with Spatial Correlation Analysis can clearly show the spatial distribution rule of the landscape fragmentation and its relationship with related spatial factors on a local scale.(2) Although there was almost no relationship between the selected factors and the Change of DIVISION (CODIV) from the view of the whole range of study area, when comparing the latter with the former intervals, it proved that the human activities of plant building led to the increase in the DIVISION value.3) Correlation analysis after reclassification of the selected factors showed that high positive relationship existed between slope (SLP) and CODIV, where r was 0.857, while the relationship between distance from river line (DFR) and CODIV is high negative correlation, where r was —0.837. Medium negative relationship lay between elevation (ELV) and CODIV, while there was no obvious linear relationship between Distance from Qiaoqi Reservoir (DFQ) and CODIV.