Study On The River Ecological Integrity Under The Threat Of Hydropower Development In LRGR

Dam construction is considered as the major factor contributed to the significant modifications of river ecosystems. The main concerns on the disturbances of dam construction are the changes of hydrology, river morphology and habitat. Longitudinal Range-Gorge Region (LRGR), composed of Hengduan Mountains that are related to Tibetan uplift and the adjacent mountain-valley regions on south-north direction, lies in Southwestern China. Three famous transboundary rivers flow through the LRGR, i.e., Nu-Salween River, Lancang-Mekong River and Yuan-Red River. All of them have been planned to have or have had cascade hydropower dams constructed on the mainstreams. The development of cascade hydropower dams may alter the structure and functions of river ecosystems. The objectives of this paper are to (1) predict the changes of key ecological factors after the construction of cascade hydropower dams on the Lancang River; (2) develop a general model for predicting the changes of river ecosystem integration REI after the construction of cascade hydropower dams; (3) estimate the changes of REI of the Nu River and Yuan River after the planned construction of cascade hydropower dams; (4) determine the thresholds of pressures due to the constructions on the Lancang River, Nu River and Yuan River; (5) analyze the effects of ecosystem management measures.The main conclusions are as follows:(1) The concept of the ecological integrity was defined form the general and narrow sense by analyzing the relationships between the ecosystem structure and function. The ecosystem integrity requires not only the integrity and rationality of the structures, but also the natural and general functions. The ecological integrity is a concept to describe the ability to maintain ecosystem health and continuous evolution. It has both the natural attributes and the social attributes.(2) The drive mechanism of the dams and the response mechanism of the rivers were analyzed based on the single factors and the whole ecosystem. The cumulative effects of the dams were also analyzed. The source of the driving effects of the dams is the division effects. Factors of the river ecosystem then changed based on the driving source. The cumulative ecological effects of the dams were caused by the congested time or space. There are three types of cumulative effects.(3) Prediction of the change of the key indicators after the Lancang Cascade development. The main conclusions are as follows:â‘ Soil erosion intensity. Soil erosion intensity in the 2km buffers of the Lancang river increases as the development of the Lancang Cascade. The average annual soil erosion modulus will reach 3222 t/km2·a in 2050.â‘¡Width-depth ratios of the river section. There is a significant decrease of the width-depth ratios after the development of the Lancang Cascade. The average width-depth ratios before and after the cascade construction are 14.42 and 2.29, respectively. â‘¢Sediment accumulation. The theoretical trapping efficiency and the sediment accumulative quantities of the cascade dams on Lancang River were calculated. The highest and the second theoretical trapping efficiency were found at Nuozhadu and Xiaowan dams, with the values of 92.2% and 91.87%, respectively. The largest quantities of sediments to be trapped by 2050 were also found at Xiaowan and Nuozhadu reservoirs, with the values of 9.62 and 13.58×108m3, respectively. By 2050, all of the dead storage in Manwan reservoir will be disappeared.â‘£River densities. The river densities in the 2km buffers have a significant change before and after the development of Lancang Cascade. The river densities were found between 0.50 and 0.90 before the cascade constructions and between 0.30 and 0.70 after the cascade constructions. The average river densities before and after the cascade construtions were 0.66 and 0.52, respectively.⑤Habitat function. After the development of the Lancang Cascade, the habitat environmental index will degenerate from 1.00 to 0.33 and the habitat diversity index will degenerate from 1.00 to 0.52.â‘¥Water resources regulating function. The total effective storage of the reservoirs increases continuously due to the development of cascade dams. The total effective storage of all the reservoirs is 296.5×108m3.⑦Storage coefficient. The average storage coefficient of the Lancang Cascade was 0.06. The total storage coefficient was 0.75.⑧Runnability. The length of the natural running water will be only 3% after the Lancang Cascade construction.(4) The ecological integrity of Lancang River degenerates rapidly as the cdevelopment of Lancang Cascade. The REI index decreased from 0.825 (excellent) before the cascade construction to 0.309 (bad) after the cascade construction. The scale and the number of the dams were selected to represent the pressures. Fitted regression curves were used to describe the REI changes as the increasing pressures due to the dam construction. The regression model (DRIM) wasâ–³S=2.295P2+0.685P+0.141.(5) The study drew on the ideal prediction model of ecological integrity changes under the drive of the dam constructions (IDRIM). Suppose the only drive is the dams. REI will decrease at an accelerated speed at the period of continuous construction of dams (â–³S=f1(t)=ak02t2+bk0t+c (â–³P>0)) and at a uniform speed after the construction of dams (â–³S=f₂(t)=(2ak₀²m₁t₀+bk0)(t-m₁t₀)+ak₀²m₁²t0+bk₀m₁t₀+c (â–³P=0)). After the decommission of dams, REI will decrease at a decelerated speed (â–³S=f3(t)=-ak02t2+(2ak02(m1-m2)t0+bk0)t-ak02(m12+m22)t02+c (â–³P<0)). The river ecosystem will restore at this period if there is effective human protective measures.(6) Before the development of the Lancang Cascade, the REI indices of the Nu River, Lancang River and Yuan River were 0.844, 0.825 and 0.719, respectively. After the development of the Lancang Cascade, the REI indices of the Nu River, Lancang River and Yuan River were 0.356, 0.309 and 0.389, respectively.(7) The threshold values of the pressure caused by the dams of the ideal river, Lancang River, Nu River and Yuan River were 0.480, 0.417, 0.424 and 0.374, respectively. (8) Three ecological goals of rivers in LRGR were set in the paper. The ERI values of the highest, middle and low goals were 0.34～0.35, 0.40～0.41 and 0.48～0.51, respectively. Fiver sensitive factors were picked up for ecological regulation. Suppose the pressure caused by the dams is fixed. The regulating range of the ecological factors should be 10%～15%, 40%～50% and 70%～80%, respectively. Suppose the human response is fixed. The pressure value caused by the dams should be 0.26～0.27, 0.23～0.23 and 0.16～0.18 for Lancang River, 0.27～0.28, 0.24～0.24 and 0.18～0.20 for Nu River, 0.20～0.21, 0.16～0.17 and 0.08～0.11 for Yuan River.