Global river systems have undergone major changes due to intensive anthropogenic activities, such as land use/cover change, irrigation, water diversion, and dam building and operations. According to incomplete statistics, more than half of the 300 biggest river systems around the world are controlled by or under the impact of dams [1
]. Up to now, ≈2.8 million dams (with reservoir areas >103
) have been built and more than 3700 hydropower dams (>1 MW) are currently planned or under construction worldwide [2
]. Undoubtedly, dams or reservoirs play an important role in contributing water for domestic use and agriculture, sustaining transportation corridors, and enabling power generation and industrial production. However, they also affect many fundamental processes and functional characteristics of healthy rivers, such as the flow regimes [3
], sediment conditions [6
], and thermal characteristics [9
], thus leading to the rapid decline of aquatic biodiversity and essential ecosystem services [12
]. The alterations of natural river conditions could influence the critical habits of species [15
], since each species may have an “optimal environmental window” for reproductive success and recruitment maximization [17
]. Spawning is a vital and sensitive time for most fish, and this period is thus often deemed as a representative pivotal habit for them, whose breeding is closely related to hydrological factors [20
The Three Gorges Reservoir (TGR), located over the Yangtze River main stream, is the largest hydropower project in the world (Figure 1
). It was built for flood control, navigation, and hydropower generation. It started operations in June 2003, with its dam bottom release and the normal reservoir water level being at 135 m. With its dam heightening and reservoir storing water level being upgraded to 175 m, the world’s attention on the Yangtze River keeps increasing. The endangered Chinese sturgeon (CS) and the endemic four major carps (FMC) are the two critical taxa in the Yangtze River, whose propagation and nursing habitats are located mainly in the middle reach of the Yangtze. Take the CS for instance, its spawning takes place just downstream of the TGR. It is well known that both the CS and the FMC have strict spawning requirements. Since the FMC and the CS populations indicate the health status of the Yangtze River ecosystem, they have received much needed attention during the (quasi-)normal stage of the TGR. Wang et al. [5
] pointed out that the TGR had well-documented negative impacts on the river ecosystem, especially for species of migratory carp in the Yangtze River. Xu et al. [21
] investigated the spawning activities of FMC and their responses to the TGR operation and found that the flood amplitude and river transparency were significantly positively correlated with the egg abundance of the FMC. Yu et al. [22
] examined the flow regime at four key hydrological stations along the Yangtze River and concluded that the operations of the Gezhouba Reservoir (GR) and the TGR had altered the water rising and falling regime to some degree, which is particularly crucial for the spawning of CS and FMC. Yi et al. [23
] proposed that the minimum discharge needed for the FMC in the middle reach of the Yangtze River is 3000 m3
/s. Wang et al. [24
] emphasized that only the cumulative temperature for gonad development and the minimum temperature for FMC spawning (18 °C) are both satisfied, and the occurrence of a flow increase would produce a successful spawning event. Chang et al. [25
] found out that a delay in the decrease in water temperature caused by the TGR and the few numbers of reproductively mature individuals were suspected to have contributed to the failure in natural breeding of the CS. Shen et al. [26
] disclosed that the water temperature increase from the TGR impoundment primarily induced the habitat degradation and spawning time delay of the CS, and the effect of the water temperature increase was eight times that of a decrease in discharge. Ban et al. [27
] revealed that the discharge and water levels dominated the FMC reproduction when the change of sediment concentration was kept stable; however, the sediment concentration played a leading role in the spawning while the sediment decreased over a limited range.
It can be seen that many previous studies have assessed the impacts of the TGR on the FMC or the CS during a (quasi-)normal stage; however, due to data collection difficulties, most of those studies often focused on the relationship between the spawning and the flow discharge characteristics [5
], some on the river thermal regime [24
], a few on flow discharge–water temperature [26
], or a few on flow discharge–sediment [27
]. In fact, spawning of the FMC and the CS is sensitive and restricted to complex hydrological factors that include flow, sediment, and thermal conditions. It is clear that few studies have considered the three hydrological factors together to investigate the impact of the TGR on the FMC or the CS during their spawning seasons. Besides, almost related studies focused on the impact of the TGR only on the spawning of the FMC or the CS but not both of them. Since the spawning seasons of the FMC and the CS just fall within the rising–warming flow period and the falling–cooling flow period, respectively, it is of great significance to examine the impact of the TGR on the spawning of the FMC and the CS in the Yangtze River simultaneously. Additionally, the data series of previous studies were time limited. This paper, with the most updated data from 1950 to 2017, first presents a systematic investigation of the influences of the TGR on the spawning of the FMC and the CS from the flow–sediment–thermal integrated perspective.
The aim of the study was to: (1) quantify regime changes in flow, sediment, and water temperature due to dam regulation; and (2) assess the suitable hydrological conditions and their changes for the spawning of the FMC and the CS during pre- and post-dam periods. This paper is organized as follows. First is the brief introduction, second is the data and methodology, third is the results section, and last is the discussion and conclusions. It reveals that the impact of the TGR and the GR on their downstream hydrological regimes varied with reservoir storage capacities and operation modes, with the sediment regime being most significantly affected. The two reservoirs also had different degrees of impact on the spawning conditions of the CS and the FMC, and the present status of the sediment condition is no longer suitable for the reproduction of the CS.
2. Study Region
The TGR is located on the main stem 44 km above the end of the upper Yangtze River. Its operations were done in four stages: the initial stage began in June 2003, with the reservoir water level increased to 135 m above sea level; the transitional stage started in October 2006 with the water level increased to 156 m; the reservoir water level rose to 173 m in November 2008 during the quasi-normal stage; and then to the designed reservoir water level of 175 m in October 2010 during the normal stage with the total storage capacity of 39.3 billion m3
]. According to the operational scheme of the Ministry of Water Resources [32
], the TGR stores 22.15 billion m3
of water to reach normal water level of 175 m from 145 m at the end of flood season (generally from 15 September to 31 October, lasting 47 days; it begins refilling from 10 September if the year is relatively dry). The GR, a run-of-river reservoir, located on the main stem of the Yangtze River, 38 km below the TGR. Its capacity is about of 1.58 billion m3
, partly operated in 1981. The spawning of CS takes place just downstream of the GR, and many spawning grounds of FMC also are also located in the downstream of the two reservoirs. To investigate the impacts of reservoir construction and operation on the spawning conditions of these two critical taxa, the two key hydrological stations of Cuntan and Yichang were selected as study sites (Figure 1
). The Cuntan station, the farthest end of the backwater of the TGR, is located 608 km above the TGR. The Yichang station, an important monitoring station for outflow from the TGR, is immediately downstream of the TGR, approximately 44 km below the TGR.
3. Data and Methods
Due to the ≈86% runoff at Yichang that comes from Cuntan and the alteration of the flow regime by human activities being limited in the region between Cuntan and Yichang, the Cuntan station just above the TGR was selected as a reference station to investigate the impacts of the TGR on its downstream hydrological regime in the Yangtze River basin. Since the spawning of the FMC and the CS occurs primarily during May–June and October–November, respectively, the time series of daily discharge, sediment concentration, and water temperature during these two periods, both at Cuntan and Yichang, were obtained from the Changjiang Water Resources Commission (CWRC), China (Table 1
). The data homogeneity and reliability have been firmly checked and assured by the CWRC before its release. It should be noted that the daily water temperature during 2001–2006 and 2010–2012 were averaged to a ten-day time scale for this analysis. Due to the scarcity of fish data, only annual numbers of the CS during 1981–2014 were collected from published references [33
]. All these data for the CS were collected from the same spawning ground downstream of the GR during its spawning season.
Based on the long-term historical data of the FMC (1997–2002) and the CS (1982–2006) from the Jianli river cross-section downstream of the GR in the Yangtze River, Guo [40
] investigated the spawning characteristics of these two taxa in the Yangtze River, and concluded that the suitable discharge, velocity, and water temperature for the FMC’s spawning fall within the range of 7500–12500 m3
/s, 0.2–0.9 m/s, and 18.6–25.5 °C, respectively; for the CS, the suitable discharge, velocity, sediment concentration, and water temperature fall within the ranges of 10,000–15,000 m3
/s, 1.0–1.2 m/s, 0.1–0.3 kg/m3
, and 18–20 °C, respectively.
To investigate the impacts of the GR and the TGR on the propagation of the FMC and the CS, the study period was divided into five sub-periods, i.e., pre-GR period (before 1981), post-GR period (1981–2002), post-TGR period (2003–2005 for the first stage, 2006–2008 for second stage, 2009–end for the third stage). Frequency distributions of the daily discharge, sediment concentration, and water temperature during May–June and October–November were found and compared among the different sub-periods. The satisfying degrees, i.e., the suitable daily discharge/sediment concentration/water temperature for the two taxa, and the flow rising characteristics during May–June, as well as the falling ones during October–November, were also examined. In addition, the flow rising (falling) difference between Yichang and Cuntan during different sub-periods was also tested using the two-sample t
-test at a confidence level of 95% [41
]. Definitions of frequency, satisfying degree, and flow rising and falling characteristics were provided as follows.
The frequency of the daily discharge/sediment concentration/water temperature was estimated according to the empirical frequency expectation formula [42
is the data from a given streamflow/sediment concentration/water temperature series and
is the frequency estimator of
is the rank of observations
arranged in rising order, and
is the number of data points.
The satisfying degree () was defined as: . For the FMC, the was for the days of daily flow or water temperature within the range of 7500–1000 m3/s and 18.6–25.5 °C, respectively, while the is for the total days from May to June; for the CS, the was for the days of daily discharge or sediment concentration or water temperature within the range of 10,000–15,000 m3/s or 0.1–0.3 kg/m3 or 18–20 °C, respectively, and was for the total days from October to November.
The flow rising (falling) characteristics included the rising (falling) count, rising (falling) magnitude, and rising (falling) duration. The rising (falling) count was defined according to the following algorithms: . If remained positive (negative) for at least three days in a row, then one rise (fall) was recorded; and the related rising (falling) magnitude for this rise (fall) was defined as , while the days is defined as the related rising (falling) duration for the rise (fall).
Generally, the GR exerted no obvious effects on the frequency distribution of the downstream discharge in both May–June and October–November, whereas the TGR decreased its downstream flow significantly during May–June and October–November. What is more important is that the influence from the TGR was more pronounced in October–November, relative to May–June, since the TGR began storing water from the middle of September to the end of October or early November. Generally, the higher water storage in the TGR reservoir led to a larger decline, with the maximum impact during the TGR’s normal stage in May–June and October–November. The GR and the TGR also affected the downstream flow rising and falling characteristics, with a larger impact on the falling characteristics than the rising ones. Correspondingly, the TGR and the GR had negative impacts on the spawning of the CS and the FMC.
Significant trapping of sediment was induced both by the GR and the TGR, particularly by the TGR during the refilling operations due to the higher water level. For instance, the average daily sediment concentration at Yichang reduced from 0.406 kg/m3
over 1981–2002 to 0.032 kg/m3
, 0.010 kg/m3
, 0.004 kg/m3
during the first stage, second stage, and third stage, respectively, of the post-TGR period. The low sediment in the clear water with high energy can trigger serious riverbed scouring and riverbank collapse in the downstream channels [8
]. In addition, low sediment may also harm the spawning of Chinese sturgeons as they are rather sensitive to the sediment condition during their propagation. The present study suggested that the sediment condition in the downstream of the TGR was not suitable anymore for the spawning of the CS after 2006. A similar conclusion for the FMC by Ban et al. [27
] disclosed that the sediment concentration played the leading role in the spawning while its reduction was beyond the limited range. It is worthy to note that the dramatic decline of sediment could also be partly attributed to the sediment trapping above the Cuntan station since several other large reservoirs (i.e., Xiluodu reservoir, Xiangjiabang reservoir, Pubugou Reservoir) were built in recent years. This is one of the most important reasons that the Cuntan was chosen as a reference station to investigate the impact of the TGR and the GR on their downstream hydrological regimes.
The GR had a very mild thermal hysteresis effect on its downstream water temperature for both the May–June and October–November periods. Comparatively, the hysteresis impacts of the TGR were much more significant, i.e., hindering the warming trend in water temperature in the warm season of May–June and impeding the cooling trend in the cooling season of October–November. The thermal hysteresis effects were amplified significantly with increasing reservoir storage volumes. The larger the reservoir capacity, the longer the residence time of the water inside the reservoir, as a result of more severe stratification of the water. Since the spawning of the FMC and the CS are closely associated with the water temperature, the lower temperatures in the water released from the TGR, as a result of the cooling effect, delayed the FMC spawning timing to middle May, and the spawning period was shortened from April–July to May–June [46
]. Du et al. [47
] also reported that the timing of the water temperature reaching 20 °C (the upper limit of the optimal water temperature for the CS) was postponed by 27 days.
The population of adult CS may directly reflect the ecological health degree of their spawning conditions. The number of the adult CS downstream of the GR spawning site significantly reduced from 1131 during 1981–2002 to 295 during 2003–2005, with a peak population of ≈2200 in 1984. The population kept decreasing to 153 during 2006–2008, and further dropped to 111 during 2009–2014, with a historical minimum of 57 in 2013 (Figure 9
). Moreover, the Ministry of Agriculture (MOA) also reported that no CS spawning activity was detected in 2013 and 2014 [48
]. This evidence implies that integrated changes/variations in flow, sediment, and water temperature regime due to dam construction and operation have severely affected the CS spawning ground and habitat, leading to a significant reduction in the Chinese sturgeon population. It is worth noting that due to difficulties in fish data collection, only the annual population of adult CS was obtained from different publications. Although these data series may not be very homogenous, they are from the same CS spawning ground during the spawning season, and their variation could serve as a reliable reference for the change in CS spawning habitat between the pre and post-dam construction.
To alleviate the adverse effects of dam regulation, the TGR has advocated releasing an experimental flow increase process to simulate the natural flow in duration, magnitude, and daily increasing rate during the spawning season to enhance the natural population of the FMC since 2011 [23
]. These ecological experimental operations have been launched continuously over the past seven years and the 11th ecological operation test was carried out by the Changjiang Flood Control and Drought Relief Headquarter on 19 May 2018 [49
]. It was reported that the water temperature and rising characteristics were considered reasonable during this experimental operation. For the spawning of the CS, there is not as much information of ecological experimental operations as that for the FMC. However, the spawning condition of the CS is much more complicated and sensitive to the environment than that of the FMC. Since the CS is in a near-extinct condition, the ecological operation for the CS’s spawning should be the highest priority and should at least take flow, sediment concentration, water temperature, and flow falling characteristics into account simultaneously. Therefore, an urgent ecological operation, including at least three sub-operations, i.e., ecological flow, sediment concentration, and thermal condition, is necessary for the GR and the TGR in the near future. Furthermore, we also emphasize the need for long-term monitoring of the FMC and the CS after the TGR commenced operation in order to understand the ecological health responses to hydrological alterations for effective resource management in regulated rivers [50
This study investigated the impact of the GR and the TGR on their downstream spawning of the FMC and the CS in the Yangtze River using frequency distribution, satisfying degree, and rising/falling characteristics of hydrological conditions. Results revealed that the GR had no obvious impacts on the frequency distribution of its downstream discharge, a weak impact on water temperature in the spawning season of the FMC and the CS, but a significant impact on the sediment concentration in the CS spawning season. Due to GR operations, the satisfying degree of the suitable discharge and water temperature for the FMC propagation increased, while that of the suitable flow, sediment concentration, and water temperature for the CS decreased. The TGR significantly reduced the downstream flow and sediment concentration in the spawning season of these two species, with a very dramatic decrease in the CS’s season. The TGR had a significant hysteresis effect on the downstream water temperature by hindering the warming trend of the water temperature during the FMC spawning season while impeding the cooling trend in the CS’s season. During the post-TGR period, the satisfying degree change showed a pronounced ascending tendency in the suitable flow while displaying a remarkable descending trend in the water temperature for the FMC; the satisfying degree change presented a significant decreasing trend for the suitable discharge, sediment concentration, and water temperature for the CS, and the importance of each effect on the decrease was ranked as sediment concentration > flow discharge > water temperature. The TGR and the GR had negative impacts on the spawning of the CS and the FMC in terms of the rising/falling characteristics. The impact of the TGR was generally much more significant than that of the GR, and the impact of the TGR increased with the reservoir storage.