The lower Jingjiang reach of the Yangtze River is known for its meanders with a high degree of channel sinuosity [1
]. Due to a series of natural and artificial cut-offs since the 1880s, several oxbow lakes remain along the river bank on the floodplain [3
]. With the implementation of some major water projects, including the Three Gorges Dam and the Gezhouba Dam [5
], as well as river regulation and flood control projects [7
], the hydrologic regime in the lower Jingjiang reach and the oxbow lakes have been greatly altered in recent decades.
In general, in-situ data obtained from gauged hydrological stations can be applied to analyze the hydrological changes in rivers and lakes. Limited by the underdeveloped infrastructure and the complex underlying surfaces, hydrological station networks in the middle reach of the Yangtze River Basin are sparse and poorly maintained [9
]. In these ungauged basins and regions, especially in the oxbow lakes, historical or long-time series of hydrological data are either lacking or difficult to obtain. Given the lack of hydrologic data, the hydrologic changes caused by human activities have rarely been acknowledged in oxbow lakes. Nevertheless, many oxbow lakes in the middle reach of the Yangtze River Basin maintained abundant endangered species. For example, Père David’s deer (Elaphurus davidianus
) and the Yangtze finless porpoise (Neophocaena asiaeorientalis
). Despite the fact that hydrologic changes have a great influence on the habitats of these endangered species, no study has fully addressed the hydrologic changes in this area.
Owing to the development of satellite remote sensing technology, nearly every process of the terrestrial hydrological cycle can be observed/inferred directly or indirectly [12
]. In particular, the measurement of water surface area and water level by remote sensing in ungauged areas has become more accurate and reliable. In addition, remote sensing can provide instantaneous “snapshots” of rivers and lakes, thus allowing the direct and convenient extraction of data regarding water surface area. Water level can be directly measured by using a radar altimeter and can be inferred by referring to some ground measurement data, such as the in-situ water level, discharge, or topographic data with water surface area [13
]. However, the radar altimeter approach must ensure that the return echo data comes from water bodies instead of land [15
]. The larger the water bodies are, the more echo data are returned, and the better the accuracy of the water surface elevation measurement [14
]. As a consequence, the water level of rivers and some small bodies of water, such as oxbow lakes, cannot be measured directly by a radar altimeter. Remote sensing also provides two indirect approaches for water level extraction [13
]: the water boundary elevation integrated with topographic data (WBET) method and the level–surface area relationship curve (LRC) method. The WBET method measures the water boundary line elevation integrated with ground topographic data. Through the WBET method, topographic maps and satellite images are used to obtain the water level data of the reservoirs [17
] and rivers [18
]. The LRC method correlates satellite-derived water surface area data with in situ water level data to construct an empirical rating curve. This method has also been widely employed in the extraction of the water level data in many water bodies [19
In the present study, we focused on the application of remote sensing technology in determining the hydrologic pattern changes of an oxbow lake in the middle reach of the Yangtze River. Two remote sensing approaches (WBET and LRC methods) were applied to estimate the water level fluctuation in the ungauged oxbow lake. Then, the hydrologic pattern changes caused by human activities during 1992–2015 were analyzed. The specific aims of this study are as follows: (i) to verify the effectiveness of the remote sensing method in characterizing the hydrologic changes of ungauged ecological zones; and (ii) to quantitatively analyze the variation characteristics of hydrologic patterns in the Tian-e-Zhou oxbow lake due to the impact of human activities.
2. Study Area
A series of oxbow lakes have formed in the abandoned channel of the middle reach of the Yangtze River after the channel migration process. The Tian-e-Zhou wetland (112°31′N–112°37′N, 29°46′E–29°51′E) is a typical oxbow lake located at the north bank of the Yangtze River, adjacent to the south of the Jianghan (Yangtze–Han River) floodplain (Figure 1
). Located about 180 km downstream of the Three Gorges Dam, this wetland was once a section of the middle reach of the Yangtze River before being separated from the main stream through natural realignment of the channel in 1972 [21
]. This oxbow lake is approximately 21 km long. The width ranges from 1–2 km and the surface area varies from 10–20 km2
with the seasonal changes. The Tian-e-Zhou oxbow lake shown in Figure 1
is at its normal water level with a water surface area of approximately 16 km2
. A subtropical monsoon climate prevails in this area and the average annual temperature and precipitation are 16.5 °C and 1200 mm, respectively. Due to its geological and meteorological conditions, this area is prone to constant flooding [7
]. Therefore, to prevent floods, the well-known Great Jingjiang Levee and many other minor levees have been constructed along the middle reach of the Yangtze River Basin [4
]. The hydrological connection in the flood season between the oxbow lake and the Yangtze River was absolutely cut off by the construction of the Shatanzi Levee in 1998. At present, the lake is only connected to the river channel by a narrow “cecum reach” and the Tian-e-Zhou sluice. We defined the minor levee around the oxbow lake as the boundary of the Tian-e-Zhou area. The upstream channel of the oxbow lake is entirely channelized and diked to control floods, and the rate of scouring and silting in the oxbow lake is very small.
Due to the successful reintroduction of Père David’s deer (Elaphurus davidianus
] and the ex situ conservation of the Yangtze finless porpoise (Neophocaena asiaeorientalis
], the Tian-e-Zhou oxbow lake has attracted increasing attention. The Yangtze finless porpoise (Neophocaena asiaeorientalis
) is the world’s only freshwater porpoise. It was classified as Endangered in 1996 and raised to Critically Endangered in 2013 on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. Père David’s deer (Elaphurus davidianus
) is listed as Extinct in the Wild on the IUCN Red list. The two national natural reserves are present in the oxbow lake to protect these endangered species. The numbers of these species in the two reserves account for 20% and 5.8%, respectively, of the total population in the world [24
]. Hydrological changes caused by the drastic channel migrations and intensive human activities have a significant influence on the ungauged habitats of these two endangered species.