Morphological Characteristics of Cambodia Mekong Delta and Tonle Sap Lake and Its Response to River-Lake Water Exchange Pattern

Tonle Sap Lake is the largest river-connected lake, buffer area and ecological zone of Mekong River, which plays a huge role in dispelling flood peak and compensating water, and the conservation of biological diversity. The riv-er-lake relationship between Mekong River and Tonle Sap Lake is unique and has always been a major focus in the international community. The land terrain and under-water topography were used to analyze the morphological characteristics of Cambodia Mekong Delta and Tonle Sap Lake. Long series of hydrological data of river-lake controlling stations were used to analyze the water level variation characteristics and water volume exchange pattern between Mekong River and Tonle Sap Lake, and the response relationship to river-lake morphological characteristics were also researched. The results show that: Cambodia Mekong Delta and Tonle Sap Lake Area is low-lying and flat with gentle channel gradient and water surface gradient, making the relationship between water level and area (or volume) smooth. The channel storage capacity of Mekong River and Tonle Sap River is not enough compared to the inflow, so vast flooding plain is extremely prone to be inundated, making the flood relationships between the left and right banks become very complicated. Tonle Sap Lake is a seasonal freshwater lake with water flowing in and flowing out, and the timing and intensity of water exchange with Mekong River are closely related to the water flow resistance at the exit section of Tonle Sap Lake and the cross-sectional area of Tonle Sap River, which can be reflected by the river-lake exchange intensity between Mekong River and Tonle Sap Lake is great. Tonle Sap Lake not only stores 14.4% of flood volume (39.7 billion m 3 ) from the Mekong River every year, but also supplies 29.7% of dry water (69.4 billion m 3 ) to the Mekong River. Influenced by the adjustment of the floodplain, the water level fluctuation of Mekong River and Tonle Sap Lake is slow, and the rising and droop rates of water level are positively correlated with the floodplain storage area. The research results will help to understand the relationship mechanism between Mekong River and Tonle Sap Lake and provide a scientific basis for the comprehensive governance of Cambodia Mekong Delta and Tonle Sap Lake Area.


Introduction
Both of Mekong Delta and Tonle Sap Lake Area are in the tropical monsoon climate; more than 90% of the rainfall are from the rainy season. The unique hydrological environment and geographical location determine the special morphological characteristics of the Mekong Delta and Tonle Sap Lake Area and the exchange pattern of them, laying the essential role that Tonle Sap Lake plays in the wetland ecosystem of the Mekong River Basin and the world. The catchment area of the junction of Tonle Sap River and Mekong River is about 749,000 km 2 , accounting for 92% of the total basin area. Since then, more than 95% of the runoff has transferred to the Mekong River Basin [1]. year, and the flooding lasts for a long time, and the overland flood volume is vast [2] [3]. Apart from the influence from the upstream, it is also affected by the downstream tidal roof, causing the high flood level of Mekong River mainstream and the Tonle Sap Lake last for a long time, and the flooding process subsides slowly [1].
Affected by topography and upstream inflow conditions, the relationship between the Mekong River and Tonle Sap Lake is complicated, and the water ex-Sap Lake during the rainy season. Tonle Sap Lake is the largest storage place for the Mekong River floods, significantly reducing the flood threat of Mekong River. The slow-flowing-out water of Tonle Sap Lake in the dry season maintains a certain amount of water volume and water level in the lower reaches of the Mekong River downstream Phnom Penh, which is an indispensable water source for the navigation and irrigation of the Mekong Delta. As a result, the water level, area and volume of the lake in the flood and dry season of Tonle Sap Lake are quite different. The highest water level measured in Tonle Sap Lake is 10.54 m, the corresponding area is 15,261 km 2 , and the volume is about 78.7 billion m 3 . The measured minimum water level is 1.11 m, the relevant area is 2053 km 2 , and the volume is about 800 million m 3 , and the extreme ratio of area between the maximum and minimum values is 7, and that of volume reaches 101 [4].
The Tonle Sap Lake Area is the most complex large freshwater wetland in the Cambodian water system. It is an essential flood storage and ecological frontage in the Mekong River Basin. It is a paradise for waterfowl and aquatic animals. It is called the "heart of Cambodia" and "lake of life" by the Cambodian people. The Tonle Sap Lake Area wetland is significant for the conservation of international biodiversity and the ecological security of the Mekong Delta. The unique morphological characteristics of the Mekong Delta and Tonle Sap Lake and the study of water exchange patterns between them will help to understand the relationship mechanism between the Mekong River and the Tonle Sap Lake and provide a scientific basis for the comprehensive management of the Mekong Delta and Tonle Sap Lake Area.
At present, the research results are few and lack systematic. Based on different sources of topographic data and various methods, the water level-area (volume) curves of Tonle Sap Lake were constructed [4]- [11], the ability of Tonle Sap Lake to regulate the flood of Tonle Sap Lake basin and Mekong River mainstream was studied [5], the time and magnitude variation characteristics of the water exchange between the Mekong River and the Tonle Sap Lake and the corresponding hydrological conditions were analyzed [12] [13], and the flood control planning schemes for the Cambodia Mekong Delta and Tonle Sap Lake Area were proposed [13] [14]. The Yangtze River Water Resources Commission, Cambodian Ministry of Water and Meteorology, and Korean government proposed a comprehensive water resources planning plan for the Cambodia Mekong Delta and the Tonle Sap Lake Area [2] [15] [16]. The influence of water facilities on the water level of the Mekong River and Tonle Sap Lake was studied [17], the effects of Mekong River flow changes on the Tonle Sap Lake flood pulse were analyzed [18], and the impact of flood pulse changes on the Tonle Sap Lake ecosystem was investigated [19]. However, these results have not revealed the correlation between river-lake water exchange pattern and the morphological characteristics of the Mekong Delta and Tonle Sap Lake Area. Therefore, it is necessary to carry out in-depth research on the morphological characteristics of the Mekong Delta and Tonle Sap Lake and river-lake exchange pattern, revealing the

Study Area
The Lancang River originates in the Zadu County of Yushu Tibetan Autonom-  [1]. The entire area of the Lancang-Mekong River Basin is 812,400 km 2 , the total length of the mainstream is around 4880 km, and about 5060 m drop of the mainstream with the average gradient 1.04%. In addition, the whole basin area of Mekong River is 648,800 km 2 , which occupies 79.8% of the entire basin area of the Lancang-Mekong River, the length of mainstream is 2719 km, which occupies 55.7% of the total length of the Lancang-Mekong River, and the natural drop is 477 m, which occupies 9.4% of the whole drop of the Lancang-Mekong River. The Mekong River is divided into upstream, midstream, downstream and delta by Vientiane, Pakse, and Kratie. Firstly, the drainage area of upstream is 134,600 km 2 with 1107 km of the length; secondly, the midstream has 246,600 km 2 of drainage area with 753 km of distance. Next, the downstream has 106,200 km 2 of drainage area with 331.3 km of length. Eventually, the drainage area of delta is 161,200 km 2 (including Tonle Sap Lake) with 527.8 km of distance [14]. The river network of the Mekong Delta is dense, and the water system is created by variability, complexity, and instability. The rivers above the Phnom Penh are flooding, especially in the rainy season, which overflows the banks of the Mekong River, and enhances the flood plains up to 50 km wide for several weeks [20] [21] [22]. The Mekong River is linked to the Tonle Sap Lake, which is the largest freshwater lake in Southeast Asia's with 120 km long Tonle Sap River at Chaktomuk in Phnom Penh. The Tonle Sap Lake has a drainage area of 86,000 km 2 and a multi-year average lake area of 6176.8 km 2 with a volume of 15.14 billion m 3 . The Mekong River is divided into the Lower Mekong River and the Bassac River by Phnom Penh, and then divided into nine sea rivers in Vietnam. The overview of the Mekong River Basin is shown in Figure 1.

Terrain Data Sources and Processing
The 1:50,000 land-based terrain in Cambodia, the 1:100,000 underwater terrain of Tonle Sap Lake Area, and the 1:20,000 underwater topographic of Mekong River and the Tonle Sap River is used as data resources. ArcGIS is used to vectorize and bond the terrain data of Mekong Delta and Tonle Sap Lake Area, and construct DEM to perform elevation simulation and analyze its morphological characteristics.   3) Cross section morphology The Mekong Delta has branching sections and single sections of the cross-section. The single section cross-section is narrower than the branching section. The cross-section of branching section has the following three characters: first, more branching section, and more complicate section shape. Second, compared with single section, its section shape is wider and shallower. The width and depth increase with the size of curvature of the branching section. Third, the branching section remains the characters of riverbed, and the main slot is clearly tilted to one side in the curve branch. The shoreline of Mekong Delta River is generally in a natural state with low elevation. The current flood control capacity is 5 -10-year return period, and the two banks are broad flood plains. The typical section morphology is shown in Figure 3.

4) Longitudinal profile morphology
The mainstream of the Mekong River and Bassac River in the Cambodia Mekong Delta is about 297 km and 80 km long, respectively, and the average river  Figure 4. It can be seen the longitudinal profile of the riverbed in each river section is very different, showing several apparent twists and turns.

Morphological Characteristics of Tonle Sap Lake Basin 1) Topographic features
The mainstream of the Mekong River is connected to the Tonle Sap Lake, Southeast Asia's largest freshwater lake, through the 148-km Tonle Sap River in Phnom Penh. The western junction of the Tonle Sap Lake basin is the Elephant Mountains and the Cardamom Mountains. The southwest narrowed to the Gulf of Thailand, and the northern part is the Dângrêk Mountains which separated Tonle Sap Lake basin from the Khorat Plateau. Tonle Sap Lake basin has a drainage area of 86,000 km 2 , including 81,700 km 2 in Cambodia. The west, north, and east parts of Tonle Sap Lake basin are mountainous area with the terrain higher than 50 m above sea level, accounting for 54% of the Tonle Sap Lake basin. The Cardamom Mountains in the southwest are over 1700 m above sea level, and the Dângrêk Mountains in the north are 500 m above sea level. The central and southern parts are plain areas with low elevations, altitudes below 50 m, and gentle slopes. The plain region accounts for 54% of total area of Tonle Sap Lake basin. The floodplain with a height of less than 11 m is about 1.62 km 2 , accounting for 19% of Tonle Sap Lake's watershed area. The topographic distribution of the Tonle Sap Lake basin is shown in Figure 5.
The area and volume transition of Tonle Sap Lake at different water levels is flat, as shown in Figure 6, further illustrating the flat terrain of the Tonle Sap Lake Area. Journal of Water Resource and Protection

2) Planform morphology
The Tonle Sap Lake basin consists of Tonle Sap Lake, Tonle Sap River, 15 tributaries and floodplains. Among them, the branches with a catchment area of over 5000 km 2 are Stung Sen, Stung Sreng, Stung Chinit, Stung Sangker, Stung Pursat, Stung Sisophon and Stung Mongkol Borey. According to the Tonle Sap Lake Area topographic map, when the water level of Tonle Sap Lake is 10 m at Kampong Luong, the lake is 350 km long, the largest lake width is 89 km, and the smallest lake width is 12 km, with the corresponding area of 13,111.38 km 2 and volume of 56.73 billion m 3 , as shown in Figure 7.
3) Cross section morphology In the upper reaches of the Tonle Sap River at Samraong (20 km from the Tonle Sap outlet), the morphology phenomenon of lacustrine facies under high water level and fluviatile facies under low water level is apparent. While the water level is below 7 m, the water flow belongs to the deep trough, the average river width is about 1640 m, and the average river bottom elevation is about −8.9 m; when the water level is higher than 8 m, the water flows up to the beach, and the river width increases rapidly; when the water level is 10 m, the average river width reaches 16 km. The cross-section of Phnom Penh Port (1 km from the Tonle Sap outlet) and the Prek Kdam station (32 km from the Tonle Sap outlet), the Kampong Chhnang station (102.5 km from the Tonle Sap outlet), and the Kampong Luong station (171.76 km from the Tonle Sap estuary) are shown in Figure 8.

4) Longitudinal profile morphology
The lengths of the trail channel of Tonle Sap Lake, Tonle Sap Lake, and Tonle Sap River are about 90 km, 130 km, and 129 km respectively. The average river channel drops of them are 0.0988‰, 0.0024‰, and 0.0354‰, respectively. The thalweg cross-section of the riverbed is shown in Figure 9. It can be seen the

Distribution of the Hydrology Stations and Water-Level Stations in the Researched Reaches
The Mekong River and Tonle Sap Lake meet in Phnom Penh, the capital of Cambodia, and then diverted into the Lower Mekong River and the Bassac River downstream of Phnom Penh. The distribution map of hydrological and water-level stations of the Mekong River, Tonle Sap Lake, and Tonle Sap River in the researched reaches is shown in Figure 10. The Mekong River has three hydrological stations: Kampong Cham, Chruy Changvar and Neak Luong. Bassac River has two hydrological stations: Chaktomuk and Koh Khel. Also, the catchment area of Kampong Cham Station is 660,000 km 2 , with the distance of around 103 km from the exit of Tonle Sap River in Phnom Penh, and the distance of about 448 km from the estuary. Additionally, the catchment area of Chruy Changvar Station is 663,000 km 2 , with the distance of approximately 2 km from the exit of Tonle Sap River. The distance of Neak Luong station and Chaktomuk station from the estuary is 227 km and around 325 km, respectively, and Koh Khel station is 9 km downstream of Chaktomuk station.
Tonle Sap Lake Area has 2 water-level stations, including Kampong Chhnang and Kampong Luong. Among them, Kampong Chhnang Station is about 109 km from Mekong River, and Kampong Luong Station is about 172 km from Mekong River.
Tonle Sap River is the connecting river between Tonle Sap Lake and the Mekong River. The Tonle Sap River has two stations, Phnom Penh Port and Prek Kdam. The Prek Kdam station is a hydrological station and the Phnom Penh Port is a water-level station. The Phnom Penh Port station is located at the end of the Tonle Sap River exit section, about 1km from the exit; Prek Kdam station is the flow controlling station of Tonle Sap Lake, about 32 km from the exit of Tonle Sap River.

Selection of River-Lake Controlling Stations and Hydrological Data
1) Selection of river-lake controlling station The Kampong Cham station in the mainstream of the Mekong River controls the inflow of more than 81% of the catchment area of the whole basin, and there are few tributaries between Kampong Cham and Phnom Penh, the exit of the Tonle Sap Lake. The Mekong River is divided into the Bassac River and the lower Mekong River below Phnom Penh. Kok Khel station controls the inflow of Bassac River, and Neak Luong hydrological station controls the inflow of lower Mekong River. Kampong Cham, Neak Luong, and Koh Khel Stations were used as the water level and flow standing stations of the Mekong River.
There are three stations at the exit of Tonle Sap River in Phnom Penh, Chruy Changvar, Phnom Penh Port and Chaktomuk. These three stations are very close to each other on three rivers. Among them, Chruy Changvar Station is located in the lower Mekong River and 2 km upstream of the exit of Tonle Sap River, Phnom Penh Port Station is located at the end of the Tonle Sap River, Chaktomuk Station is located in the Bassac River and about 20 km downstream of the exit of Tonle Sap River, and the location relationship of them is shown in Figure 10.
The water level relationship is shown in Figure 11. It can be seen that: the water levels of Phnom Penh Port station, Chruy Changvar station and Chaktomuk station are strictly linearly related. Considering that the water level data series of Chaktomuk station is longer than the other two stations, so the Chaktomuk Station was used as the water level representative station at the intersection of the Mekong River and the Tonle Sap River.
Kampong Chhnang station of Tonle Sap Lake Area is located at the intersection of Tonle Sap Lake and Tonle Sap River, so its water level cannot be representative for Tonle Sap Lake and Tonle Sap River. Meanwhile, its vertical datum

Water Level Characteristics of the Mekong River 1) Inter-annual variation
According to the measured daily water level data from 1999 to 2011 of Kampong Cham, Chaktomuk, Neak Luong and Koh Khel stations, the average annual water level, annual maximum water level, and annual minimum water level were counted. The statistical results are shown in Table 1.   Tonle Sap Lake is different from that of the Mekong River.
2) Annual variation The monthly average water level distribution at Prek Kdam and Kampong Luong stations in Tonle Sap Lake Area is shown in Figure 13.
It can see that the highest water level of both stations occurred in October, the average water level of this month was 8.72 m and 9.00 m, respectively, and the water level variation range was 2.88 m and 3.00 m respectively. The lowest water level occurred in April and May respectively, the average water level is 1.16 m and 1.57 m, respectively, and the water level variation range is 0.39 m and 0.74 m,  respectively. Besides, the water level fluctuation process of the two stations in Tonle Sap Lake Area is consistent. From May to September, the water levels from the bottom to the top of Prek Kdam and Kampong Luong decreased accordingly. Meanwhile, the water level of the Mekong River was higher than that of Tonle Sap Lake, which caused the flood of the Mekong River reverse flow to the Tonle Sap Lake. After the flood season from October to April, the water level of the Mekong River is lower than that of Tonle Sap Lake, which makes the water of Tonle Sap Lake flow out to the Mekong River. During the highest water level period from the end of September to the beginning of October, the water levels of the two stations are in the same, reflecting the river-lake relationship between the Mekong River and the Tonle Sap Lake.

Response Relationship of River-Lake Water Level Characteristics to Morphological Features
Kampong Cham Station is used as the hydrological representative station of Mekong River upstream Phnom Penh. Neak Luong and Koh Khel are the hydrological representative stations of the Mekong River downstream Phnom Penh. The Chaktomuk Station is the water level representative station at the intersection of the Mekong River and the Tonle Sap River. Prek Kdam Station is the hydrological representative station of Tonle Sap River, and Kampong Luong Station is the water level representative station of Tonle Sap Lake. Based on long series of hydrological data of river-lake controlling station, the water level variation characteristics of the Mekong River and Tonle Sap Lake and their response to the morphological characteristics of them were analyzed.

Response Relationship of the Water Level Fluctuation of Mekong River to the River-Lake Morphological Characteristics
The Mekong Delta is a flat, silt-type river with a small stream gradient. Affected by the regulation of floodplain of the Mekong Delta and the Tonle Sap Lake, the water surface ratio tends to be gentle. The average water surface ratio of the Kampong Cham to Chaktomuk section of the Mekong River is 0.0198‰, far less than the average river channel ratio of 0.038‰; the average water surface ratio of the Chaktomuk to Koh Khel section of Bassac River is 0.0198‰, which is less than the average channel ratio of 0.033‰.
Due to large catchment area, abundant precipitation, small stream gradient, large storage capacity of river channel, flood plain and lake, the flood process of Mekong River is characterized by high peak, large volume, long duration, flat peak shape and slow fluctuation. Taking the year 2000 as an example, the floodplain area of the Mekong Delta is increasing along the river. The area of the floodplain above the Kampong Cham section, the Chaktomuk section (including the Tonle Sap Lake) and the Cambodian-Vietnamese border section is about 1322 km 2 and 20,148 km 2 and 25,909 km 2 , respectively. Correspondingly, the water level fluctuation along the course tends to be gentle, and the ratio of the water withdrawal days to the water increases days decreases from 1.  Figure 14, which indicates that the water level fluctuation positively correlated with the floodplain storage area, and the water increase is greater than the water withdrawal.

Response Relationship of Water Level Fluctuation to the Morphological Characteristics in Tonle Sap Lake Area
Influenced by the floodplain of the Tonle Sap Lake Area, the average water surface slope of Kampong Luong-Prek Kdam section is 0.00598‰. Of which, the average water surface slope during the water-rising stage is 0.0057‰, and the maximum water surface slope is 0.048‰. During the water-falling stage, the average water surface slope is 0.0063‰, and the maximum water surface slope is 0.0181‰, which was much smaller than the average stream gradient of 0.0555‰.
After the adjustment of the Tonle Sap Lake, the water level in the Lake area Station in Tonle Sap Lake and the Tonle Sap Lake water level-area (volume) curve, the variation characteristics of water level, area, and volume of Tonle Sap Lake are analyzed, as shown in Table 3.  It can see that the area and volume of the Tonle Sap Lake during the flood and dry seasons are quite different. The average annual water level of Tonle Sap Lake is 4.82 m, the corresponding lake area is 6620 km 2 , the volume is 16.2 billion m 3 ; the measured maximum water level is 10.54 m, the corresponding lake area is 15261 km 2 , and the volume is 78.7 billion m 3 ; the measured minimum water level is 1.11 m, the corresponding lake area is 2053 km 2 , and the volume is 800 million m 3 . During the year, the minimum water level variation range is 6.62 m, the corresponding area variation range is 9038 km 2 , and the water storage volume variation range is 40.8 billion m 3 . The maximum water level variation range during the year is 9.27 m, the relevant lake area variation range is 13,028 km 2 , and the water storage capacity variation range is 77.6 billion m 3 . The multi-year average annual water level variation range is 7.92 m, the relevant lake area variation range is 11,097 km 2 , and the water storage variation range is 59.5 billionm 3 .

Response Relationship of River-Lake Water Exchange Pattern to River-Lake Morphological Characteristics
The water exchange pattern between Mekong River and Tonle Sap Lake and its response relationship to the river-lake morphological characteristics were studied. The water exchange intensity between the Mekong River and the Tonle Sap Lake is enormous, and the effect of peak cutting and water replenishing on the Mekong River runoff is noticeable, and the response relationship with the river-lake morphological characteristics is good.

Exchange Pattern between the Mekong River and Tonle Sap Lake
Tonle Sap Lake is the largest connected lake in the lower reaches of the Mekong River, with a drainage area of 86,000 km 2 , accounting for 10.6% of the area of the Mekong River. Tonle Sap Lake has a tremendous difference of area and volume between flood season and dry season. The average water level, area, and volume of Tonle Sap Lake increased from 1.51 m, 2487 km 2 , and 1.68 billion m 3 in May to 8.70 m, 12,768 km 2 and 52.88 billion m 3 in October, respectively. Tonle Sap Lake has a substantial regulation effect on the Mekong River flood.
With the change of the water level between the Mekong River and the Tonle Sap Lake, the Mekong River reverse flow into the Tonle Sap Lake from the beginning of May to October, and the water of Tonle Sap Lake flow out to the Mekong River from the middle to late October to April in the next year. The water exchange intensity between the Mekong River and the Tonle Sap Lake is enormous, and the effect of peak cutting and water replenishing on the runoff of the Mekong River is visible. Taking the year 2000 to 2001 for example, the measured discharge of the Prek Kdam station is shown in Figure 19. It can be seen the Mekong River flood reverse flow to Tonle Sap Lake from May 16th to When the water level of Mekong River is lower than that of Tonle Sap Lake, Tonle Sap Lake will replenish water to Mekong River with a long duration and a large volume. According to the flow data of the Prek Kdam station of Tonle Sap River from 1999 to 2011, the average annual water replenishment duration is 238 days, and the amount of water replenished is 69.4 billion m 3 , accounting for 29.7% of the inflow in the lower reaches of Mekong River, which is 2.6 times as large as the area ratio, and 1.77 times as large as the reverse volume of Mekong River. The water replenishment mainly occurred from October to January, accounting for 83% of the total volume of the whole year.
In summary, Tonle Sap Lake is not only a vital buffer zone in the Mekong Delta but also can accommodate a large amount of flood in the upper reaches of the Mekong River during the rainy season, significantly reducing the flood threats in the lower reaches of the Mekong River. It can also serve as an important supplementary source of water during the dry season of the Mekong River, keeping the Mekong River adequate amount of water and water level to ensure the regular operation of navigation and downstream agricultural irrigation and prevent seawater intrusion in Vietnam Mekong Delta during the dry season.

Response Relationship of River-Lake Water Exchange Pattern to
River-Lake Morphological Characteristics Tonle Sap Lake has a relatively large lake area during the flood season compared to its narrow exit of Tonle Sap River. While the water level is low, the water flow resistance of the Tonle Sap Lake water flowing out to the Mekong River or the Mekong River flood flowing into the Tonle Sap Lake is relatively large compared to other parts of the lake area. The width of the Tonle Sap River is very different between low and high water levels, making the morphology phenomenon of lacustrine facies when the water level is high and fluviatile facies when the water level is low is apparent.
Take the Prek Kdam section as an example, while the water level is lower than 7 m, and the water flow belongs to the deep trough. The river width corresponding to the water level of 1 m and 6 m is 1015 m and 1419 m, respectively, increasing the river width for the unit river depth only 80.8 m; the river channel section area corresponding to the water level of 1 m and 6 m are 5285 m 2 and 11,374 m 2 , increasing the channel section area for the unit water depth only 1218 m 2 . When the water level is higher than 7 m, the water flows out to the beach. The width is rapidly increased, and the river width corresponding to the water  Table 4, and the relationship between the water level difference and discharge of Prek Kdam Station is shown in Figure 20. It can be seen that: the water level difference in the exit section of the Tonle Sap Lake is larger at low water levels and larger flows. When the water level of Prek Kdam Station is at 3.5 -4.5 m, 4.5 -5.5 m, 5.5 -6.5 m, 6.5 -7.5 m, 7.5 -8.5 m, the  When the backflow is small, the water level difference needed is very low. During the period of water replenishment from Tonle Sap Lake to Mekong River, the relationship between the discharge of Prek Kdam Station and the water level difference between Prek Kdam and Phnom Penh Port and the water level of Prek Kdam station is similar, and it's not described in detail this time.
Above all, the main hydrological conditions of reverse flow from Mekong River to Tonle Sap Lake and water replenishment from Tonle Sap Lake to Mekong River are the river-lake water level difference and the water level of Tonle Sap River. Of which, river-lake water level difference can be reflected by the water level difference between Prek Kdam and Phnom Penh Port, and it reflects the flow resistance at the outlet section of Tonle Sap Lake. The water level of Tonle Sap River can be reflected by the water level of Prek Kdam station, and it reflects the cross-sectional area of Tonle Sap River. The backflow and replenishing flow increase with the increase of river-lake water level difference and expand with the rise of the water level of Tonle Sap River.

Conclusions
According to the topographic data and long series of hydrological data of the Mekong Delta and the Tonle Sap Lake, the morphological characteristics, water level variation characteristics and the response relationship between them were analyzed, and the river-lake water exchange pattern and its response relationship to river-lake morphological characteristics were studied. The results show: 1) The terrain of the Mekong Delta and the Tonle Sap Lake is flat. The Mekong River and Tonle Sap Lake interact with each other strongly, resulting complicated hydrological situation. The highest and lowest water levels in the Tonle Sap Lake occur in October and May respectively. The highest and lowest water levels in the Mekong Delta happened in September and April respectively. Affected by the morphological characteristics of Mekong River and Tonle Sap Lake, the water exchange pattern between them has the following features: the Mekong River reversely flows into the Tonle Sap Lake from June to September every year, and the storage water of Tonle Sap Lake flows out to the Mekong River in the dry season from October to May. The water exchange intensity of them is enormous, which makes the Tonle Sap Lake have a significant effect of the peak cutting and water replenishing on the Mekong River. The reverse flow lasts an average of 127 days per year, reducing flood peak discharge by 6857 m 3 /s, whose peak reduction rate is up to 15.6%, stores flood voume totaling 39.7 billion m 3 , which accounted for 14.4% of flood voume of Mekong River. The forward flow lasts an average of 238 days per year, and the amount of water supply to Mekong River is 69.4 billion m 3 , accounting for 29.7% of the inflow in the lower reaches of the Mekong River, which is 2.6 times as large as the area ratio, and 1.77 times as large as the reverse volume.
2) Both of Cambodia Mekong Delta and Tonle Sap Lake Area are low-lying land. Currently, the flood control capacity of most reaches is 5 -10-year return period with converging flow into main channel in the dry season and flood overbanking in the rainy season; the flood relationships between the left and right banks and between Mekong River and Tonle Sap Lake are complicated. Besides, overland flood of Mekong Delta often occurs at Kampong Cham-Phnom Penh section, where the average overland flood volume in June-November is about 31.2 billion m 3 , accounting for 8.7% of the total volume at the upstream Kampong Cham Station. The overbank flood mainly concentrates from August to September, and the flood volume accounts for 65% -72% of the total volume. Tonle Sap Lake is the floodplain of the Mekong Delta, influenced by the reverse flow of the Mekong River and the inflow from the Tonle Sap Lake basin, the area and volume of the Tonle Sap Lake during the flood and dry seasons are quite different. The water level of Tonle Sap Lake increases gradually from May to Oct., and reaches the maximum value in Oct., when the average water level is 8.70 m, and the corresponding area is 12,767.9 km 2 and volume is 52.88 billion m 3 . The lowest water level of Tonle Sap Lake sees in May, when the average water level is 1.51 m, and the corresponding area is 2486.5 km 2 and volume is 1.68 billion m 3 . The multi-year average annual water level variation range is 7.92m, the relevant lake area variation range is 11,097 km 2 , and the water storage variation range is 59.5 billion m 3 .
3) Affected by the function of dispelling flood peak and compensating water of the floodplain in the Mekong Delta and the Tonle Sap Lake, the water level variation range of Mekong River declines from top to bottom. The annual average water level variation range in Kampong Cham, Chaktomuk, Neak Luong and Koh Khel are 2.59 m, 2.27 m, 1.56 m, 1.40 m. Additionally, the water surface ratio tends to be gentle and far less than the river channel ratio, the average water surface ratio of the Kampong Cham-Chaktomuk section in Mekong River, Chaktomuk-Koh Khel section in Bassac River and Kampong Luong-Prek Kdam section in Tonle Sap Lake Area are 0.0198‰, 0.0198‰ and 0.00598‰. The water level fluctuation tends to be gentle, the average daily water rising rate of Mekong River decreases from 0.16 m/d in the Kampong Cham section to 0.09 m/d in the Phnom Penh section and 0.07 m/d in the Koh Khel section, and the average daily water rising rate of Tonle Sap Lake decreases from 0.08 m/d in the Prek Kdam section to 0.06 m/d in the Kampong Luong section. Also, the annual average daily water withdrawalling rate of Mekong River is reduced from 0.09 m/d in the Kampong Cham section to 0.06 m/d in the Phnom Penh section and 0.05 m/d in the Koh Khel section, and annual average daily water withdrawalling rate of Tonle Sap Lake decreases from 0.053 m/d in the Prek Kdam section to 0.0459 m/d in the Kampong Luong section. Besides, the water level rising rate and withdrawalling rate are positively correlated with the floodplain storage area.
4) The timing and intensity of water exchange between the Mekong River and the Tonle Sap Lake are closely related to the water level difference between them and the water level of Tonle Sap River. When the water level of the Mekong River is higher than that of Tonle Sap Lake, and the flood from the Mekong River will reversely flow to the Tonle Sap Lake. When the water level of the Mekong River is lower than that of Tonle Sap Lake, the water of Tonle Sap Lake will replenish to Mekong River. The amount of water for the reverse and replenishing flows increase with the rise of the water level difference. Under the same water level difference conditions, the volume of backflow and replenishing flow increase with the rise of the water level of the Tonle Sap River. The river-lake water level difference shows the water flow resistance at the exit part of Tonle Sap Lake. The water level of the Tonle Sap River reflects the changing characteristics of the cross-sectional area of the Tonle River during the flood season and dry season.