Mekong River Commission

2.7 Vientiane to Pakse - the regional tributary systems

Figure 18. Vientiane to Pakse - schematic of the major Mekong tributary systems
indicating the sites at which discharge data are available. (Numbers refer to those
in Table 6.)

In this reach the large left bank tributaries in Lao PDR begin to exert their dominant influence on the flood hydrology of the Mekong, starting with the entry of the Nam Ngum 50 km downstream of Vientiane. The combined mean annual flow of all tributaries between Vientiane and Pakse is 171 km3, almost 40% of the Mekong total. The major contributions to this figure are made from the left bank by the Nam Ngum, Nam Kading/Nam Theun and the Se Bang Hieng, while the Mun – Chi Basin accounts for by far the greater part by far of the floodwater arising from the right bank in Thailand. The major tributary systems are indicated schematically in Figure 18, which also shows the functional hydrometric network. Tables 5 and 6 provide further summary information.

Within this reach:

• The tributary catchment areas are generally very large such that flash floods do not dominate the flood hydrology as they do in the tributaries upstream of Vientiane. They are locally important on some of the smaller, steeper tributaries and in the upper reaches of the larger rivers and conditions can arise on the larger tributaries of extremely rapid increases in discharges, followed by equally fast flood recessions.
• In their lower reaches backwater effects from the confluence with the Mekong mainstream can extend very considerable distances upstream and exacerbate the depth and duration of inundation during extreme events.
• The accelerating development of hydropower schemes in this sub-region of the LMB will in time significantly modify the flood regime of many of the major tributaries. The impact upon flood peaks and volumes will depend upon the scale of reservoir development and whether any such storages are operated in a way that mitigates the flood hazard, by the provision of operational flood storage during the flood season, for example.
• It is anticipated that one of the major changes to the flood regime will be a delay in the start of the flood season and the onset of the higher discharges associated with it. Early season flood water will be withheld in the reservoirs which will typically be drawn down at the end of the dry season.
• The cumulative impacts of the numerous proposed schemes will in time modify the flood regime of the Mekong mainstream itself.

Table 5. Major tributaries to the Mekong mainstream — Vientiane to Pakse.

 

Table 6. Vientiane to Pakse — tributary sites at which functional hydrometric data are available.
(Sequence numbers refer to those in Figure 18.)

Sketch	Station No.	River	Site	Area  (km2)		Mean annual flow (cumecs)
1	230205	Nam Lik	Muong Kasi	375		15
2	230201	Nam Lik	Ban Hin Heup	5115		265
3	230401	Nam Song	Vang Vieng	865		50
4	230110	Nam Ngum	Ban Na Luong	5220		130
5	230104	Nam Ngum	Ban Tha Lat	8280		590
6	230101	Nam Ngum	Ban Pak Kanhong	14,300		600
7	230102	Nam Ngum	Tha Ngon	21,995		680
8	250101	Nam Nhiep	Muong Mai	4305		170
9	260101	Nam Sane	Muong Borikhane	2230		130
10	270101	Nam Kading	Ban Phone Si	14,200		340
11	270901	Nam Theun	Kham Kheut	5650		290
12	270903	Nam Theun	Ban Signo	3370		220
13	290102	Nam Songkhram	Ban Tha Kok Daeng	4650		150
14	310201	Nam Pung	Ban Tham Hai Br	1070		10
15	320107	Se Bang Fai	Mahaxai	4520		230
16	320101	Se Bang Fai	Se Bang Fai	8560		440
17	330103	Huai Bang Sai	Ban Na Khom Noi	1220		15
18	350105	Se Bang Hieng	Tchepon	3990		170
19	350101	Se Bang Hieng	Ban Keng Done	19,400		520
20	350301	Se Pon	Ban Muong Chan	1980		65
21	350401	Se Thamouak	Highway Br	636		20
22	350201	Se La Nong	Muong Nong	2,010		80
23	350501	Se Xangxoy	Ban Phalane	880		25
24	350602	Se Champone	Dong Hen	1525		45
25	350601	Se Champone	Ban Keng Kok	2640		60
26	370210	Nam Pong	Si Chompu	1260		10
27	370805	Lam Choen	Ban Tha Dua	1500		10
28	371101	Huai Rai	Ban Non Kiang	1370		15
29	371203	Huai Pa Thao	Ban Tad Ton	326		4
30	371509	Nam Yang	Ban Na Thom	3240		30
31	370122	Nam Chi	Ban Chot	10,200		55
32	370104	Nam Chi	Yasothon	43,100		230
33	380134	Nam Mun	Rasi Salai	44,600		175
34	380103	Nam Mun	Ubon	104,000		610
35	380127	Nam Mun	Kaeng Saphu Thai	116,000		770
36	380111	Nam Mun	Pak Mun	117,000		backwater
37	381206	Huai Khayuong	Ban Huai Khayuong	2900		35
38	381503	Lam Dom Yai	Ban Fang Phe	1410		20