There are many glacial lakes in the Pumqu (Arun) basin, but only selected number of them were studied by Sino Nepalese investigation during 1987 (LIGG/WECS/NEA 1988). Brief description of those lakes are given on the basis of their field investigations.

Zonggyaco Lake
The Zonggyaco Lake is situated at the latitude 28o 27’ N and longitude 87o 39’ E at the headwaters of the Natanque River (Figure 10.1). The lake was dammed by Neoglaciation moraine. The lake is 2.5 km long, 1.391 sq. km of area with maximum depth of 25.5m and 0.0195 km3 of water reserves. The Lake outlet is at 4,934 masl with the water level is at 4,935 masl. The maximum free board of the end moraine dam is 27m. The outer slope of the moraine around 18 percent and the inner moraine slope is around 9.8 percent. A mother glacier (1.4 km long) is in contact with the lake and it covers an area of 1.37 sq. km (Figure 10.2). The distance between the main glacier and the back margin of the lake is only 3.5 km. Three series of end moraine ridges are found about one kilometre below the glacier, corresponding to the Little Ice Age moraines and the Zonggyaco Lake may be a relic of the Neoglaciation. This moraine has partly cemented and is made to form an undulating hilly landscape. The Zonggyaco Lake has two terraces: the first terrace is 2.5 m higher than the lake water level and is 5-10m in width and the second terrace is 4.5m higher than the lake water level and is 6-7m in width.


Figure 10.1: Elongated Zonggyaco Lake in the head water of Natanque River.

The lake water surface is situated far away from the back glacier and cannot affect greatly by the glacier advance. The mild slope, partly cemented and dry moraine dam represents the stable condition of the dam, the lake does not seems in the conditions for bursting. It is an end moraine-dammed lake in a stable and retreating state.


Figure 10.2: Scree slope and knife-edge crest beside Zonggyaco Lake

Chemical analysis showed that the lake water quality belongs to the HCO3- -- Ca++ type; total dissolved solids is 16.72 mg/1; and the pH 6.95. It is a freshwater lake nourished by snow melt water and precipitation.

Riowpuco Lake
The Riwopuco Lake is named after the Riwo Village (7km west) and lies at latitude 28o 04’N and longitude 87o 38’E at an elevation of 5,470 masl (Figure 10.3). The Lake is 500m long and 100m wide with a surface area of 0.049 sq. km. The lake is elongated oval in shape and extended into the south-east direction. The lake is in contact with the cirque type hanging glacier in one side and other side dammed by lateral and end moraine.

The damming feature of the lake represents a recent end moraine. The dam is about 25m wide at the base and about 20m high. At 2 km downstream a 40m high breaching at the terminal moraine can be seen, which represents the outburst events in the past and reduced the size of the lake but no written documents. This lake is seems safe and even in out burst of this lake, there will be no damage at the downstream.


Figure 10.3: Riowpuco Lake and its surroundings

Abmachimaico Lake
The Abmachimaico Lake is situated at latitude 25o 06’N and longitude 87o 38’E at the headwaters of the Natangqu River (Figure 10.4). The lake is 1.8 km long and 0.3 km wide with an area of 0.565sq. km and 0.0194 km3 of estimated water reserves. The lake was formed due to damming by Neoglaciation moraine. The total height of the dam is 118m with the elevation range of 5,220 masl and 5,102 masl. The freeboard of the lake is about 20m with the slope of about 12.5 percent and at dry side increases up to 29.5 percent.

The moraine ridge is about 250m away from the cliff of the glacier. The shape of the glacier is not well defined owing to long-term water erosion and destruction. There are some bedrocks protruded in the end moraine ridges.

A small valley glacier having a length of 3.8km and 1.66 sq. km of area is connected at the left bank of the lake. The ice cliff of about 5.255m is exposed at the height of 40 to 50m at the lake side. An upheaval of frontal end of the ice cliff is recognised, which was formed by the pushing lake ice under the action of the advancing glacier in winter. From the actual movement it has been reported that the glacier advanced at least 3-4m in the past winter.


Figure 10.4: Abmachimaico lake at the headwater of the Natangqu River


The water of Abmachimaico Lake, with HCO3- as preferential anion and Mg++ as preferential cation, belongs to the bicarbonate-magnesium type water. It is a freshwater lake with a total dissolved solid of 26.24mg/1, nourished by snow and ice melting water. Snow and glacial ice are also fresh water, having mineralization degrees than the water sample of the lake, with C1- as preferential anion, Ca++ and Mg++ as preferential cations.
 

The Abmachimaico Lake seems stable, the seasonal change of water is 0.3m, whereas the perennial water level change is 0.5-0.6m. The mother glacier is a normal movement glacier, and its advance cannot bring about a collapse because the advancing or retreating amplitude of the glacier is limited. The end moraine dam at the frontal margin has partly cemented and is in a stable state.

Gelhaipuco Lake
The Geilhaipuco Lake is located in the latitude 27º 58’ N and longitude 87º 49’ E at an elevation of 5,270 masl in the headwater of the Natangqu River (Figure 10.5). It is an end moraine-dammed lake, which was burst out on 21 September 1964, the details of the outburst is given in the Chapter 9. Before the burst, Gelhaipuco Lake was 1.4 km long with 0.548 sq. km area and about 25.45 million m3 of water reserves The water level of the lake was dropped by 40m after the lake burst in 1964. The slope of the exposed lake bed is 0.6% and it is 0.2 km away from the glacier margin. The present condition of the lake indicates stability. But if the glacier advances forward again, the possibility of another burst cannot be ruled out.

Generally, bank erosion and/or landslides at the narrow sections and large volume of debris were deposited on the wide valley. Large deposits of sand, gravel and big boulders are seen near its confluence with the Natangqu River. The riverbed has been eroded up to 6m at certain portions of the Gelhaipu Stream.


Figure 10.5: Gelhaipucho Lake and its hanging mother glacier

The lake is too small after outburst and less chances of further refilling of the lake. The lake can be considered safe. However, it is recommended that the lake should be monitored on a regular basis.

Qangzonkco Lake
The Qangzonkco Lake is situated at latitude 27o 56’N and longitude 87o 46’E at the headwaters of the Qumaqie Gully in the Natangqu River (Figure 10.6). In 1987 the lake was measured 2.1 km in length with an area of 0.763 sq. km. The lake dimension measured from the topographic map of 1974 shows only 1.4 km in length and 0.425 sq. km in area. From this it has the indication of lake expansion 0.026 sq. km per year and correspondingly, the glacier behind the lake has been retreated about 700m. The lake is dammed by end moraine ridge of the Little Ice Age.

The mother glacier in contact with the lake is a complex valley glacier with 6.6 km length and 10.09l m2 of area. Its cliffy terminus has immersed into the lake, the visible height of the cliff is 20-25m. The upheaval belt formed by pushing lake ice under the frontal end of advancing glacier. There are huge ice masses in clusters floating on the surface of the lake, 1475 -1675m away from the ice cliff (Figure 10.6). The exposed height of the greatest ice mass is 4 to 5m and 3 to 4m in width would be 208-270m3 in volume of ice. These might have originate only from the collapses of the retreating glacier.

The lateral moraine of the lake is forming 110 -190m higher than the water level. The moraine outer slope is 65 -110% and a lot of debris cones spread at the foot of the slope. At both sides of the adjacent glacial terminus, "dead ices" are exposed. These factors add to the instability of this lake. Three series of moraine ridges at the frontal end are found. There is an outlet at the right side of the end moraine ridge where the water flows slowly, but while it flows out the end moraine ridge, it rushes down along the slope. The inner two series end moraine ridges are fragmented: the gaps between ridges have become inter-lake passages due to long-term lake water erosion and destruction. The lake was divided into two lakes by three lines of end moraine ridges: the inner lake is the biggest one with a maximum depth of 69m; the outer lake has a maximum depth of 7 to 8m and covers an area of 0.020 sq. km. The volume of water stored in the lake is 0.0217 km3 from the measurement in 1987.


Figure 10.6: Frozen Qangzonkco Lake and its associated glacier

The mother glacier is retreating and some ice masses are continuously falling into the lake, though the quantity is not very large. Three series of end moraine ridges could play an important role in the stability of the lake. As for the inner series of ridges although their height exposed above the water surface are not very high, they could prevent safety to the outer dam. The depth of the lake water gradually decreases from inside to outside. The lake seems stable, however, if the glacier advanced to a large degree, or if the glacial tongue collapsed suddenly along with the ice bed, the strong shock waves produced and large amount of ice masses falling into the lake might lead to a burst. Therefore, it is felt necessary to monitor the dynamic changes of the mother glacier.

Analyses of the water samples of Qangzonkco Lake indicate that the lake water has HCO3- as preferential anion and Ca++ as preferential cation and it belongs to bicarbonate-calcium type water. The total dissolved solids is 34.90mg/1 and the pH is 6.75, thus being an excellent freshwater lake.

DAROCO LAKE NO. 5O191B0001( IN THE POIQU BASIN)
 
According to Xudaoming' s investigation, Daroco, a glacial erosion lake which is near the outlet of Congduipu Gully (LIGG/NEA/WECS 1988), had burst in 1955. The present field investigation indicates that the lake is 1.1 km long and covers an area of 0.517 km². It is a closed glacial erosion lake. The lake water level is 35 m below the outlet saddle. From this it is concluded that the lake probably never burst since its formation. The debris flow of 1955 might have been a burst of a glacier lake at the headwater of a branch gully flowing along the south side of Daroco Lake, or a debris flow formed by glacier ablation. Rather fresh debris flow sediments can be seen in that valley.

Daroco Lake is a beautiful ice-free body of water with no discernible inflow, now at approximately 4390 m a.s.1. A former outlet (the east saddle) now dry, is approximately 30 m above the lake water level. There is some evidence of exposed beaches indicating receding lake levels. Its catchment does not include any glacier, and during the inspection in mid June, not much snow was present on the peaks immediately around it. Inflow creek beds were dry.

It is concluded that, barring an extraordinary event, this, not being a glacier lake today, is not a potential source of outburst floods. However, the large moraine east of the lake shows some signs of small local rock slides along Congduipu Gully.

LAKE NO.5O191B004 ( IN POIQU BASIN)

Glacier Lake No. 5O191B004 of the Poiqu River lies across the Congduipu Main Gully of the Poiqu River, latitude 28º12' N and longitude 85º51' E. The altitude of lake surface is approximately 4400 m a.s.1. It is 0.8 km in length, the area being 0.332 km² and the water reserves estimated is 0.0083 km³. It is an end moraine-dammed lake with end moraine ridge of the Little Ice Age.

The adjacent glacier behind the lake is Glacier No.21 (LIGG/NEA/WECS 1988) in the Poiqu River. It is 5.5 km long and covers an area of 5.01 km2, being a cirque-valley glacier. The glacial terminus connects to the lake bank through an ice cliff. The glacier consists of two firn basins that on converging form an ice fall. The ice layer is broken and the glacial tongue, with moraine material covering its surface, is gentle. The glacier behind the lake has steep slopes and well-developed crevasses, and thus could collapse.

The lake moraines bounding it have partially blocked the drainage flow from upper valley reaches of the Congduipu Main Gully, where other lakes may also exist. The gully flow around Lake No.5 (LIGG/NEA/WECS 1988) moraine is substantially higher than that overflowing its end moraine. Between Lake No.5 (LIGG/NEA/WECS 1988) moraine and the larger and higher latter moraine separating the two valleys (Congduipu and Jiapu), flows the river which is eroding the toe of the latter moraine. This moraine is very steep and high and right across the lake where the obstruction is. It has suffered severe undercutting and loose material is falling on the river.

Although small, it is considered that Lake 5O191B004 is potentially dangerous as it has a steep glacier as its source; is overflowing; is now constricting the Congduipu Main Gully channel; a gully blockage may occur if large moraine landslides develop (a distinct possibility); being at a relatively low elevation, the lake exhibits an ice-free surface for longer periods, thus longer exposure to serious water surface disturbances; and, its drainage valley is relatively straight and steep, with Nyalam just about 18 km downstream.

It is concluded that, pending further study, Lake 5O191B004 is potentially hazardous and deserves further, close surveys, investigations and monitoring. These should also include a survey of the moraines, lakes and glaciers northwest and upstream of the lake whose drain waters constitute the source of the Congduipu Gully.

LAKE NO. 5O191B0026 (IN POIQU BASIN)

Lake No. 11(LIGG/NEA/WECS 1988) at approximately 5000 m a.s.l., is smaller than Lake No. 5O191B0026 and lies S-SW and not far from it. This lake does not seem to have a major feeder nor was a drainage outlet observed. It is likely that it flows into Lake 5O191B0027 along a very mild channel. It is perhaps a small shallow lake with a remote likelihood of bursting as it is trapped around very wide and flat morainic remains.


LAKES NO. 5O191B0027 AND 5O191B0028(IN POIQU BASIN)
 
Lake 5O191B0027 at approximately 5000 m a.s.l., is about one fifth of the surface area of Lake 5O191B0030, a few hundred meters southwest from the latter, and almost circular in shape. It has a permanent, very wide overflow drainage channel flowing into drain from Lake 5O191B0030 (Jiapu Gully). The overflow waters are very clean, which is not the case of the overflow from Lake 5O191B0030 as pointed out. The drain channel is long and has a relatively mild slope. No glaciers lie directly around the lake; it might be fed by a seepage through a western moraine holding 5O191B0028. Its terminal moraine in the east is low and flat.

Lake 5O191B0028, due west of Lake 5O191B0027, seems to be smaller and is perhaps higher in elevation. No major glacier is around it and it is surrounded by talus; snow and ice may fall from the higher sections of the surrounding range.

Based on this scanty intbrmation, it is concluded that the only possible threat to Lake 5O191B0027 would exist if somehow Lake 5O191B0028 bursts, a remote possibility. Moreover the lake volume seems to be relatively small and the drop in elevation from lake waters to valley is of the order of 20 to 25 m. The possibility of bursting is then considered to be nonexistant.

LAKE NO.5O191B0030 ( IN POIQU BASIN)
 
Glacier Lake 5O191B0030 in the Poiqu (Bhote-Sun Koshi) River, at approximately 5030 m a.s.l., is situated at the headwaters of the Jiapu Gully, the northern tributary of Congduipu Gully; its latitude is 28º19 ' N and longitude 85º50 ' E. At present the lake is 2.1 km long and covers an area of 1.684 km2, being the largest end moraine-dammed lake in the Poiqu Basin.

Calculated from 1974 aerial photograph and corresponding map, the lake was 1.3 km long and had an area of 0.875 km². It was shown on those records that a sandy stretch of land, 0.5 km in length, is located between the upper margin of the lake and the glacial terminus. In addition, there was a small lake with an area of 0.04 km², 0.3 km in length, lying in the branch gully of the big lake to the west. This investigation found that the glacier behind Lake 5O191B0030 has retreated 0.3 km since 1974, and the glacial terminus has connected to the lake waters, the former sandy land being now submerged by the raised lake water. The small lake is now connected to the big lake. Thus, the length of Lake 5O191B0030 and its area have increased by 0.8 km and 0.809 km² respectively, and its water level has also correspondingly raised. If the average depth of the glacier lake is 30 m, its water reserve may reach 0.0505 km³ (no measurements of depth were made during the inspection).

There is a complex valley glacier with a length of 9.5 km and an area of 15.41 km² developed behind this lake. This glacier originated from the southeastern side of Mt. Xixabangma and consists mainly of two ice-flows. From the headwaters to the glacial tongue area the ice surface has a steep slope, forming two ice falls, where ice layers are broken. The glacial tongue, formed by two ice-flows is rather gentle with morainic material covering its surface. The glacial terminus connects to the glacier lake through an ice cliff. The vertical altitude difference between the source of Glacier No.35 (LIGG/NEA/WECS 1988) and its terminus is 2900 m; the positive difference of glaciation is 2280 m, whereas the negative difference of glaciation is 630 m. The glacier is mainly nourished by snowfall, snow-avalanche and snow drift, and under the influences of the climate, advances or retreats.

The end moraine ridge at the frontal margin of the lake was formed in the Little Ice Age. As compared with the outside river bed the altitude difference is approximately 80 m, and the slope on the dry side is 25%. The moraine ridges, ranging from 5050 to 5075 m a.s.l., have not cemented into rocks and have a poor stability, because they were formed in recent times.

Lake 5O191B0030 is overflowing at the south end and also permanent seepage was observed at the foot of the terminal moraine. Such seepage and overflow have dug their own channels and seem to have been there for some time. Overflow is along the moraine in two steps, the lower one being more abrupt, for a total drop from lake to valley of about 40 to 50 m. Seepage water is very clean whereas overflow water is milky. This lake seems to be a threatening one and is cause for concern. Based on qualitative observations during visit, it is over-flowing; it is seeping; the dead-ice area seems to be receding quickly; the two bodies of water being connected, an ice or rock fall from the adjacent Glacier No.34 (LIGG/NEA/WECS 1988) over the small bay may create disturbances powerful enough to cause a burst of the terminal moraine.

It is then preliminarily concluded that Lake 5O191B0030 is a potential candidate for creating a GLOF, and that therefore it deserves further and deeper investigations, surveys and monitoring, including its glaciers.

The water of Lake 5O191B0030 with HCO3¯ as preferential anion and Mg++ as preferential cation, belongs to bicarbonate-magnesium-type water. The total dissolved solids is 25.46 mg / 1; pH 7.21, being a low-mineralized fresh water lake. The lake was found to be frozen over in early June but its margin cover had partly melted. From this it is inferred that the lake is frozen over in winter and spring, its ice thickness likely reaching 0.8-1.0 m.

KUNGCO LAKE NO. 5O191B0045 ( IN POIQU BASIN)
 
This is the largest (surface wise) water body within the basin below or above Lake 5O191B0030, measuring approximately 2.7 km along its NE-SW axis. It is surrounded by moraines(0n NE and SW corners) and relatively mild sloping hills.

There are no visible glaciers within its catchment area and no inflow was observed along the S, W and N perimeters. The west lake shore was inspected and it displays a steep beach formed of sand and, mostly, of boulders of up to about 1 foot in diameter. High water marks along this beach are now from 10 to 15 m, above the current lake surface.

The above evidence indicates that a deeper lake was present but has receded over the last decades, as the beach marks suggest; the moraine saddles are much higher than the water surface and it is possible that in former times it drained south along Jiapu Gully east of Lake 5O191B0030, as a higher moraine separates those two features.

It is concluded that this, not being a glacier lake today, poses no threat as far as bursting. Evidence points to this being a dying lake perhaps not too deep at this time.

GANGXICO LAKE NO. 5O191C0011 ( IN POIQN BASIN)
 
This lake, at approximately 5300 m a.s.l., in the headwaters of a basin lying north of the Congduipu Gully and flowing into the Poiqu River in a general east direction (thus its water following a longer path into Nepal), was inspected for just a few minutes. It was reached from the south across steep moraines between this lake and Kungco.

The 1.5 km lake is completely surrounded by moraines which exhibit very steep and loose lake-side slopes; its feeder is a long glacier from the slopes of Mt. Xixabangma, due east of Gangxico Lake. The frozen surface exhibited a number of icebergs or trapped glacier ice chunks spread all over. No evidence of overflow was seen from the only vintage point from which observations were made.

The quick observations made do not allow for even a preliminary conclusion on this lake' s potential hazard. The steep eroding moraine slopes on the lake side may be a result of a large range of water level fluctuations and / or due to wave action generated by ice calving of the glacier tongue in contact with the water.

It is, therefore, concluded that further investigation of glacier and lake, particularly of the exit area of this lake, is necessary to fully assess its GLOF potential.