GLACIER LAKE STUDIED IN PUMQU BASIN
There are many glacier lakes in the Pumqu (Arun) Basin, but only selected number of them were surveyed or inspected. Sino Nepalese investigation during 1987 (LIGG/NEA/WECS 1988) carried out field study of some selected glacial lakes in Pumqu basin. The following is a brief account of each of these lakes as of 1987 field investigation.

Zonggyaco Lake
Zonggyaco Lake, situated at the headwaters of the Natanque River (latitude 28o 27’ N and longitude 87o 39’ E), is a lake with Neoglaciation moraine as its damming feature (Photo 6-5). According to actual measurements, the lake has an area of 1.391 km2, is 2.5 km long and has 0.0195 km3 of water reserves.

The altitude of the Zonggyaco Lake outlet is 4934 m a.s.l., the water level being at 4935 m a.s.l. The maximum altitude of the end moraine dam is 4962 m a.s.l.; the altitude difference between the dam and the valley bed is about 40m. The moraine slope in the outside is 18%, whereas the inside slope is 9.8%. The glacier (no. 20) behind the lake covers an area of 1.37 km2, is 1.4 km long and belongs to the cirque-hanging type of glacier. At present the distance between the glacier and the back margin of the glacier lake is 3.5 km. Three series of end moraine ridges are found about one kilometre below the glacier, corresponding to the Little Ice Age moraines. From this it can be concluded that the end moraine damn of 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; the second terrace is 4.5m higher than the lake water level and is 6-7m in width.

This lake, with few small size glaciers at its headwaters, is in the retreating state due to insufficient meltwater flowing into the lake. The maximum depth of the lake is 25.5m, much less than the Abmachimaco and the Qangzonkco Lakes (see below). The water surface of the Zonggyaco Lake, situated far away from the back glacier, cannot at present be affected greatly by the glacier advance. With the lower height of the end moraine dam, partly cemented moraine and mild slope of the dry side, the lake does not present conditions to bursting. It is an end moraine-dammed lake in the stable and retreating states.

Chemical analysis showed that the water quality of the lake belongs to the HCO3- -- Ca++ type; total dissolved solids is 16.72 mg/1; pH6.95. it is a freshwater lake nourished by snow melting water and precipitation.

Riowpuco Lake
Riwopuco Lake lies west of the Riwo Village, at an eleviation of 5470m a.s.l. The coordinates of the lake based on the topographical map are 28o 04’N and 87o 38’E. The surface area is 0.049 km2, with an axial length of 500m and a width of 100m. The shape of the lake is elongated oval and extends in the SE direction at its outlet. The lake is connected with the cirque-hanging glacier No.17 upstream. The lake is about 7km west of the nearest settlement, Riwo, and about 8km from the Natangqu River.

This lake is surrounded by lateral and terminal moraines of recent ages. The rocks at the top of mountains west of the lake consist of schists of the Nyalam Group. The lake is probably formed by moraine damming and glacier melt water only.

The blocking feature of the lake is a recent end moraine, which is unstable in nature. The base width of this retaining structure is about 25m, and it is about 20m high. A few hundred years ago, this lake could have been bigger in size and it could have burst. A 40m high breach at the terminal moraine can be seen about 2 km downstream of the present lake, an evidence of the occurrence. But no written document on this event exists. At present, this lake is safe and, in the event of bursting, no damage is expected downstream.

Abmachimaico Lake
Abmachimaico Lake is situated at the headwaters of the Natangqu River, latitude 25o 06’N and longitude 87o 38’E. The end moraine-dammed lake, with an area of 0.565km2 is 1.8 km long and 0.3 km wide: water reserves are estimated to be 0.0194 km3 (see Photo 6-6 and Appendix 1, Map No.4).

Till deposits of Neoglaciation constitute its dam. The greatest altitude of the dam is 5220m a.s.l.; the altitude of the foot of the outside slope is 5102m a.s.l., with altitude difference of 118m. The slope of the dry side is thus 29.5%. The greatest height difference between the end moraine dam and lake water level is 20m, with a slope of 12.5%. The moraine ridge of the Little Ice Age is close, about 250m away from the cliff f the glacier. Its shape is not well defined owing to long-term water erosion and destruction. There are some bedrocks exposed between the end moraine ridges of the Little Ice Age.

There is a single valley glacier developed behind Abmachimaico Lake, with an area of 1.66 km2 and 3.8km in length (No.18). The terminus of the glacier is connected to the lake bank through an ice cliff. The top height of the ice cliff is 5255m; the height difference between the cliff and the lake surface is 40-50m. Upheaval belt at frontal end of the ice cliff is found, which was formed by the pushing lake ice under the action of the advancing glacier in winter. From the actual movement it has been found that glacier advanced at least 3-4m in the past winter.

Chemical properties of Abmachimaico Lake are shown in Table 6-3. For comparison, the analytical results of snow and glacial ice are also listed in the table. 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 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.

It appears that Abmachimaico Lake, with a stable water level, has not burst since it was formed. According to water level investigations, the seasonal change of water is 0.3m, whereas perennial level change is 0.5-0.6m. The glacier behind the lake 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 of the lake was formed in an earlier time, has partly been cemented and is in a stable state. Although the above-mentioned analysis showed that the lake is a stable end moraine-dammed lake, there is, however, a tectonic fault line passing though its upper margin. If it reactivates, there may be an earthquake that would strengthen the glacial activity, perhaps leading to a burst of the glacier lake with disastrous consequences downstream.

Gelhaipuco Lake
Geilhaipuco Lake is located on the left bank of the Natangqu River at elevation if 5270m a.s.l. This lake burst on September 21, 1964, causing enormous damage to the roads, bridges, houses, as well as human loss along the Gelhaipu stream, the Natangqu River and the Arun River in Nepal (see 6.3.1).

Large masses of ice and ice blocks from the huge hanging glaciers No. 43 and No. 32 behind the lake could have very quickly plunged into the lake and created a wave, which overtopped the terminal moraine, thereby triggering the breaching of the retaining structure.

Prior to the outburst, the terminal moraine should have been 50m to 55m above the present water level. As the GLOF occurred at the end of the rainy season, the lake would have been full at the time of breaching. Minor traces of the flood mark on the left and right banks indicate that the wave was not every high at the terminal moraine. At the beginning of the surge, possible a great amount of water could have over flown through the depressed area at the right bank and the water level would have stabilized there for a while leaving a flat terrace about 40m above present water level of the lake. However, the terrace could be the mark of the low water level.

As a result of rapid rise of the water level, the terminal moraine would have breached at its center, thereby generating a catastrophic debris flood. A large mass of rock, debris and moraine material moved down and deposited along the valleys of the Gelhaipu Stream. Shock waves at the banks and high velocities would have undermined the toe of the valley walls resulting in landslides.

Generally, bank erosion or landslides are seen at the narrow sections of the Gelhaipu Stream. Large volumes of deposits are observed in the valley, while severe riverbed erosion took place in the downstream reaches. At certain portions of the Gelhaipu Stream, the riverbed has been eroded up to 6m. Large deposits of sand, gravel and big boulders are seen near its confluence with the Natangqu River.

People of the Gelhaipu Village, witnesses of the 1964 GLOF, reported that the debris flood lasted for about half and hour. At first, a huge mass of big boulders and stones moved along the Gelhaipu Stream creating a big surge that was instantly followed by a high flood, eroding the banks and riverbed. The surge produced a big noise due to colliding of stones. The surge moved from one bank to another, the banks were trembling and the houses were shaking.

Local people have also provided valuable information regarding the location of the eroded area and flood mark levels. At present, a study is being conducted by WECS/NEA and LIGG to estimate the magnitude of the flood at various downstream reaches. This simulation work will provide some important clues to understanding the behaviour of this GLOF.

The result of the analysis will be incorporated in a separate report2.

Currently, the size of the lake is too small and further refilling of the lake is a remote possibility. In the present conditions, this lake is considered safe. However, it is recommended that the lake is inspected on a regular basis and that further studies are conducted aimed at collecting more information to understand the general GLOF phenomenon, based on the 1964 occurrence.

Qangzonkco Lake
Qangzonkco Lake is situated at the headwaters of the Qumaqie Gully in the Natangqu River, latitude 27o56’N and longitude 87o46’E. It is an end moraine-dammed lake with end moraine ridge of the Little Ice Age as its dam (Photo 6-7). According to the present observations, the lake has an area of 0.763 km2 and is 2.1km in length. As measured from the map complied by 1974, it covers an area of 0.425km2 and is 1.4km in length. In the thirteen years since 1974, it has thus expanded 0.338km2 in area and extended 0.7km in length. Correspondingly, the glacier behind the glacier lake has retreated about 700m (Appendix 1, Map No.5)

The glacier (No.51) behind the Qangzonkco Lake is complex valley glacier with an area of 10.09lm2 and 6.6 km length. Its cliffy terminus has immersed in the lake, the visible height of the cliff being 20-25m. Similarly, the upheaval belt formed by pushing lake ice under the action of advancing glacier at the frontal end of the glacier, and ice mass fallen down from the ice cliff, can be seen. There are huge ice masses in clusters floating on the surface of the lake, 1475-1675m away from the ice cliff. The exposed height of the greatest ice mass is 4-5m and 3-4m in width. Calculated from this, the biggest ice mass would be 208-270m3. These might originate from collapses of the retreating glacier. Qangzonkco Lake was excavated on the ice water platform by the readvancing glacier before Little Ice Age.

The lateral moraine of the Little Ice Age along both sides of the glacier lake is 110-190m higher than the water level, its slope dense gullies, is 65-110%, and a lot of different sizes of debris cones spread at the foot of the slope. During snowmelt periods gravel and pebbles can be heard at times. At both sides of the adjacent glacial terminus, "dead ices" are exposed, which collapsed while the sun was shining on them. These factors add to the instability of this glacier lake. The end moraine ridge located at the frontal end of Qangzonkco Lake was formed in the Little Ice Age. Three series of ridges are found there. The outermost has a trapezoidal shape (the altitude difference between ridge top and valley bed is 80m, the dam base 450m). 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 ridge exhibit fragmental shapes: the gaps between ridges have become interlake 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 7-8m and covers area of 0.020km2. the depth of the lake about 2m away from the outer end moraine ridge, is only 2.4m. Calculated from actual measurements, water reserves of the lake are 0.0217km3.

At present the glacier behind the glacier lake is retreating. Although there are some ice masses continuously falling into the lake, the quantity is not very large. In addition, there were three series of end moraine ridges of the Little Ice Age. As for the inner series of ridges although their height exposed above the water surface are not very high, they could play an important part in improving the safety of the outer dam. From inside to outside the depth of the lake water gradually decreases. The above observation shows that at present the lake would not burst. However, if the glacier advanced to a large degree, or if the glacial tongue collapsed suddenly along 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 glacier behind the lake.

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