Introduction

Glacier lakes of various sizes and shapes are widely distributed in the western alpine regions of China where the present glaciers are situated. Catastrophic outbursts from glacier lakes, produced by various causes, occur frequently in these regions. Most of them have been ignored as they happened in untraversed regions. However, the known catastrophic outbursts from glacier lakes, as well as meltwater floods and debris flows, have caused severe damage with respect to life and property, farmland, water conservancy, communications, transportation, etc. For example, catastrophic outbursts from glacier lakes in the Himalaya in 1954 inundated hundreds of residential areas, including the cities of Xigaze, Gyangze and Yadong and a number of farmlands, water conservancy and traffic facilities. After the Damenhai glacier lake outburst in Gongbogyernde County in 1964, the flood rushed down with the speed of 10 m s⁻¹ from 5120m a.s.l., headed direct to the mouth of a channel at about 3400 m a.s.l. and lashed at the side of the Niyang River. The debris flow piled up to form a dam in the river valley. The dam was 20 m high, 850 m long and 150 m wide at the top, and it obstructed the river for about ten hours. The flood destroyed the Sichuan–Xinzang highway, cut off the traffic for 20 days, inundated most of four pasturelands and washed away trees on both sides of the gully. In 1981, the Zhongzangbo glacier lake burst and debris flow in Nyalam County smashed the highway for 50 km between the outlet of Zongzangbo gully and the Sun Kosi power station in Nepal. In 1982 the Jinco glacier lake outburst submerged eight villages and a large number of fields, and more than 1600 livestock were killed. In 1984, the Erkuran glacier lake of the Gez river-basin burst, and the debris flow blocked the Gez River and damaged the China–Pakistan highway and a bridge.

With the development of communication, transportation, farming and animal husbandry in the west of China, the disasters which are brought about by outburst floods and debris flows have seriously affected life and property and the economic reconstruction of these regions. Therefore, it is important and urgent to understand the distribution of the dangerous glacier lakes, in order to study the causes of their outbursts and to promote preventive measures. Hence, catastrophic outbursts from glacier lakes have been brought to the attention of scientists, and various types of outburst disasters have been reported and studied during the past ten years (Reference HewittHewitt, 1982; Reference Lai Lai, 1984; Reference Lu and Li Lu and Li, 1986; Reference Xu Xu, 1987; Reference Xu and Daoming Xu and Feng, 1988, 1989).

Types and Distribution of the Glacier Lakes

Two types of potentially dangerous glacier lakes are known in China. One is the lake dammed by an end moraine and the other is the lake dammed by a glacier. Outbursts from lakes dammed by end moraines occur mainly in the Himalaya, the middle and eastern Nyainqentanglha mountains and sections of the Hengdaun mountains in China Fig. 1. Up to the present, outbursts from lakes dammed by end moraines happen most frequently in the middle section of the Himalaya (Table 1). Nineteen outbursts from lakes dammed by end moraines have been investigated since 1935. The dams of the end-moraine lakes are made up of various terminal moraines that have formed since the Little Ice Age, of which the youngest moraine lakes, which are close to the present glaciers, are the most dangerous.

Fig. 1. Distribution of floods from glacier-dammed lakes (GDLOFs) and floods from lakes dammed by end moraines (EMDLOFs).

Table. 1. History of outbursts from lakes dammed by end moraines

Outbursts from glacier-dammed lakes are mainly scattered over the Karakoram mountains, the Pamir and the western Tien Shan mountains in the west of China Fig. 1. The famous glacier-dammed lakes, Taramkangri and Kyagar, are situated at the upper reaches of the Yarkand River in the Shaksgam valley. Taramkangri and Kyagar Glaciers, both located on northern slopes of the Karakoram mountains, extend to the Shaksgam valley and have dammed up two lakes, 5 km apart (Table 2). These two glacier lakes have burst repeatedly, as recorded at Kaqung hydrological station at the outlet of the Yarkand River since 1954. Up to now, 30 outbursts from glacier-dammed lakes have been recorded since 1956, of which there have been 18 in the Yarkand River, Karakoram mountains, six in the Kunmalike River, Tien Shan mountains, three in the Sikeshu River, Tien Shan mountains (Reference LaiLai, 1984), two in the Gez River, Pamir and one in the Qilian mountains.

Glacier lakes liable to outbursts are distributed mainly over the mountainous border regions of Qinghai–Xizang Plateau where there are frequent neotectonic movements and high altitudes. The greatest concentration of out-bursts from lakes dammed by end moraines is found in the middle Himalaya around Mount Everest. The region where outbursts from glacier-dammed lakes occur frequently is in the Karakoram where the second highest peak in the world, Mount Qogir, is located. Similar lakes in China are mostly situated between 4500 and 5200 m a.s.l. Their areas are under 3 km² for the largest ones and only 0.01 km² for the smallest ones.

Table 2. Data for Taramkangri and Kyagar Lakes (Reference Liu and SharmalLiu Jinshi, 1988, unpublished)

Investigations of Outbursts From Glacier-Dammed Lakes

The mechanism of an outburst is complicated, but the external factors that trigger it are obvious. When a glacier lake stores water, its level rises with increasing glacier ablation. Consequently, outbursts from glacier-dammed lakes often occur in years of the highest air temperature Fig. 2. The volume of most of the outbursts is between 1000 and 3000 m³ s⁻¹. The probable reason why four of the floods reached peaks between 5000 and 6000 m³s⁻¹ is because two ice-dammed lakes, Taramkangri and Kyagar, burst at the same time. Three of the four largest floods in the Yarkand River, and all floods in the Kunmalike River, happened between 20 August and 30 September, which is over a month later than the maximum ablation period in July. Failure of dams happens mostly between 20 August and 30 September when the storage capacity of the glacier lakes reaches a maximum (Table 3). It might be postulated that the catastrophic outbursts from glacier-dammed lakes can be predicted on the basis of weather forecasts, when satellite images show that the lakes have reached their maximum area.

Fig. 2. Peak discharge of glacier outbursts of the Yarkand River at Kaqun Station, mean air temperature in summer and annual precipitation at Tashikurgan Station (3090 m a.s.l.). GDLOFs are foods from glacier-dammed lakes.

In general, an ice dam is not frozen to its bed, water from the lake does not flow over the dam and the lake drains through subglacial channels. After the outburst, the dam forms again.

Potentially Dangerous End-Moraine Dammed Lakes

An end-moraine dam is different from a marginal glacier dam. The latter is composed of ice which easily melts and cracks, but the former is composed of till of various sizes and is normally much more stable than the ice dam. An inventory of existing glacier lakes of the Pumqu and Poiqu Rivers in the Himalaya was made by a Sino–Nepalese expedition investigating glacier lake outbursts in the Himalaya in 1988 (Table 4). Only 3.7% of the total number of the glacier lakes and 7.2% of the end moraine-dammed lakes are dumping glacier lakes.

Table 3 Timing of outbursts from glacier-dammed lakes and outbursts from lakes dammed by end moraines

Table 4 Glacier lake inventory in the Poiqu and Pumqu basins (Reference Liu and SharmalLiu, C. H. and Sharmal, 1988)

The following morphological features characterize a dangerous end-moraine dam: it has a closed lake basin, a dam height over 80 m, a ratio of dam height to top width less than 0.6, an angle of slope of the outside of the dam of about 23° and a water depth at the dam over one-third of its height (Reference Xu and DaomingXu and Feng, 1989). Further, the dam contains an ice core that may reduce its stability. However, the static pressure maintained by the lake is still not high enough to burst the dam without an additional external action being exerted. The investigated outbursts from lakes dammed by end moraines were all induced by ice avalanches that came from the glaciers feeding the lakes. In each case, an ice avalanche dropped suddenly into the lake; swift and violent waves travelled down the lake, creating breaches in the dam. Because an ice avalanche is associated closely with melting of ice, periods of maximum ablation also are those of frequent ice avalanches. Hence, 79% of all failures of the dams occur during the hottest months of July and August (Table 3).

Whether or not end moraine-dammed lakes are dangerous depends on the flow of the glacier that feeds the lake. In an advancing glacier, the glacier tongue is so steep that many cracks develop and ice avalanches are easily produced. Ice avalanches falling into the lake will create waves of huge surging pressure to the dam wall. On the other hand, when a glacier extends down into the lake along a gentle slope, ice sliding into the lake creates small pressures to the dam wall. The areas of the glaciers above the end-moraine lakes are usually between 1.5 and 5.5 km and the glaciers are between 1 and 4 km long. The glacier tongue is generally located in a steep depression or a valley, with the lower part in an

Table 5. Comparison of dangerous and stable moraine-dammed lakes in the Himalaya

extended state, so that cracks tend to form. Thus, it is important to understand recent developmental and morphological features of the glacier to determine whether an end moraine-dammed lake is of potential danger. Table 5 shows some typical end moraine-dammed lakes in the middle Himalaya of China. It can be seen that the potentially dangerous lakes have a larger lake area and a smaller glacier area, a shorter distance from the glacier terminus to the lake and a steeper angle of slope to the glacier tongue than the stable, non-dangerous lakes.

Long-Term Changes in Frequency of Outbursts From Glacier-Dammed Lakes in China

Reference LiuLiu (1980) presented results on changes of climate and glaciers in the Karakoram region based on tree-ring statistics (Fig. 3, lower part). Three cold periods (Cl, C2, C3) since 1740 correspond to periods of glacier advance (Al, A2, A3). Plotting the number of outbursts from lakes dammed by end moraines in China and the recorded and investigated outbursts from glacier-dammed lakes in the Karakoram mountains, we find that periods of frequent outbursts from glacier-dammed lakes are identical with periods of glacier advance Fig. 3. Figure 3 shows the total number of outbursts from glacier-dammed lakes m the Karakoram over a period of ten years, as reported by Reference HewittHewitt (1982) and by the hydrological station, for the southern and northern slopes, respectively. There have been 71 reported outbursts from glacier-dammed lakes in the Karakoram region from the beginning of the 19th century to the present. The periods when outbursts from glacier-dammed lakes occurred most frequently were 1830–50, 1890–1920 and 1960–80, which correspond to periods of glacier advance in 1820–70 (Al), 1880–1930 (A2) and 1950–90 (A3), respectively. This indicates that colder periods give rise to periods of glacier advance, which, after the response time-lag imposed by the glacier dynamics, cause an increase in the number of glacier dams and hence an increase in the number of outburst floods. For the ice-dammed lakes of Iceland, Reference ThorarinssonThorar-insson (1939) came to a similar conclusion. It can also be shown that outbursts from glacier-dammed lakes occur frequently for about 20 to 30 years and with intervals between this activity of about 40 years. Therefore, if glacier advances take place at the beginning of the next century, in response to the cold period since 1940, frequent outbursts from glacier lakes may be expected.

Fig. 3. Comparison between changes in climate, glacier advance and occurrence of outbursts from glacier-dammed lakes (GDLOFs) and from lakes dammed by end moraines (EMDLOFs).