Himachal Pradesh is a mountainous state in Indian Himalayas, covering over 55 thousand square kilometers. The state's peculiar geography results in a diversity of terrain. Snow and ice cover the area above 4,000 meters above mean sea level most of the year. Perpetual glaciers, which are the source of various rivers in the area, generate many glacial lakes. Climate change and variability has shifted dramatically in recent decades that has affected glacier lifecycle in the Himalayan zone. As a result, several extensive glaciers melted quickly, resulting in a huge number of glacial lakes. Such outburst floods also turn into even more damaging rivers of debris when the flow path is steep, the flow velocity is high, and material is available for erosion (Clague et al., 1985; Evans and Clague, 1992; Huggel et al., 2004; Chiarle et al., 2007), threatening tourists and the downstream mountain valley population. In order to know their location, spatial and temporal inventories of glacial lakes are necessary. Changes in their number and area covered, as well as related geo-hydrological properties, are critical to comprehend in order to identify flood-prone zones, reduce flood risk, and the resulting impact on the surrounding population.
The mountain lakes, created by glacial activity, are known by geographers as tarns. Tarns are found mainly in the upper reaches, above 5,500 m, of the Himalayas. The snow-melt, precipitation and springs feed high-altitude lakes. The high elevation Chandratal lake, Chandra valley, Lahaul and Spiti, Himachal Pradesh are largely oligotrophic lakes. Unlike the low altitude lakes, which are in different stages of trophic state index. The high altitude lakes also have an anthropogenic effect and a pristine climate. These glacial lakes can be classified as pro-glacier, en-glacier, sub-glacier, supra-glacier, Ice-dammed lakes and moraine-dammed lakes (Salerno et al., 2012; Zhang et al., 2015). In the Himalayas, there are two types of glacial lakes; one is moraine-dammed lakes and the rest of the ice-dammed reservoirs.
Aggarwal et al. (2016) and Campbell (2005) have respectively mapped 143 and 266 glacial lakes in the Sikkim region, and a rise in the number of glacial lakes creates the prospect of multiple hazardous glacial lake outburst floods (GLOFs) events that occur when a large volume of water is suddenly released from them (Clague and O’Connor, 2015; Veh et al., 2020). There is also high confidence that the number and area of glacier lakes will continue to increase in most regions in the coming decades, and new lakes will develop closer to steep and potentially unstable mountain walls where lake outbursts can be more easily triggered by the impact of landslides (Frey et al., 2010; ICIMOD, 2011; Allen et al., 2016; Linsbauer et al., 2016; Colonia et al., 2017; Haeberli et al., 2017). For instance, many incidents of GLOFs have happened in northern Pakistan, and massive floods have been observed over the last two decades (Amin et al., 2020). Imja Tsho was initially considered to be in the critical condition; however, several new studies categorized Imja Tsho as moderate risk (Budhathoki et al., 2010). Despite the differences on whether or not Imja lake posed an immediate threat, the results of these scientific studies provided a strong foundation for further research on people’s perceptions of GLOF risks and vulnerability.
In high mountains close to established glaciers, moraine-dammed lakes are found. During the Little Ice Age, the moraines impounding these lakes were established. Since the glaciers have then withdrawn, leaving behind closed water-filled basins. When the glacier recedes during the twentieth century, ice melting contributes to the creation of a moraine-dammed glacial lake, leading to the creation of a glacial lake end-moraine accumulation at the front. Many moraine-dammed lakes have drained suddenly, producing floods and debris flows that have caused considerable damage. Usually, failure occurring at the moraine dam is caused by a rockfall or glacier avalanche overhead. Ice core melting, piping, earthquake and other potential failure mechanisms explain the moraine dam failure.
Lakes dammed by glacier ice are referred to as ice-dammed lakes. These lakes are also known as supra-glacial lakes. In general, supra-glacial lakes occur on a glacier surface that is entirely a dense layer of debris covering it. As supra-glacial lakes grow, the ice of the glacier below melts, and moraine-dammed lakes are thus formed. Such lakes are also combined with moraine-dammed lakes or can grow into composite forms occasionally. In glacier ice-dammed lakes, the lakes dammed by glacier ice without lateral moraine is designated as lakes dammed by glacier ice.
Tools and techniques from RS and GIS make it possible to continuously monitor, analyse and track developments and modifications of glacial lakes. The aim of this study is to analyse the temporal variations in glacial lakes using time series lake (2005 - 2019) remote sensing data of Landsat and map major glacial lakes in Lahul & Spiti district.