GIS inventory of glacial lakes in the Altai Mountains within Russia, Mongolia and China

Since the Little Ice Age, the altitudinal and latitudinal range of the Earth's cryogenic formations has been steadily narrowing and expressed in a progressive reduction of the glaciosphere [1]. Subject to global climatic trends, the glaciation of the Altai (Russian, Mongolian, Chinese) has been in steady regression for a century and a half, and the glaciers of its main centres demonstrate a negative mass balance and decrease in the area occupied [2,3,4,5] . A characteristic element of landscapes of the Altai periglacial belt, as an area with actively shrinking glaciation, are complexes of polygenic and polymorphic lake water bodies, the number of which increases in proportion to the rate of glaciation reduction. The uneven distribution of lakes across the mountain-glacial basins of the Altai is dictated by their morphological features: most of the lake water bodies are confined to the gentle precipices of valley and car-valley glaciers with well-defined marginal moraine complexes, which act either as natural dams or provide conditions for the development of thermokarst limnogenesis. In basins dominated by glaciers of flat-top or slope type, there are no conditions for the formation of water bodies. A comparative analysis of the space survey materials from 1968 to 2020 shows that most of them were formed in the last half-century. Lakes of the deglaciation belt are represented by the following morphogenetic types:

1. Glacial lakes. Glacial lake basins are formed in the ablation zone of valley glaciers in thermoerosional or thermokarst depressions of the glacial terrain. This type of lakes belongs to ephemeral formations; he hydrographic characteristics of this type of lakes (water area, volume) change annually with a steady upward trend due to their thermal impact on the occupied depression and natural transformation of the glacial surface. When critical volumes are reached, after a short period of time, glacial lakes empty, with a more or less pronounced degree of catastrophism.

2. Moraine lakes are formed in intra-marine depressions at the stage of active regression of valley glaciers. Most often they are confined to areas of widening and deepening of inter-moraine flow channels. Their number on large end-marine complexes may reach several dozens. The bottoms and sides of lake depressions are composed of moraine-containing ice, ice-bearing moraine or frozen varieties of moraine sediments. Water volumes in moraine lakes can reach considerable values, depending on the local topography and balance characteristics of water bodies. The development and time of existence of this type of lakes is controlled by the intensity of thermoabrasion impact on the lake basin and cryolithological properties of sediments composing the sides and bottoms of the occupied depression.

3 Lakes dammed by rock glaciers. The origin of this type of lakes is related to blocking of the main glacier melt-water channel by the margins of the rock glaciers rising from tributary valleys of the first order or from the sides of the main valley. The vast majority of the lakes in question were formed during the maximum stage of the Little Ice Age, their stable volumes being controlled by features of the local topography, indicators of the solid runoff of the main watercourse (as the main factor of aggradation of the lake reservoir) and regime characteristics of the underrun rock glacier.

4. Moraine-dammed lakes are located hypsometrically above the marginal moraine complexes of the Little Ice Age and, as a rule, occupy the depressions excavated by valley glaciers during the transgressive stage of the Little Ice Age. Dams of these lakes are represented by moraine ramparts composed of frozen loose clastic weakly water-permeable rocks. Most of the moraine-dammed lakes have surface runoff, more rarely the source is through filtration of water through the lake dam. Moraine-dammed lakes are time-stable accumulators of regulated glacier runoff and are an important element of water redistribution in mountain-glacial basins.

5. Rigel and kar lakes occupy the lowest parts of glacial kars and cirques, separated by a rock or moraine rigel. Kar lakes are formed during the final stage of glaciation degradation and are fed by atmospheric precipitation on the surface of the catchment. The stable volume of karst lakes in time is ensured by the height of the rock or moraine transom and the regime of atmospheric precipitation.

6. Zander lakes are small and unstable in time reservoirs with insignificant depths, formed in local depressions of relief within prelimes of retreating glaciers. Glacial lakes are usually classified as distant geological hazards (in terms of distance rather than time) as they originate in high altitude areas, far from populated areas. They are predominantly located in the highlands, in uninhabited areas. Glacial lakes contain a store of water which under certain conditions can have an enormous destructive force. Therefore mountain lakes should be classified as particularly dangerous sources of natural disasters. The most dramatic event was the outburst of the Maashey moraine-dammed lake in July 2012. The outburst flood caused significant damage to the infrastructure of the Ulagan district: destroyed 4 bridges on the Maashey River and Chuya River, and indefinitely delayed the building of a hydroelectric power stationon the Chuya River, important for the local population.

There is currently no detailed catalogue of glacial lakes in the Altai, so it is crucial to carry out an inventtory and survey of glacial lakes to obtain data on potentially dangerous lakes that could trigger floods and cause harm to the population of the downstream river valleys.

For systematization, analytical processing and display of spatially coordinated data on glacial lakes in theAltai, we have developed the GIS «GL Altai» (Fig. 1). Microdem/TerraBaseII V12.0 and Global Mapper V 16.0 software package was chosen as the main GIS software. The structure of our developed GIS includes: data bank, hardware-software complex and tools for creation of operational materials for development of forecast conclusions. The databank serves as the information basis of the GIS, its composition and structure were determined by the composition of input and output data required to solve the functional tasks of the research topic.

The GIS databank consists of a map archive, thematic databases in DBase format and remote sensing materials. The vector digital maps in the GIS are in Shape-file format. The availability of a digital topographic base made it possible to bring digital thematic maps and remote sensing materials into a unified coordinate system. Aster GDEM, SRTM matrix third generation and DTED level 3 are used as digital elevation model.

Glacial lake mapping was performed by digitizing with Microdem/TerraBaseII V12.0 tools in Stream mode using raster pattern of summer scenes of multispectral Canopus-B MSS, Resurs-P Geoton ultispectral and monochrome satellite images (scenes obtained using authorized access to the file archive of Roscosmos Geoportal's DPC, https://gptl.ru/) with spatial resolution of 1 to 3.5 m for the period from 2017 to 2020. All available scenes in GeoTiff format were converted into a single map projection (46 UTM zone, WGS84), geotransformed using Aster GDEM matrices and coordinates of reference points prepared during the field expedition research. The spatial position of the lakes for the time interval of the 1960s was mapped using monochrome geotransformed and geo-referenced electronic scans of Corona satellite imagery (materials were ordered and obtained through authorized access to the USGS GDEM file archive, https://earthexplorer.usgs.gov/).

The lake coverage of the mountain-glacier basins was mapped using direct signatures from high and ultrahigh resolution multispectral and panchromatic satellite imagery. The main decoding features of lake water bodies were: smooth phototone and specific monotone or expressive structure of water images; oval shape of lakes and confinement of water bodies to low relief elements. Lakes were mapped when their shape became visible. The main hydrographic characteristics of the lakes (water area, shoreline length, and shoreline elevation) were calculated using GIS software. The lake water volumes were calculated in two ways: by C. Hugel's [6] dependence using formula (2) and by formula (3) proposed by S. Evans [7].

V = 0,104·A^1,42  (2),

V = 0,035·A^1,5  (3),

where V is water body volume, m³ , A is water body area, m² .

To verify the obtained values, a bathymetric survey of potentially hazardous lakes was carried out. Measurements were taken with GPSMAP 585 Plus sonar with built-in GPS receiver. The echo sounding results were processed in Garmin Quickdraw Contours, from which depth maps of the water bodies were generated and their true volumes were established. The discrepancy between the measured volumes and those calculated using formula (2) was on average ±15%, and ±12% using formula (3).