Methodological Problems of Studying the Underwater Relief in the River Delta from Space Images (on the Example of Mapping the Shallows of the Yenisei River Delta)

Abstract

Based on images from the Sentinel-2 satellite taken in 2017–2019, a map of shallows in the Yenisei delta was compiled. This paper highlighted the methodological problems that arise at using satellite images for research and mapping of underwater relief; the ways of their solution are offered. Consideration of the depth of penetration of sunlight into the water and analysis of the spectral brightness of underwater objects allow for the determination of the spectrum zone (red zone) optimal for interpreting sandy riverbanks and options for color synthesis. At a limited choice of cloudless images in the Arctic to have images obtained during the minimum water level, not only the imaging season (low-water period) but also interannual runoff fluctuations are taken into account. The water turbidity limits the possibility to interpret the underwater relief. Images obtained with minimal turbidity are selected, and in the case of spreading of turbid waters, multi-temporal images are compared to separate the image of volatile turbid waters from more stable bottom objects; color synthesis is applied at different times. The identification of sandbanks, ridged forms of relief, and the contours of underwater channels are performed by direct interpreting by the variations in the brightness of the image in the red zone. To identify sandy-silty and silty shallows, indicative interpreting based on the distribution of aquatic vegetation on silty deposits is used. The identification of sandbanks, ridged relief forms on them, and the contours of underwater channels are performed by direct interpreting by the variations in the brightness of the image in the red zone. To identify sandy-silty and silty shallows, indicative interpretation of the distribution of aquatic vegetation on silty deposits is used. To highlight the fuzzy boundaries between the shallows and core zones of streams, the brightness quantization of images is applied. Comparison of the Yenisei River delta shallows map compiled from modern photographs with topographic and pilot maps of the 1970–1980th shows a reduction in the area of shallows adjacent to the tips of large islands in the eastern part of the delta. This is in accordance with earlier revealed activation of the erosion of the banks of the branches of the Yenisei delta in the XXI century.