As much a sign of spring as longer days, greening plants, and melting snow, the Ob River had spread across its floodplain in far northern Russia when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured the top image on June 20, 2007. The Ob River and its tributary, the Irtysh, together form one of the longest river systems in Asia, flowing from the Altay Mountains of northern China to the Arctic Ocean. The northern reaches of the Ob flow over a flat, permafrost plain.
As spring creeps north, the snow that covered northern Russia gradually melts, and the runoff flows into the river. Swollen with spring runoff, the river flows north, where it meets ice jams on sections of the river that have not thawed. Since the river cannot cut deep channels into the frozen land, it flows out over the surrounding plain during the spring melt, creating the wide band of water seen in this image.
The lower image shows the Ob River in the fall, immediately before winter secured the region in its frozen grip. The river was a fraction of the size it would be the following spring. Both images were made using a combination of visible and infrared light. Water is black and dark blue. Snow, light blue, dusts the ground south of the Gulf of Ob in the October image and covers the peaks of the Ural Mountains west of the river in the June image. Also in the June image, a smooth pane of ice, also light blue, covers the Gulf of Ob, providing the natural dam that created the floods shown here. Pale blue, frozen lakes dot the permafrost north of the river, and darker, ice-free lakes adorn the land to the south of the river. Plant-covered land is green, and clouds are light blue and white.
In the span of three weeks, spring crept over the Siberian landscape surrounding the northern half of the Lena River. Many of the rivers in Earth’s temperate zones run high in the spring when melting snow and spring rain flood river basins. On the Lena River, however, spring flooding is almost inevitable for another reason: ice. Like other north-flowing rivers, the upper reaches of the Lena melt before their downstream counterparts. Because the northern mouth of the river remains frozen while the southern body of the river flows freely, water naturally builds behind the ice, forming a temporary reservoir that drains as the ice dwindles.
Russia’s Ob River flows from south to north, and each summer, it thaws in the same direction. The result is that an ice jam sits downstream from thawed portions of the river, which is laden with heavy runoff from melted snow.