THE WORLD DELTA DATABASE

THE LENA DELTA

Delta ID         # 18.

DISCHARGE WATER 155000M3/S

DRAINAGE BASIN AREA 3.028X106 KM2

The Lena River, with a length of 4,319 km, is the longest river in the Russia [18-i01]. A great delta has formed where it empties into the Laptev Sea, a part of the Arctic Ocean. The river rises a few kilometers west of Lake Baikal, on the western slopes of the Baikal Mountains, at an elevation of 1800 m. The average elevation in the drainage basin is 570 m, with a maximum of 1,697 m and a minimum of 90 m. The drainage basin comprises some 2,421,400 sq km, one of the larger drainage basins of arctic rivers. The tributary drainage network is quite dense (Figure 55), average drainage density is 0.19 km stream length per 500 sq km, and is controlled primarily by geologic structure. The southern rim of the drainage basin is comprised of PreCambrian and Cambrian sedimentary rocks, while the central basin is composed of Jurassic and Cretaceous sedimentary rocks. The northwest part of the drainage basin displays complex geology, but is comprised mostly of Cambrian, Ordovician, and Triassic sedimentary rocks. The Aldon Shield and the Baikal Patom fold belt lie in the southern part of the basin, whereas the Nepa - Botuoba Arch and thee Anabar - Olenek high occupies the western part of the basin. Rainfall is relatively low, the average annual rainfall being 406 mm. Maximum rainfall is 700 mm and minimum rainfall is 226 mm. Heaviest rainfall occurs in the months of July through September. For some 285 days of the year, basin temperatures are below freezing and no runoff occurs. Throughout much of its course, the river widens into broad braided stretches 3 to 4 km wide and then narrows as it cuts through more resistant strata [18-i01]. The alluvial valley is well-defined, relatively narrow, and braiding is the most common channel pattern. Numerous small lakes dot the alluvial valley. 18-i01 and 18-i04 illustrate the control that geologic structure has on the river’s course. Most of the basin is covered by Boreal forests and taiga, while the delta plain consists of ice bound tundra. Near its delta, the river narrows considerably as it cuts through the Kara Ulak Mountains, which are composed of Permian and Cretaceous sediments. Even in this stretch, narrow Pleistocene river terraces are present along the Lena's course.

Discharge is extremely high, the average annual discharge being 16,630 m³/sec, with a maximum of 73,996 m³/sec and a minimum of 1,203 m³/sec. Discharge abruptly increases from 5,578 m³/sec in May to a maximum of 73,997 m³/sec in June, when spring melt begins. Nearly every year in the late spring, ice blocks the flow of water at the mouth of the Lena River in northeastern Russia and gives rise to floods across the Siberian plains. In the spring, however, water upstream thaws earlier than water at the mouth of the river. As the southern end of the river begins to melt, blocks of ice travel downstream to the still frozen delta, pile up, and often obstruct the flow of water [18-i01]. Emerging from the highlands, this long north-flowing river crosses the taiga of Siberia, one of the most extensive forests on the Earth. After passing through the narrow gorge near its delta, the Lena cuts through broad coastal Pleistocene terraces, many remnants of which are present as isolated highs within the delta. The fan-shaped delta covers some 43,563 sq km and is a maze of small distributaries [18-i03]. The river splits off into several distributaries, the only one of significance being the Trotimorskaya, which flows into the eastern part of the delta. The delta can be divided generally into two major parts, the western delta and the eastern delta (Antonov, 1959). The western delta is relatively high, is moderately crossed by distributary channels, and has had many of its lakes tapped and drained, resulting in dry land or former lake surface over approximately 85 percent of the delta. The presently more-active eastern part of the delta is characterized by relatively low elevations and a large number of anastomosing channels and sand bars. In both parts of the delta, the ground is permanently frozen, often to depths in excess of 15 m.

At the apex of the delta, the Lena River is complexly braided, with many large braid bars and small channels. Within the delta plain, the major channels split and rejoin, forming a maze of anastomosing channels (Suslov, 1961). Near the distal ends of the distributaries, where the channels often broaden out, the river mouth bars are composed of barren sand and mudflats. Tidal range is low, generally less than 0.21 m, and does not significantly influence the sediment distribution pattern. Wave action at the coast can be considerable, with root mean square wave heights up to 0.24. As a result, small beaches and complex offshore barriers [18-i02] are present at most of the river mouths of arctic deltas. The channels in the lower delta are often quite deep, on the order of 15 to 18 m, being much deeper than the upstream segments. Sediment load is quite high during the short flood season, with the suspended load reaching 11.7 million tons per year. Offshore currents are relatively low and generally flow to the northwest, carrying large quantities of fine-grained sediment westward along the coast.

The delta plain consists of a maze of anastomosing channels, between which are broad areas of tundra surfaces containing patterned ground or polygons, freshwater lakes, and overbank splays. The tundra surface is composed of relatively large and complex patterned ground, which results from the annual freezing and thawing process. Ice wedges and other similar arctic landforms are present. As a result of thawing, many small freshwater lakes now dot the tundra surface. The freshwater lakes cover some 16 percent or 6,970 sq km of the delta plain. Because ice jams in the distributary channels cause abnormally high water in the delta, overbank splays are quite common, often filling many of the small lakes. After the major flood has subsided, water levels drop considerably, exposing a large number of sandy mid channel bars. Wind erosion removes the finer sands and forms small oriented dunes along the channel margins, often extending onto the adjacent tundra surface and into the lakes. The river traverses large taiga forests and thus carries large quantities of woody debris. This organic debris is often left stranded on the mid channel bars and within the channels.

On the Pleistocene surface to the west [18-i02], oriented lakes are present, similar to those found along most of the Arctic tundra. Pingos or "ice-core hills" are also found on this surface. Broken pack ice, which is nearly always present, can be seen offshore of the delta.