Juyan Lake Basin - 居延海

The Juyan Lake （Chinese:居延海) is a former lake in the Gobi desert. It is located in the western part of Inner Mongolia, in Ejin Banner of the Alxa League, near the border with Outer Mongolia. Goshun Nur had an area of 267 km2 in 1958, of 213 km2 in 1960, and has dried up in 1961. The eastern lake reappeared in 2005. As of August 2012, the area of the lake is 38 km2 .

The Juyan Lake basin is a rare wetland located in a desert. It covers an area of about 10,000 square kilometres. The Juyan lake is one of three former terminal lakes located at the outer edges of the Heihe River ('Black River') catchment which formed a large inland delta between the Qilian and the Gobi Altay. The Heihe River is also known as the Ruo Shui (Chinese: 弱水, literally: "weak river"), also Etsin Gol or Ruo He or Ejin River.

The basin's boundary is formed by the Mazong Shan mountains to the west, the Heli Shan and Longshou Shan mountains to the south, the Helan Shan and Lang Shan ranges to the east and the Mongolian Gobi Altay range to the north.

居延海位于内蒙古自治区阿拉善盟额济纳旗北部，形状狭长弯曲，有如新月，额济纳河汇入湖中，是居延海最主要的补给水源。居延是匈奴语，《水经注》中将其译为弱水流沙，在汉代时曾称其为居延泽，魏晋时称之为西海，唐代起称之为居延海，现称天鹅湖。
居延海的湖面因额济纳河的改道而时有变动，自元代以后分为了亦集乃、哈班哈巴儿、塔剌失三个海子（湖泊），清代以来又分成了东部的苏泊诺尔（蒙古语，意思是母鹿湖）和西部的嘎顺诺尔（蒙古语，意思是苦湖）。两湖之间相距约35公里，平均水深1.5米，湖面上碧波荡漾，湖畔芦苇丛生，湖中生长着鲤鱼、鲫鱼、大头鱼、草鱼等鱼类，天鹅、大雁、鹤、水鸭等常来此栖息。

历史上的居延海，由东、西、北三个湖泊组成。人们早年所说的居延海主要是指西居延海（嘎顺诺尔）；现在所说的居延海一般指东居延海（苏泊淖尔），位于阿拉善盟额济纳旗达来呼布镇东北约40公里的巴丹吉林沙漠北缘，为古弱水的归宿地。

额济纳河离不开祁连山。每当春季，暖风吹化祁连山上的冰雪，汇成奔腾的河流，冲进巴丹吉林沙漠；雨季到来后，补充水量的雨水进入河流。河水宛如一条晶莹的飘带、延展向额济纳旗北端，飘带尽头系着两颗洁白的“绣球”—嘎顺诺尔、苏泊淖尔，也就是史料记载的弱水流沙“居延泽”－－居延海。

该地区年蒸发量约3000毫米，而降雨量不足40毫米，完全靠额济纳河（黑河）补给，清代诗人任万年称“巨浪滔天大石浮，龙形滚滚向东流”。

居延海是在东西断裂形成的条带状洼地的基础上形成，再加上风力剥蚀作用使洼地范围扩大形成居延海湖盆。湖盆内有巨厚的侏罗纪沉积。第四纪早期造成静水沉积的环境，说明为温暖湿润的澹水湖泊。更新世-全新世以来气候转为干旱。