Ho Hoan Kiem or Lake of the Returned Sword was once a part of the Red river (song Hong). Through thousands of years of changes in the geography, the lake moved eastward to its present position many kilometers from the river. The lake was once called Luc Thuy or Green Water because the water was green year round. In the fifteen century, the lake was named Ho Hoan Kiem, based of a legend that is quite similar to King Arthur and the Lady of the Lake's legend.

Ha Noi

In the fall of 1010, King Ly Thai To (Ly Cong Uan) moved the capital from Hoa Lu to Dai La. 
On the way, the king saw a vision of a golden dragon ascending from the Red river 
( song Hong). The King decided to change Dai La to Thang Long (Ascending Dragon). 
Thang Long remained the capital city until the end of the Tran dynasty when in 1397, 
the capital city was moved to Thanh Hoa -Tay Do (Western Capital) and Thang Long 
became Dong Do (Eastern Capital). 
Vietnam was invaded by China in 1407, and the city was renamed Dong Quan. In 1428, after ten years of fighting, Le Loi liberated Vietnam and renamed the city Dong Kinh. In 1527, the city was renamed Thang Long. In 1802 when King Gia Long (Nguyen Dynasty) moved the capital city to Hue, the name Thang Long remained but, Long no longer means dragon, Long in this case means prosperity. Ha Noi, was the name given to the city by King Minh Mang in 1831. Ha means river and Noi means within - Ha Noi means within the river.