Lake Pukaki...

 Lake Pukaki is the largest of the three glacial lakes...

All three lakes were created when the terminal moraines of receding 

glaciers  blocked their respective valleys, forming moraine-dammed lakes. 

 It has become famous for its milky-blue color and as the foreground

 to Mount Cook... Which in this photo is hiding in the clouds.

 The glacial feed to the lakes gives them a distinctive blue colour, 

created by glacial flour, the extremely finely ground rock particles 

from the glaciers. Lake Pukaki covers an area of 178.7 km², and is 

518 - 532 meters above sea level.

 The lake is fed at its northern end by the braided Tasman River, 

which has its source in the Tasman and Hooker Glaciers close to  

Aoraki/Mount Cook, which today is being very elusive.

Lake Pukaki in Aoraki/Mount Cook National Park - is the 

location of Lake-town in The Hobbit Trilogy.

Read about that here. 

 The lake is now part of the Waitaki hydroelectric scheme.

     The lake's original outflow was at its southern end, into the Pukaki River.

 The outflow has been dammed, and canals carry water from Lake Pukaki 

and Lake Ohau through the Ohau A power station to  Lake Ruataniwha.
 Pukaki is also fed by the waters of  Lake Tekapo, which are
diverted through a canal to a power station on Pukaki's
 eastern shore (Tekapo B station).
The lake has been raised twice to increase storage capacity
(9m in 1952, and 37m in 1976 ), submerging Five Pound Note Island,
which once appeared on New Zealand's five pound note.