Guys have an update on the history of this lake. as i mentioned earlier, i've lived in this area for many generation and the existance of a lake completely surprised myself and anyone i spoke to locally.
have gone there couple of times since with cousins etc and was completely amazed by the site. the water is definitely clean, its not drainage, its very deep, even near the banks.
So as it happens I was speaking to some relatives and one of them said that he knows how it came to being. Basically, Bahria town had purchased this land some years ago and had sold it to some other builder, I believe Emaar. however, they ran away and now Bahria has this land. there was some chashma here locally and bahria folks build a embankment on the lower end of this small gorge. the water from chashma as well as from rains that used to drain in soan normally, started accumulating here and now we have a beautiful lake 
this happened in last couple of years, so its not too old.nonetheless, its a great picnic spot to visit. the plans from bahria are to use this water for their projects locally, not sure if they'll leave it or drain it out completely.
As for directions, as soon as you go past the security checkpoint while entering DHA, stay on your left and take the 'Orchard Ave' underpass, this road will take you till the river swan, total distance is perhaps a mile or so. there is a kacha track where the road get on the kacha track. this track goes along the river for 1/4 of a mile. after that it hits a small paved road. just make a right on that road and go uphill. go for perhaps 100 yards and you will see the lake on your left below. from there pick any track leading down to water.
Nauman
