I have been trying to photo a waterfall near me on a long exposure to make the water look loke a blanket of silk, see example of it done right.....

However, when i turn mu shutter speed up too 3 seconds for instance, and my apature all the way down to let as little light in as possible, the image still gets saturated with light, how are pics like these taken? in very low light conditions?
Please help?
