Before the dam, the water is so calm, it reflects the surrounding cliffs and trees - with Half Dome on one side and North Dome on the other.
At the dam and beyond, the water gushes through several cascades and continues flowing through the creek.
During my last visit in June of 2017, the water flow through the creek was so strong, it felt like a roaring river. Towards the end of summer, the lake mostly dries out and the creek becomes a trickle.
The lake is accessible via a very easy trail which makes it very popular and crowded. This trail starts from a paved access road on the north side. But for a really rewarding hike, take the south trail.
Both trails start from bus stop #17. There is also a parking lot close by for people driving themselves.
At the bus stop, take the dirt road into the woods and stay to the right when you reach a bridge. The trail mainly runs alongside the creek and is mostly shaded. There are some steep climbs, but it is mostly level.
When you reach the natural rock bridge, top and enjoy the contrast between the calm lake and roaring river. This a great spot for pictures.
The south trail used to connect to the north side forming a long loop around the lake, but due to a major rock slide, the loop was cut off.
Another way to visit Mirror Lake and enjoy its beauty is to start from the Majestic Yosemite Hotel (formally Ahwahnee) and continue under the Royal Arches and North Dome.