How did that "quake lake" occur?

Question

Just wondered how it came to be and wheres the water coming from? How big is the lake like in miles or feet? What the heck. Was that supposed to happen? Has it ever happened before?

Answer 1

Earthquake shook the mountainous parts of China which led to landslides. All that dirt made their way to the lakes in between the mountains, thus blocking the usual route of water. Needing to have an outlet, the blocked water is now trying to make its way via a different path which happens to be one of the severely damaged towns.

Answer 2

The earthquake triggered landslides which dammed up the river.  The lake covers many square miles and holds approximately 34 billion gallons of water and is rising at the rate of over 7 feet per day.

This type of thing is not uncommon. It happened in the 1950's along the Madison Canyon River in the Yellowstone area of Wyoming.

It doesn't necessarily require an earthquake -- it happened in the U.S. in the 1920's at a place called the Gros Ventre Slide in Wyoming.  Heavy rains and snow melt cause a large slide which block the river.  The water eventually washed out the temporary dam created by the landslide (which authorities had said posed no danger) resulting in a catastophic flood downstream.