AUSTIN (KXAN) — Lake Travis is Central Texas’ largest water storage reservoir, holding over 300 billion gallons. And twice a year, the entire lake turns over.

This process all has to do with water temperature. In the summer, long days and hot temperatures warm the surface of the lake to more than 80°F, while the deep water sheltered from sunlight and warm air, stays colder.

Since cold water is heavier than warm water, the lake is stable in the summer and retains this temperature profile.

As summer turns to winter, days get shorter and air temperatures get colder. This cools off the surface of the lake, making the water heavier. This cold, heavy water sinks — forcing the deep water upward toward the top of the lake. As this process repeats itself during the colder autumn months, and gusty winds further mix the lake, the lake eventually becomes the same temperature throughout. The water is just as chilly on the surface of the lake as it is at the bottom. This creates free mixing.

But why does it smell bad?

Decaying organic matter at the bottom of the lake creates hydrogen sulfide gas. When the lake mixes freely during the overturning process, that gas is able to rise from the bottom of the lake to the top, and escape into free air.

Fish feast on the nutrients released, but we smell rotten eggs.

As winter eventually turns back to summer, the days again get longer and the surface lake temperature warms. This returns the lake to a stable temperature profile, and the swimming season resumes.

Typically, Lake Travis turns over once in November or December, and again in April or May. The autumn overturning is much more dramatic and rapid than the spring.