LAKE TAHOE, Calif. (KERO) — Lake Tahoe's water level has dropped to a four-year low.

It was hovering around the natural rim but this week it fell just below it.

At that point water stops flowing out into the Truckee River.

One researcher says the water level is decreasing about a quarter of an inch a day and it's something to keep an eye on.

"So the water level will come back up over the rim probably some time in the next few months *but then if it's just an average winter, it's going to go below the rim a lot earlier and go a lot lower than this winter," said Geoff Schladow, director of UC Davis-Tahoe's Environmental Research Center.

He says water evaporation is increasing because of dryer winters.

And adds that without a lot of precipitation, the lake is expected to evaporate at a rate of about one to one-and-a-half inches a week. That amounts to about four to six billion gallons a week.