LAS VEGAS (KLAS) — A big snowstorm in California is welcome news, as long as floods and mudslides aren’t wrecking lives.

Snow and rain bring some relief from the drought, but it matters where the snow falls. The current storms are drenching central California and dumping snow in the Sierra Nevada, the beautiful mountain range that’s home to national parks and scenic mountain trails and roads. That snow is never going to make a difference at Lake Mead — the focal point of the 23-year “megadrought” gripping the desert Southwest.

The Sierra Nevada feeds rivers, lakes and springs that supply water to two-thirds of California’s population by way of the California State Water Project (SWP). About 25 million Californians get water from the SWP and about 750,000 acres of farmland receive irrigation water from the SWP. The map below shows the drainage from the Sierra: It flows to the west.

(USDA/U.S. Forest Service graphic)

Lake Mead gets water from the Colorado River, which starts in the Rocky Mountains. And lately, the Sierra Nevada and the Rockies have both been suffering through dry years.

For the first 11 months of 2022, California was going through its second-driest year on record, but an atmospheric river that started in December turned it to only the ninth-driest year on record for California, NOAA climate monitoring chief Karin Gleason told The Associated Press.

A wet winter in one mountain range won’t solve problems that belong to another mountain range. Will the rain and snow make a difference? It depends on where it happens.

Water flows downhill. It doesn’t matter whether that’s north, south, east or west. It just goes downhill. The maps that show how that water flows define “watersheds” — the area of land that drains or “sheds” water into a lake, a reservoir or an ocean. The Colorado River Basin collects water and takes it downstream.

(U.S. Geological Survey map)

A big rainstorm or a driving blizzard won’t matter unless it happens inside the drainage area for the Colorado River.

The future of Lake Mead is tied directly to the snowpack that builds up each year in the Rockies. Water flows down the Colorado River, through Lake Powell and onto Lake Mead, the biggest reservoir in the U.S.

While storms have helped so far this year in the Sierra, there’s no guarantee it will continue. A weather pattern known as La Niña is entering its third year. La Niña — which affects the air’s moisture content — has typically meant drier weather in the desert Southwest.

And measurements of snowpack, while encouraging right now at 142% of normal, aren’t guaranteed to hold through the spring. That’s when snowpack measurements really matter. Until then, we’re just watching the weather and hoping for the best. Snowpack for the Upper Colorado River Basin hasn’t changed over the past week.

“A normal snow year doesn’t mean normal runoff,” according to Kyle Roerink of the conservation group Great Basin Water Network.

Sunken speedboat now points skyward after Lake Mead water level drops. Aug. 14, 2022 (KLAS)

“From what we’ve seen in tributaries in La Niña conditions, the percentage of runoff will be somewhere in the ballpark of 10-20% less than the percentage of snowpack. It is an inexact science. But I think it is important to note that a 100 percent snowpack does not equal 100 percent runoff,” he said.

Roerink says the current method of managing water needs to change.

“We cannot repeat this mistake in the future. The purpose of our collective efforts — funded at great taxpayer expense — should not just be filling reservoirs. It needs to be about changing behaviors,” Roerink said.