I often hear these words treated as if they were interchangeable.

Problem:
The nouns “avalanche” and “landslide” are not synonyms.

Explanation:
The noun “avalanche” means a large mass of snow, rocks, or debris suddenly detaching from a mountainside and then falling or sliding down.

The noun “landslide” means a mountainside falling or sliding down.

Winter sports enthusiasts have to watch out for avalanches, whereas many people in California suffer from landslides.

“Avalanche” should make one think of individual items coming down a mountainside. “Landslide” should make one think of the entire mountainside coming down.

If one were on a mountain during an avalanche, one might be able to stay put with little or no damage. If one were on a mountain during a landslide, one definitely would move with the mountainside.

Solution:
Use “avalanche” when referring to something coming down a mountainside. Use “landslide” when referring to the entire mountainside detaching and coming down.