Complex sentence: Moisture from the Pacific Ocean streams on shore in the form of clouds and humid air between the Sierras and Cascades into Snake River Plain where it is channeled though most of Rocky Mountains with no high plateaus nor mountain ranges to impede its progress.
Split into simpler sentences: Moisture from the Pacific Ocean streams on shore in the form of clouds and humid air. It passes through the gap between the Sierras and Cascades and on into Snake River Plain where it is channeled though most of Rocky Mountains with no high plateaus nor mountain ranges to impede its progress.