Most of the water resources in the western USA come from precipitation over mountain ranges, mainly during winter. The orographic lifting of the air increases the precipitation amounts over the high elevations with respect the upwind low lands, and so concentrates the precipitation over the mountain where it runs off easily and becomes useful for exploitation as water resources. A trend of decreasing of this orographic enhancement factor is noted during the last century at all the places that experienced significant population growth within that period. Increasing levels in the small pollution aerosols that can produce small cloud drops that are slower to merge into precipitation is suspected to be the culprit. No alternative meteorological explanations were found so far. The precipitation trends are reflected in similar trends of the river flows, which are the main water sources for California and much of western USA. Remote sensing and aircraft in situ measurements that were conducted during February 2005 confirm the detrimental impact of particulate air pollution on precipitation forming processes over the Sierra Nevada.