Global warming could have serious effects on both the quality and quantity of freshwater, according to a report released yesterday by the Pew Center on Global Climate Change. In mountainous watersheds, global warming will increase the ratio of rain to snow and lead to earlier and greater spring runoff, the report says, which could improve water quality in some streams but could add more pollutants from farms, factories, and homes to others. In arid regions, climate change can be expected to fuel more demand for freshwater. Climate change could also increase water temperatures, making it harder for many species of fish to survive.