Snowpack in the mountain valleys where the Colorado River originates is only a little below normal, marking one of the few bright spots in an increasingly grim drought gripping much of the West. Measurement stations in western Colorado showed the snowpack yesterday at 90 percent of the long-term average. By contrast, many stations in the mountains of California, Oregon and Washington have shown snowpack at 50 percent or less. Some detected no snow at all. Mountain snow in Colorado is closely monitored because a half-dozen Western waterways, including the 1,400-mile Colorado River, start in the area. The Colorado River and its tributaries supply water to millions of people in seven states and Mexico. Much of the river comes from mountain snow that accumulates during winter and melts in the spring.