After one to five years roaming the ocean and building up powerful muscles, mature chinook salmon return to the streams of their birth each fall. Taking advantage of water levels raised by fall rains, they thread their way upstream against swift currents and surging waterfalls to reach ancestral spawning grounds. All across Northern California -- from tiny riffles in secret ravines to mighty rivers crashing out of the mountains -- salmon gather to lay eggs on silt-free gravel bars where clean waters cool the eggs. Because each locality hosts a unique run of salmon with its own calendar and spawning strategy, many complex genetic forms have evolved. If a spawning ground succumbs, so does a distinctive fish.