Downstream migration of age 0+ chinook salmon from stream channels with rock and rubble (good) or gravel or shale (poor) substrates and constant or declining water temperatures was assessed during the fall months of 1970 and 1971. As water temperatures declined, juvenile chinook left stream channels with gravel or shale substrates or moved into available hiding spaces in stream channels with rock and rubble substrate. Fish initially emigrated as temperatures declined below 10 C. The number of emigrants reflects the holding capacity of the substrate and the density of the fish population. The migration response varied with size and race of fish. Most of the response difference between races is attributed to differences in fish size. Juvenile spring chinook find rock and rubble substrate and important component of their winter habitat.