This paper reexamines Lewis and Clark's population figures for the native inhabitants of the lower Columbia river valley and discusses them in relation to the region's resource base and social organization. Two versions of Lewis and Clark's estimate exist: the first, easily accessible, has been in print for over 170 years, while the second remains in manuscript and has not been used by most researchers. The two estimates, apparently collected by Lewis and Clark in October to November 1805 and March to April 1806, differ significantly in numbers for lower Columbia villages. We argue that together both estimates accurately depict seasonal variations in lower Columbia populations. A comparison of the two sets of figures suggests the distinctive southern Northwest Coast pattern, known from other sources, of populations resident in winter but mobile the rest of the year. The lower Columbia drainage is well known for great temporal and spatial variation in the availability of desired resources. It seems likely that such seasonal movement, facilitated by a widespread network of social ties, would have had the effect of balancing, on a regional basis, people and resources. In developing our argument, we present ecological data on seasonal and local variations in the resource base and ethnohistorical materials on social organization and the use of resources, and we suggest comparisons with similar regions elsewhere