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In many Puget Sound streams, summer low flows have declined in recent decades, and are projected to decline further. Concerns that humans may be responsible have focused on two main causes: anthropogenic climate warming and aspects of development, including urbanization and the abstraction of groundwater. Difficulty in distinguishing their relative impacts has hindered the conception and design of strategies intended to restore and enhance future low flows. We analyzed trends in low flows over recent decades, separating the effects of these factors in two steps. First, low flow variation was assessed in 23 basins that are minimally disturbed by development. Low flows varied over time, and with elevation, in complex ways, consistent with the loss of snowpack at elevations >~800 m. Second, low flow trends in developed lowland basins were compared with trends in a minimally developed lowland reference basin. Flows in developed basins deviated from a purely climate‐driven pattern in unique ways, reflecting unique histories of development. In 21 lowland basins, there was no consistent decline in low flows with increasing impervious land cover, at least between 2001 and 2019. Effects on low flows of private wells alone could be assessed in only one basin, but no impact was evident. An assessment of projected relative impacts on low flows of urbanization, rural development, and anthropogenic warming suggested that the latter will be the greatest. [ABSTRACT FROM AUTHOR]