Dam-building behaviour is a distinctive feature of beavers; yet they often occupy streams or sections of streams where they do not construct dams. This study was designed to determine how stream depth and stream width influence dam-building behaviour. Stream depth and stream width were measured at 39 beaver lodges, which did not have dams associated with them and at 74 dams. The mean water depth at the lodges was 1.16 m (range=0.55–2.0 m), while the mean water depth at the dam sites was 0.36 m (range=0.10–0.85 m). The stream width was greater at lodge sites (mean=11.15 m, range=1.0–40 m) than at dam sites (mean=2.5 m, range=0.5–6.0 m) A discriminant analysis classified 93% of the sites correctly as either lodge or dam sites.