Anthropogenic strath terrace formation caused by reduced sediment retention.

Across North America, human activities have been shown to cause river incision into unconsolidated alluvium. Human-caused erosion through bedrock, however, has only been observed in local and isolated outcrops. Here, we test whether splash-dam logging, which decreased in-stream alluvial cover by removing much of the alluvium-trapping wood, caused basin-wide bedrock river incision in a forested mountain catchment in Washington State. We date incision of the youngest of four strath terraces, using dendrochronology and radiocarbon, to between 1893 CE and 1937 CE in the Middle Fork Teanaway River and 1900 CE and 1970 CE in the West Fork Teanaway River, coincident with timber harvesting and splash damming in the basins. Other potential drivers of river incision lack a recognized mechanism to cause T1 incision or are not synchronous with T1 incision. Hence, the close temporal correspondence suggests that reduced sediment retention triggered by splash damming led to the observed 1.1 mm⋅y-1 to 23 mm⋅y-1 of bedrock river incision and reduction of the active floodplain to 20% and 53% of its preincision extent on the Middle and West Forks, respectively. The development of such anthropogenic bedrock terraces may be an emerging, globally widespread physiographic signature of the Anthropocene.