EXAMINING ALTERNATIVE HYPOTHESES FOR HOLOCENE STRATH TERRACE FORMATION: THE ROLE OF SEDIMENT RETENTION AND WOODY DEBRIS

The pace of river incision, which sets the local base level for hillslopes and thus is a major control on landscape evolution, is widely recognized to be modulated by temporal variations in alluvial cover. These variations are strongest at glacial-interglacial transitions and so many fluvial landforms resulting from incision rate changes, such as strath terraces, are thought to represent climate variations. However, recent studies in the Pacific Northwest show that alluvial cover has varied historically due to decreased frequency and magnitude of large wood jams, which block sediment transport and force an alluvial cover. River incision rates are currently elevated in these areas and abandoning strath terraces. Is this historic strath terrace formation, triggered by decreased sediment retention and deforestation, a global phenomenon? Here, we use a global compilation of strath terrace studies to examine the timing and nature of strath formation and test our hypothesis that deforestation decreased sediment retention, increased river incision, and formed strath terraces. For each study, we compiled the strath abandonment age, attributed trigger of incision, bedrock lithology, and study location, as described originally by the authors, for a total of 389 terraces. We find that climate is the attributed trigger for 50% of the dated strath terraces, although at the end of the Last Glacial Maximum at 9-12 kya, climate accounts for 74% of strath terraces. The greatest frequency of strath terrace ages is found from 0-4 ky, and only 13% of these terraces are attributed to climate while 65% have unknown triggers and potentially formed in response to deforestation and decreased sediment retention. Regionally, strath terrace and deforestation ages are coincident; in North America, 30% of the 0-4 ky terraces with unknown triggers formed 100 ya, at the same time as large-scale deforestation and stream-clearing in the study areas. In Taiwan, the location of 47% of the 0-4 ky terraces, large scale deforestation began coincident with terrace formation at approximately 4 kya. Our results suggest that reduced sediment retention occurred globally and the resulting increase in river incision rates has led to the formation of strath terraces at least 4 kya and continuing to the present in some locations.