Multiple paths to straths: A review and reassessment of terrace genesis

Abstract

Strath terraces, an important tool in tectonic geomorphology, have been attributed to climatic, tectonic, volcanic, and human activity, yet the pathways connecting external forcings to the channel response leading to terrace formation are highly variable and complex. To better understand variability and controls on the pathways between forcing and terrace formation, we created a comprehensive database of 421 strath terraces from peer-reviewed literature and noted the strath age and rock type, the ascribed forcing (climate, tectonics, volcanoes, or humans) or whether the cause was unascribed, and the pathway between forcing and strath incision or planation. Study authors identify climate, tectonics, volcanoes, and humans as the forcing for 232 (55%), 20 (5%), 8 (2%), and 5 (1%) strath terraces in our compilation respectively. A forcing was not identified for the remaining 156 (37%) terraces. Strath terraces were dated using 14 different methods: 71% of terraces in our database are dated using methods, such as radiocarbon and optically stimulated luminescence, that date planation and give a maximum age of incision; 16% of terraces are dated with methods that give a minimum age of incision; and 14% use a variety of methods for which a generalization about incision age cannot be made. That the majority of terrace studies use planation ages to understand terrace formation highlights the necessity of knowing the relative timescales of incisional and planation phases, which has so far been quantified in only a handful of studies. In general, rivers in arid regions plane straths in interglacial periods when discharge and sediment transport capacity increase, whereas temperate rivers plane in glacial or interglacial periods when sediment supply increases. Heterogeneities in rock strength between watersheds further control how sediment is produced and when straths are planed. Globally, these regional and watershed controls result in strath planation and incision during all parts of the glacial cycle. Terraces with no identified forcing in our database reach a maximum frequency during the late Holocene (4 kya–present) and could potentially be explained by regional deforestation and increased anthropogenic fire frequency, regionally active tectonics, and climate fluctuations. Deforestation and fires, by reducing the supply of wood to streams, decrease instream sediment retention and could convert alluvial channels to bedrock, thus promoting strath incision. The regional and watershed controls on strath formation highlighted in our database, as well as the possibility of anthropogenic forcings on strath terrace formation in the late Holocene, illustrate the importance of explicitly establishing the pathway between forcing and strath terrace formation in order to accurately interpret the cause of strath formation.