Thresholds of stability in incised “Anthropocene” landscapes

Channel incision occurs in the “Anthropocene,” where natural river processes and climate variation increasingly interact with human activity. Causes of “Anthropocene” incision include landuses that change the ratio of discharge to sediment load, lower baselevels, or human activities that otherwise alter fluvial systems, such as channelization. This paper reports a field study of an alluvial channel incised into valley fill within the northern Coast Ranges of California. At this site, channel slope adjustments associated with incision, indicated by bank heights of ∼5–8 m, increased transport capacity and excess shear stress by over 20%. The incision exposed Holocene valley fill in eroding channel banks. Results of field surveys enabled development of a dimensionless metric “relative incision,” ht/de, that quantifies the ratio of terrace height (ht) relative to effective flow depth (de). The ratio is predicted to be near a threshold value of 1.0 in stable alluvial channels; in incised alluvial channels the ratio is predicted to exceed 1.0. Further application and testing will provide information to aide in managing incised systems where complex feedbacks in coupled human–landscape systems may promote or dampen incision.