River floodplain abandonment and channel deepening coincide with the onset of clear‐cut logging in a coastal California redwood forest

Abstract

Changes in both land use and climate can alter the balance of transport capacity and sediment supply in rivers. Hence, the primary driver of recent incision or aggradation in alluvial channels is often unclear. The San Lorenzo River on the central coast of California is one location where both climate and land use—specifically, clear‐cut forestry of coastal redwoods—could explain recent vertical incision and floodplain abandonment. At our field site on the San Lorenzo, we estimate the magnitude of recent incision using both the ratio of bankfull to critical Shields numbers ( τbf*/τc*) and the geomorphically effective discharge, calculated from historical gauge data. The Shields number ratio suggests that the normalized bankfull stress of the San Lorenzo River is in the upper 1–2% of West Coast rivers, and the effective discharge corresponds to flow depths ~2–4 m below current bankfull conditions. Radiocarbon ages from detrital charcoal in floodplain sediment reveal active floodplain deposition during the 1600s and possibly into the 1800s, constraining the timing of incision to the last few centuries. Multiple hanging tributaries above the mainstem San Lorenzo River, along with patterns in vegetation on terrace surfaces, corroborate our estimates of the magnitude and timing of incision. Taken together, our findings suggest that floodplain abandonment in this reach was mainly due to methods employed during logging that increased shear stress on the channel bed and reduced sediment storage capacity. We suggest that direct channel modifications in rivers can counterbalance increases in sediment delivery due to clear‐cutting, resulting in channel incision rather than aggradation. Today, a young, lower surface appears to be forming adjacent to the San Lorenzo River, which we interpret as an incipient floodplain that is in equilibrium with modern sediment supply and transport capacity.

The Influence of Knickpoint Development and Channel Incision on Riparian Vegetation in Semi‐Arid River Corridors

“…Channel incision can result from a change in transport capacity relative to sediment supply due to sediment capture behind dams (Heine & Lant, 2009; Jacobson et al., 2009), large flood events (Brogan et al., 2019), or instream gravel mining (Comiti et al., 2011). Incision can also occur due to increased shear stress caused by logging activities (Chapman et al., 2021), or following aggradation caused by increased sediment supply due to land use change within watersheds (Baena‐Escudero et al., 2019; Dearman & James, 2019; Dow et al., 2020; Merritts et al., 2011). Although our study highlights an understudied interaction between geomorphic change and riparian vegetation because it explores the impact of knickpoint evolution on incision and subsequent changes to vegetation, our results can inform the influence of incision on riparian vegetation regardless of the underlying cause.…”