Knickpoint and knickzone formation and propagation, South Fork Eel River, northern California

The South Fork Eel River, northern California (United States), displays a prominent knickzone in its longitudinal profi le that may represent a perturbation that is propagating upstream. We investigated two tributary basins (Standley and Bear Pen Creeks) located downstream from this major trunk-stream knickzone to document the presence of knickzones within tributary and subtributary streams and to explore their correlation to the South Fork Eel River knickzone. We utilized LIDAR (light detection and ranging) derived digital elevation models to identify more than 100 major knickpoints and knickzones along 103 streams within these 2 tributary basins. Major knickpoints are located at clear infl ection points separating two reaches of concave-upward stream profiles. These knickpoints can be delineated at breaks in the regression relation of channel slope versus drainage area for these two tributaries. Using the slope-area relation, we recreate paleolongitudinal profi les to represent the pre-incision profile of main stem tributary channels, as well as the pre-incision elevations of subtributary outlets. Knickpoint distribution throughout the two basins indicates that the channels are responding to pulses of incision initiated through base-level fall along the South Fork Eel River. However, most of the major knickpoints identifi ed do not correlate with the current, prominent knickzone along the South Fork Eel River. Rather, knickpoint distribution within the study area indicates that there have been multiple instances of base-level fall along the South Fork Eel River, each triggered by the upstream passage of knickzones that are no longer preserved in the South Fork Eel River profi le. INTRODUCTION Base-level fall at the mouth of a drainage basin can initiate an upstream-propagating wave of incision (e.g., Gardner, 1983). The manner by which incision migrates up a stream channel is an observation fundamental to understanding the process of river base-level evolution, especially in bedrock-dominated channels. Such migrating incision can take the form of transitory knickpoints and knickzones (Seidl and Dietrich, 1992; Crosby et al., 2005; Wobus et al., 2006; Crosby and Whipple, 2006; Haya kawa and Oguchi, 2009). A knickzone is a locally high-gradient reach between lower gradient reaches (Hayakawa and Oguchi, 2006, 2009). The knickpoint is the distinct infl ection point between a knickzone and an upstream, lower gradient reach (Seidl and Dietrich, 1992; Wobus et al., 2006; Crosby and Whipple, 2006). In addition to transient knickpoints, there are also stationary knickpoints, where an erosionally resistant substrate in the channel locally impedes incision. We believe that a prominent knickpoint present on a trunk stream has propagated past multiple tributary junctions, initiating tributary response to a rapid drop in base level at each tributary confl uence. The tributary response is manifest as knickpoints in the longitudinal profi les of these tributary basins. This study documents propagation of knickpoints into tributary basins. In the past such documentation required exhaustive fi eld surveys; however, with the recent availability of LIDAR (light detection and ranging) data from which 1-m-resolution digital elevation models (DEMs) are produced, we can now observe detailed channel-incision response to base-level lowering. We select an ideal fi eld setting for such an opportunity and utilize LIDAR data in two adjacent, similarsized tributaries to document recent knickpoints and knickzones as well as geomorphic evidence of older instances of base-level lowering. Our objectives were to compile an inventory of knickpoints on two tributaries and associated subtributaries of the South Fork Eel River in northern California, United States (Figs. 1 and 2) using LIDAR-derived DEMs, and then determine whether major knickpoints in the tributaries are correlative with the major knickpoint in the trunk stream. We discuss, based on knickpoint attributes and correlation, knickpoint formation and propagation.