Geology and geomorphology control suspended sediment yield and modulate increases following timber harvest in temperate headwater streams

Abstract

a b s t r a c tSuspended sediment transport is an important contributor to ecologic and geomorphic functions of streams. However, it is challenging to generalize predictions of sediment yield because it is influenced by many factors. In this study, we quantified the relevance of natural controls (e.g., geology, catchment physiography) on suspended sediment yield (SSY) in headwater streams managed for timber harvest. We collected and analyzed six years of data from 10 sites (five headwater sub-catchments and five watershed outlets) in the Trask River Watershed (western Oregon, United States). We used generalized least squares regression models to investigate how the parameters of the SSY rating curve varied as a function of catchment setting, and whether the setting modulated the SSY response to forest harvesting. Results indicated that the highest intercepts (a) of the power relation between unit discharge and SSY were associated with sites underlain primarily by friable rocks (e.g., sedimentary formations). The greatest increases in SSY after forest harvesting (up to an order of magnitude) also occurred at sites underlain by the more friable lithologies. In contrast, basins underlain by resistant lithologies (intrusive rocks) had lower SSY and were more resilient to management-related increases in SSY. As such, the impact of forest management activities (e.g., use of forested buffers; building of new roads) on the variability in SSY was primarily contingent on catchment lithology. Sites with higher SSY, or harvest-related increases in SSY, also generally had a) lower mean elevation and slope, b) greater landscape roughness, and c) lower sediment connectivity. We used principal component analysis (PCA) to further explore the relationship between SSY and several basin physiographic variables. The PCA clearly separated sites underlain by friable geologic units from those underlain by resistant lithologies. Results are consistent with greater rates of weathering and supply of sediment to headwater streams in catchments with more friable lithologies, and limited sediment supply in catchments underlain by resistant lithologies. We hypothesize that a similar framework may aid in predicting the overall SSY of a catchment as well as its susceptibility to increases in SSY following forest harvesting.

Relative Influence of Landscape Variables and Discharge on Suspended Sediment Yields in Temperate Mountain Catchments

“…The standard deviation of slope was furthermore strongly correlated with standard deviation of TPI—a roughness metric. Bywater‐Reyes et al () found similar results in watersheds located in the Oregon Coast Range (PNW, USA) with suspended sediment yield varying as a function of variability in slope and a terrain roughness metric similar to standard deviation of TPI used here. Bywater‐Reyes et al () argued that the association between yields and landscape roughness metrics is likely a result of roughness being a proxy for sediment supply.…”

“…Bywater‐Reyes et al () found similar results in watersheds located in the Oregon Coast Range (PNW, USA) with suspended sediment yield varying as a function of variability in slope and a terrain roughness metric similar to standard deviation of TPI used here. Bywater‐Reyes et al () argued that the association between yields and landscape roughness metrics is likely a result of roughness being a proxy for sediment supply. Bywater‐Reyes et al () found a peculiar association between high index of sediment connectivity and lower suspended sediment yields.…”

“…Bywater‐Reyes et al () argued that the association between yields and landscape roughness metrics is likely a result of roughness being a proxy for sediment supply. Bywater‐Reyes et al () found a peculiar association between high index of sediment connectivity and lower suspended sediment yields. Here some highly connected watersheds (e.g., WS01, WS02, and WS03) had high sediment yields, whereas for others there was no consistent relationship between the index of connectivity and sediment yields.…”

“…Physiographic characteristics included (slope, elevation, roughness, and index of sediment connectivity) for each watershed from a high‐resolution light detection and ranging (LiDAR)‐derived (1 m) digital elevation model (Cavalli et al, ; Crema et al, ; Jenness et al, ; Spies, ). We selected these physiographic variables because they have previously been used to explain differences in suspended sediment yields across catchments in western Oregon (Bywater‐Reyes et al, ). Briefly, relative elevation describes the shape of each watershed (hypsometry).…”

“…In many cases, changes in forest practices that include smaller harvest areas and retention of riparian buffers have reduced the relationship between forest harvesting and increased suspended sediment yields compared to historical practices (Klein et al, ; Reiter et al, ; Terrell et al, ; Warrick et al, ). However, contemporary practices have not always been effective—others have continued to observe elevated suspended sediment after forest harvesting, even with the use of contemporary best management practices (Arthur et al, ; Bywater‐Reyes et al, ; Wear et al, ).…”