Variable hillslope‐channel coupling and channel characteristics of forested mountain streams in glaciated landscapes

Abstract

Channel morphology of forested, mountain streams in glaciated landscapes is regulated by a complex suite of processes, and remains difficult to predict. Here, we analyze models of channel geometry against a comprehensive field dataset collected in two previously glaciated basins in Haida Gwaii, B.C., to explore the influence of variable hillslope–channel coupling imposed by the glacial legacy on channel form. Our objective is to better understand the relation between hillslope–channel coupling and stream character within glaciated basins. We find that the glacial legacy on landscape structure is characterized by relatively large spatial variation in hillslope–channel coupling. Spatial differences in coupling influence the frequency and magnitude of coarse sediment and woody material delivery to the channel network. Analyses using a model for channel gradient and multiple models for width and depth show that hillslope–channel coupling and high wood loading induce deviations from standard downstream predictions for all three variables in the study basins. Examination of model residuals using Boosted Regression Trees and nine additional channel variables indicates that ~10 to ~40% of residual variance can be explained by logjam variables, ~15–40% by the degree of hillslope–channel coupling, and 10–20% by proximity to slope failures. These results indicate that channel classification systems incorporating hillslope–channel coupling, and, indirectly, the catchment glacial legacy, may present a more complete understanding of mountain channels. From these results, we propose a conceptual framework which describes the linkages between landscape history, hillslope–channel coupling, and channel form. © 2018 John Wiley & Sons, Ltd.

Spatial and temporal patterns of sediment storage over 45 years in Carnation Creek, BC, a previously glaciated mountain catchment

“…Slope failures have been common within the catchment and are a major source of sediment to the stream channel (Zimmermann et al, 2004). As with many glaciated basins in this region (e.g., Brardinoni and Hassan, 2007;Hassan et al, 2018), the catchment profile displays a stepped structure (Figure 1b), with an alternating sequence of steep and shallow gradients. As a result, the channel contains multiple depositional zones along its length and varying degrees of hillslope-channel coupling.…”

“…This second catchment contains characteristics found widely throughout deglaciated regions (e.g., Beylich and Laute, ; Hassan et al. ).…”

“…This can be dictated by the specific location of mass movement or steep tributary input (e.g., Owczarek, ), which can be influenced by a catchment's glacial history (Hassan et al. ), or by other channel features, such as logjams, should sediment be remobilized and deposited farther downstream (Sutherland et al. ).…”

Does variable channel morphology lead to dynamic salmon habitat?

“…Suitable hydraulic conditions are essential for allowing juveniles access to resources, such as low velocity pool areas (Bjornn and Reiser, 1991;Beecher et al, 2002) which are sufficiently deep to allow for passage, foraging and cover (Nickelson and Reisenbichler, 1977;Bjornn and Reiser, 1991;Beecher et al, 2002). Previous research highlights the connection between channel morphology and topography and larger-scale sediment supply dynamics, such as those imposed by channel-hillslope coupling (Hoffman and Gabet, 2007;Hassan et al, 2019). For a given river discharge, hydraulic conditions forming habitat for juvenile salmonids are shaped by a channel's topographic structure and LW (Abbe and Montgomery, 1996;Earth Surf.…”

“…For a given valley gradient, a range of channel morphologies is possible as a function of sediment supply and calibre (Montgomery, 1997;Church, 2006;Hassan et al, 2008). In many regions, such as northwestern North America, historical glaciation, deep soils, and steep hillslopes result in large and episodic delivery of sediment and wood to stream channels (Roberts and Church, 1986;Imaizumi and Sidle, 2007;Hassan et al, 2019), which commonly leads to spatially and temporally changeable channel morphology (Hoffman and Gabet, 2007), often of relevance for salmonid habitat (Hogan, 1986). Changes in sediment loads from debris flows are common in mountain landscapes, and have also been shown to influence bar and pool characteristics downstream (Madej and Ozaki, 1996;Hoffman and Gabet, 2007;Pryor et al, 2011).…”