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

Abstract

Suspended sediment is an important regulator of stream habitat quality but notoriously difficult to predict and regulate. This difficulty arises because of high natural variability in suspended sediment yield in space and time. Here we quantified associations between suspended sediment yields and discharge, watershed setting (i.e., physiography and lithology), and disturbance history for 10 temperate mountain watersheds (8.5–6,242 ha) in the U.S. Pacific Northwest (H.J. Andrews Long‐Term Ecological Research, LTER) over an ~60‐year period. Annual suspended sediment yields varied almost 4 orders of magnitude across space and time. A linear mixed effects model indicated that much of the variation in yields could be explained by the random effect of site (conditional R2 = 0.74) with additional variation explained by the fixed effects (marginal R2 = 0.67) of cumulative annual discharge (p < 0.001) and the variability (standard deviation) of watershed slope (p < 0.001). Two annual sediment yield data points were model outliers, that each occurred within a decade after forest management activities and a large‐magnitude storm event at sites with high variability of catchment slope. Other sites had low sediment yields for a range of conditions, including management or flood disturbance. Taken together, our study shows that watersheds with high slope variability have higher suspended sediment yields and may be more vulnerable to increases in sediment yields following disturbances.

Thirty years of tephra erosion following the 1980 eruption of Mount St. Helens

“…These rates are within the range of suspended sediment yields measured from forested watersheds lacking active glaciers in the Cascade Range of southwestern Washington and northwestern Oregon (Czuba et al, 2011;Wise and O'Connor, 2016;Bywater-Reyes et al, 2018), and is similar to the conclusion of Major et al (2018) that sediment yield from the Green River basin declined to pre-eruption levels within about 5 years. These rates are within the range of suspended sediment yields measured from forested watersheds lacking active glaciers in the Cascade Range of southwestern Washington and northwestern Oregon (Czuba et al, 2011;Wise and O'Connor, 2016;Bywater-Reyes et al, 2018), and is similar to the conclusion of Major et al (2018) that sediment yield from the Green River basin declined to pre-eruption levels within about 5 years.…”

“…Within about 6 years after the 1980 eruption, the catchmentscale sediment flux dropped into the range of 10-250 t km À2 year À1 (assuming a soil bulk density of 1000 kg m À3 , this is equivalent to hillslope lowering rates of 0.01-0.25 mm year À1 ). These rates are within the range of suspended sediment yields measured from forested watersheds lacking active glaciers in the Cascade Range of southwestern Washington and northwestern Oregon (Czuba et al, 2011;Wise and O'Connor, 2016;Bywater-Reyes et al, 2018), and is similar to the conclusion of Major et al (2018) that sediment yield from the Green River basin declined to pre-eruption levels within about 5 years. By that time, tephra erosion rates were approaching the limits of measurement ( Figure 8A, Table III), suggesting that the contribution of tephra erosion to streams had declined to the background level of the surrounding undisturbed landscape and that the 1980 tephra had entered the sediment supply system driven by fast and slow mass wasting.…”

Geomorphic and Sedimentary Effects of Modern Climate Change: Current and Anticipated Future Conditions in the Western United States

“…Knight & Harrison, 2014), which dominate the record for a decade or more thereafter, obscuring any possible long‐term trend. For example, ~60 years of suspended‐sediment records beginning in the 1950s from 10 mountain watersheds in Oregon showed that each catchment's sediment yield varied over three orders of magnitude (10 0 to 10 3 t/km 2 ), with decadal‐scale increases due to a large storm and to forest management, but no long‐term trends (Bywater‐Reyes et al, 2018). Many western U.S. watersheds with sediment‐flux data have a history of logging and associated dirt‐road use, which elevated fluxes for decades (e.g., Madej & Ozaki, 1996; Reid & Dunne, 1984).…”

Effects of forestry on summertime low flows and physical fish habitat in snowmelt‐dominant headwater catchments of the Pacific Northwest

“…Although many previous studies have documented how forestry affects streamflow and water quality (e.g., Bladon, Segura, Cook, Bywater‐Reyes, & Reiter, ; Bywater‐Reyes, Bladon, & Segura, ; Gomi, Moore, & Hassan, ; Moore, Spittlehouse, & Story, ; Moore & Wondzell, ; Perry & Jones, ), EFN assessments have historically been concerned with regulation and consumptive use of water resources, such as power generation and agriculture, and have neglected to consider the impacts of forestry on the natural flow regime (e.g., Lewis, Hatfield, Chilibeck, & Roberts, ). In the context of EFN, the impacts of forest harvesting on summertime low flows are of particular interest.…”