Streambed scour of salmon spawning habitat in a regulated river influenced by management of peak discharge

Abstract

In the Pacific Northwest of the United States, salmon eggs incubating within streambed gravels are susceptible to scour during floods. The threat to egg‐to‐fry survival by streambed scour is mitigated, in part, by the adaptation of salmon to bury their eggs below the typical depth of scour. In regulated rivers globally, we suggest that water managers consider the effect of dam operations on scour and its impacts on species dependent on benthic habitats. We instrumented salmon‐spawning habitat with accelerometer scour monitors (ASMs) at 73 locations in 11 reaches of the Cedar River in western Washington State of the United States from Autumn 2013 through the Spring of 2014. The timing of scour was related to the discharge measured at a nearby gage and compared to previously published ASM data at 26 locations in two reaches of the Cedar River collected between Autumn 2010 and Spring 2011. Thirteen percent of the recovered ASMs recorded scour during a peak‐discharge event in March 2014 (2‐to 3‐year recurrence interval) compared to 71% of the recovered ASMs during a higher peak‐discharge event in January 2011 (10‐year recurrence interval). Of the 23 locations where ASMs recorded scour during the 2011 and 2014 deployments, 35% had scour when the discharge was ≤87.3 m3/s (3,082 ft3/s) (2‐year recurrence interval discharge) with 13% recording scour at or below the 62.3 m3/s (2,200 ft3/s) operational threshold for peak‐discharge management during the incubation of salmon eggs. Scour to the depth of salmon egg pockets was limited during peak discharges with frequent (1.25‐year or less) recurrence intervals, which managers can regulate through dam operations on the Cedar River. Pairing novel measurements of the timing of streambed scour with discharge data allows the development of peak‐discharge management strategies that protect salmon eggs incubating within streambed gravels during floods.

Recent advances in environmental flows science and water management—Innovation in the Anthropocene

“…ELOHA, DRIFT—Downstream Response to Imposed Flow Transformation, King, Brown, & Sabet, ) recognise the need to understand geomorphic, habitat and other sources of variation that generate the environmental context for ecological responses to flow variability and environmental flow strategies (Poff et al., ; Reidy Liermann et al., ). For example, peak discharges from dams can be managed to minimise streambed scour and protect salmon eggs incubating within streambed gravels during floods (Gendaszek, Burton, Magirl, & Konrad, ). Mierau et al.…”

“…Downes, ; Olden & Naiman, ). Several papers in this special issue illustrate how the influence of flow on hydraulic processes can be linked to desirable ecological outcomes (Gendaszek et al., ; Mierau et al., ). A more process‐based (“mechanistic”) understanding of such relationships is needed, integrated with the influence of limiting factors and stressors unrelated or indirectly related to flow (Nestler, Stewardson, Gilvear, Webb, & Smith, ; Palmer et al., ; Poff, ). Improve understanding of spatial variability and cumulative catchment effects: Relatively, little work has been carried out on cumulative and interactive effects of flow regime alterations and the loss of spatial connectivity at various positions in the catchment (e.g.…”

“…Downes, 2010;Olden & Naiman, 2010). Several papers in this special issue illustrate how the influence of flow on hydraulic processes can be linked to desirable ecological outcomes (Gendaszek et al, 2018;Mierau et al, 2018). A more process-based ("mechanistic") understanding of such relationships is needed, integrated with the influence of limiting factors and stressors unrelated or indirectly related to flow (Nestler, Stewardson, Gilvear, Webb, & Smith, 2016;Palmer et al, 2010;Poff, 2018).…”

Evaluating and managing environmental water regimes in a water‐scarce and uncertain future

“…Research and case studies that evaluate the effectiveness of environmental water programmes at achieving their desired hydrological and ecological objectives or evaluating complex scenarios with multiple interacting stressors. Papers in this theme illustrate successes, challenges and provide evaluations of how well current programmes have worked (Stewardson & Guarino, 2018); how well conceptual models have performed relative to expectations (Gendaszek, Burton, Magirl, & Konrad, 2018);…”

“…Research and case studies that evaluate the effectiveness of environmental water programmes at achieving their desired hydrological and ecological objectives or evaluating complex scenarios with multiple interacting stressors. Papers in this theme illustrate successes, challenges and provide evaluations of how well current programmes have worked (Stewardson & Guarino, ); how well conceptual models have performed relative to expectations (Gendaszek, Burton, Magirl, & Konrad, ); the influence of drought on flow–ecology relationships (Lynch, Leasure, & Magoulick, ); and recommendations for ways to improve both the science and implementation of environmental water practice (Wheeler, Wenger, & Freeman, ). This theme also includes recommendations for refinements that can improve the ability of flow–ecology tools to distinguish anthropogenic effects from changes due to climate variability (Hain et al., ).…”