How riparian and floodplain restoration modify the effects of increasing temperature on adult salmon spawner abundance in the Chehalis River, WA

Abstract

Stream temperatures in the Pacific Northwest are projected to increase with climate change, placing additional stress on cold-water salmonids. We modeled the potential impact of increased stream temperatures on four anadromous salmonid populations in the Chehalis River Basin (spring-run and fall-run Chinook salmon Oncorhynchus tshawytscha, coho salmon O. kisutch, and steelhead O. mykiss), as well as the potential for floodplain reconnection and stream shade restoration to offset the effects of future temperature increases. In the Chehalis River Basin, peak summer stream temperatures are predicted to increase by as much as 3°C by late-century, but restoration actions can locally decrease temperatures by as much as 6°C. On average, however, basin-wide average stream temperatures are expected to increase because most reaches have low temperature reduction potential for either restoration action relative to climate change. Results from the life cycle models indicated that, without restoration actions, increased summer temperatures are likely to produce significant declines in spawner abundance by late-century for coho (-29%), steelhead (-34%), and spring-run Chinook salmon (-95%), and smaller decreases for fall-run Chinook salmon (-17%). Restoration actions reduced these declines in all cases, although model results suggest that temperature restoration alone may not fully mitigate effects of future temperature increases. Notably, floodplain reconnection provided a greater benefit than riparian restoration for steelhead and both Chinook salmon populations, but riparian restoration provided a greater benefit for coho. This pattern emerged because coho salmon tend to spawn and rear in smaller streams where shade restoration has a larger effect on stream temperature, whereas Chinook and steelhead tend to occupy larger rivers where temperatures are more influenced by floodplain connectivity. Spring-run Chinook salmon are the only population for which peak temperatures affect adult prespawn survival in addition to rearing survival, making them the most sensitive species to increasing stream temperatures.

How does habitat restoration influence resilience of salmon populations to climate change?

“…Increasing temperature generally had larger effects on each species/run by late century, ranging from −12% for fall-run Chinook salmon to −87% for spring-run Chinook salmon (Table 1) (Fogel et al, 2022). The extent of salmon habitat restoration in the Chehalis River basin to date has been relatively small compared with the scope of habitat loss and degradation, but restoration effort has been increasing due to a new focus on habitat restoration in the basin under the Chehalis Basin Strategy, initiated in 2017.…”

“…Small-stream spawning habitat (channels ≤20-m bankfull width) Estimated from data relating wood abundance to spawning gravel area (Beechie et al, 2021) Small-stream rearing habitat Extrapolated from 339 reach surveys, stratified by channel slope and adjacent land cover (Beechie et al, 2021) Beaver ponds Current pond area estimated from recent surveys; historical pond area modeled based on stream power (Pollock et al, 2004) Large river spawning habitat (channels >20-m bankfull width) Digitized from aerial imagery; modified by wood abundance for historical condition (Beechie et al, 2021) Large river rearing habitat Digitized from aerial imagery; modified by wood abundance and bank armor removal for historical condition (Beechie et al, 2021) Floodplain habitat Current condition from National Hydrography Dataset Plus (NHD+); historical condition from General Land Office surveys in the late 1800s and reference site data (side channels) (Beechie et al, 2021) Migration barriers Modified from Washington Department of Fish and Wildlife barrier database (Beechie et al, 2021) Bank armor Digitized from aerial imagery; armor removed for historical condition (Beechie et al, 2021) Riparian shade Calculated from lidar and aerial imagery inventory of riparian tree heights (Fogel et al, 2022;Seixas et al, 2018) Stream temperature Modeled from stream temperature loggers distributed across the basin; modified by shade and floodplain connectivity (Fogel et al, 2022) Fine sediment Modeled based on shear stress index and forest road density (Beechie et al, 2021) Increasing abundance of beaver dams increases pond area in small streams, and also reduces the area of pools and riffles where beaver ponds inundate free-flowing stream reaches. Floodplain reconnection increases the area of floodplain ponds, marshes, and side channels and decreases stream temperature via hyporheic exchange.…”

“…In this paper, we compare the potential effectiveness of these strategies for three species of Pacific salmonids with four different life histories in the Chehalis River basin of the Pacific Northwest, USA. We use the Habitat Assessment and Restoration Planning (HARP) model (Beechie et al, 2021; Jorgensen et al, 2021), which was designed to evaluate the effects of alternative habitat restoration actions and climate change on salmonid populations (Fogel et al, 2022; Jorgensen et al, 2021; Nicol et al, 2022). The HARP model quantifies degradation and potential improvement in habitat conditions, translates environmental or habitat conditions into life stage parameters for each species, and uses salmon life cycle models to evaluate change in population size as a function of modeled habitat changes (Beechie et al, 2021).…”

“…The HARP model quantifies degradation and potential improvement in habitat conditions, translates environmental or habitat conditions into life stage parameters for each species, and uses salmon life cycle models to evaluate change in population size as a function of modeled habitat changes (Beechie et al, 2021). Previously we used the HARP model to identify restoration action types with the greatest potential to increase salmonid abundance under current climate conditions (Beechie et al, 2021; Jorgensen et al, 2021), and to model the individual effects of future increases in flood flow and stream temperature (Fogel et al, 2022; Nicol et al, 2022). In this paper, we add a novel set of analyses that compares the potential for individual habitat restoration action types and combinations of actions to increase resilience of the four salmonid species/runs to the combined climate change effects of increasing flood flow, decreasing low flow, and increasing stream temperature.…”

“…Steeper slopes indicate greater sensitivity per 1% increment of change, and greater total magnitude of change indicates greater restoration potential. Model uncertainties are briefly discussed in Appendix S2, and additional details on uncertainties are in prior HARP model publications(Beechie et al, 2021;Fogel et al, 2022;Jorgensen et al, 2021;Nicol et al, 2022).…”