A process-based assessment of landscape change and salmon habitat losses in the Chehalis River basin, USA

Abstract

Identifying necessary stream and watershed restoration actions requires quantifying natural potential habitat conditions to diagnose habitat change and evaluate restoration potential. We used three general methods of quantifying natural potential: historical maps and survey notes, contemporary reference sites, and models. Historical information was available only for the floodplain habitat analysis. We used contemporary reference sites to estimate natural potential habitat conditions for wood abundance, riparian shade, main channel length, and side channel length. For fine sediment, temperature, and beaver ponds we relied on models. We estimated a 90% loss of potential beaver pond area, 91% loss of side-channel length, and 92% loss or degradation of floodplain marshes and ponds. Spawning habitat area change due to wood loss ranged from -23% to -68% across subbasins. Other changes in habitat quantity or quality were smaller—either in magnitude or spatial extent—including rearing habitat areas, stream temperature, and accessible stream length. Historical floodplain habitat mapping provided the highest spatial resolution and certainty in locations and amounts of floodplain habitat lost or degraded, whereas use of the contemporary reference information provided less site specificity for wood abundance and side-channel length change. The models for fine sediment levels and beaver pond areas have the lowest reach-specific certainty, whereas the model of temperature change has higher certainty because it is based on a detailed riparian inventory. Despite uncertainties at the reach level, confidence in subbasin-level estimates of habitat change is moderate to high because accuracy increases as data are aggregated over multiple reaches. Our results show that the largest habitat losses were floodplain and beaver pond habitats, but use of these habitat change results in salmon life-cycle models can illustrate how the potential benefits of alternative habitat restoration actions varies among species with differing habitat preferences.

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

“…where ΔT is the change in temperature in degrees Celsius and Δθ is the change in canopy opening angle (Beechie et al, 2021). We used two different metrics for T in the model, the 7-day average daily maximum temperature (7-DADM), which affects juvenile summer rearing and spring-run Chinook adult holding, and the June 1-21 average daily maximum (Jun1-21 ADM), which affects the late portion of the juvenile Chinook outmigration (Jorgensen et al, 2021).…”

“…Changes in forest road density alter percent fine sediment (<0.85 mm) in spawning gravels. Percent fine sediment is predicted by a two-stage model in which reaches with low shear stress index (reach slope × bankfull width ≤0.05) have consistently high fine sediment, and reaches with high shear stress index (>0.05) have fine sediment level predicted by changes in forest road density (Beechie et al, 2021).…”

“…The geospatial analysis focuses on estimating habitat area and quality for both current and natural potential conditions, and the difference between the two is the "restoration potential." Current conditions are estimated from recent empirical data for habitat area or quality for each 200-m-long reach in the stream network (Table 2) (Beechie et al, 2021). For the Chehalis River basin, some data sets have extensive coverage with reach-level resolution (e.g., shade, floodplain habitat), and confidence in reach-level values is relatively high.…”

“…Other data sets are extrapolated from a subsample of reaches (e.g., habitat surveys, stream temperature, fine sediment), and for those attributes reach-level confidence is lower. Natural potential conditions are estimated from historical data, contemporary reference sites, or models (Beechie et al, 2021). As with current conditions, reach-level confidence is higher for those attributes with extensive coverage and lower for attributes that are extrapolated from a subsample of sites.…”

“…Wood augmentation can increase both habitat area and quality. In small streams (≤20-m bankfull width), areas of spawning gravel and rearing pools increase as wood abundance increases, using a linear scaling between current conditions (low wood abundance) and historical conditions (high wood abundance) (Beechie et al, 2021). In large rivers (>20-m bankfull width), wood augmentation increases spawning gravel area and rearing habitat quality in edge habitats.…”