Aquatic and riparian ecosystem recovery from debris flows in two western Washington streams, USA

Abstract

An exceptionally powerful storm struck southwestern Washington in December 2007 causing large debris flows in two adjacent streams. The two affected streams had been studied prior to the storm, providing a rare opportunity to examine ecosystem recovery. We monitored the streams and their riparian zones for six years after the disturbances to determine whether recovery rates of biota, physical habitat, and water temperature differed, and if so, what factors affected resilience. Along both streams, the debris flows removed wide swaths of soil, rock, and coniferous riparian forests, widening the active channel and increasing solar exposure and summer water temperatures. Initially depauperate of vegetation, after four years red alder trees dominated the riparian plant communities. The warmer water, greater solar radiation, and unstable substrates likely contributed to variable benthic insect and tailed frog tadpole densities over time, although benthic insect communities became more similar after three years. The debris flows also decreased channel slopes and removed channel step barriers such that cutthroat trout were able to rapidly occupy habitats far upstream, but sculpins were slower to recolonize and both fish species exhibited some differences in recovery between the two streams. Crayfish were severely impacted by the debris flows; this may be due to attributes of their life history and the timing of the flows. Overall, we found that recolonizing aquatic species exhibited varying levels of resilience and recovery after the disturbances being related to the influence of physical habitat conditions, species dispersal ability, and the presence of nearby source populations.

Aquatic Biological Diversity Responses to Flood Disturbance and Forest Management in Small, Forested Watersheds

“…Despite these initial differences, invertebrate community richness by 2016 was similar across disturbance categories, suggesting a substantial recovery of the macroinvertebrate assemblage towards pre-storm conditions within eight years, even at the highly disturbed D reaches. This timeline is consistent with what has been reported elsewhere [58,59,64].…”

Variation in riparian and stream assemblages across the primary succession landscape of Mount St. Helens, U.S.A.

“…On mudflow deposits in a stream just east of MSH undergoing secondary succession, 18 years of riparian development showed early dominance by red alder ( Alnus rubra ), followed by recruitment into gaps by western hemlock ( Tsuga heterophylla ; Weber et al., 2006). Elsewhere in western WA, two forested streams impacted by catastrophic debris flows (not of volcanic origin) showed red alder with over 50% canopy cover after only 3 years of secondary succession (Foster et al., 2020). In contrast, young riparian zones on the Pumice Plain 36 years after the eruption had only occasional canopy closure (six of the 21 sites had canopy cover ≥50%), and the dominant canopy species were Sitka alder and Sitka willow, with few red alders, and no evidence of recruitment by western hemlock.…”

“…Benda et al., 2005; Fisher, 1983; Lytle & Poff, 2004; Poff et al., 1997). However, even major disturbances in streams in most cases do not devastate entire ecosystems; the physical channel usually remains and recolonisation is often rapid (Bellmore et al., 2019; Foster et al., 2020; Grimm & Fisher, 1989). Most natural disturbances in streams occur within established channels (Resh et al., 1988) and often have short‐lived effects, with biotic recovery driven by recolonisation from undisturbed tributaries, upstream reaches, or hyporheic refuges (Blum, 1956; Parkyn & Smith, 2011; Vander Vorste et al., 2015).…”

Rapid colonisation post‐displacement contributes to native fish resilience

“…Fish may require years to recover following large‐scale disturbances (e.g. mining, logging, debris flows or channelisation) that induce long‐term changes to physical habitat (Detenbeck et al, 1992; Foster et al, 2020; Niemi et al, 1990). For example, in 2012 a pipeline spill occurred in Dry Piney Drainage and mottled sculpin were absent at affected sites for 4 years following the spill event.…”