ABSTRACT:

Wide, low-gradient segments within river networks (i.e., beads) play a critical role in absorbing and morphologically adapting to disturbances, including wildfires and debris flows. However, the magnitude and rate of morphological adjustment and subsequent hydraulic conditions provided by beads compared to pre-disturbance conditions are not well understood. This study analyzed trajectories of river morphology, flood attenuation, and fish habitat following the 2020 Cameron Peak Fire and July 2022 debris flow and flood at Little Beaver Creek, Colorado, USA. Using repeat aerial imagery, ground-based surveys, and hydrodynamic modeling, we assessed morphological changes in a 600-m-long bead of Little Beaver Creek. We used remotely sensed imagery for pre- and post-disturbance geomorphic metrics in rates of floodplain destruction and formation, changes in channel width, and channel migration. Metrics of floodplain destruction and formation and channel migration greatly increased in magnitude after the first post-fire runoff season but returned to the historical range of these metrics three years after the fire. The 2022 flood deposited sediment, infilled side channels, reduced pool area, and increased the area of bars and islands. The assessed functions of the system did not show clear improvement or impairment despite more rapid changes in system geometry, geomorphic unit abundance, and geomorphic unit location. The ability of the site to attenuate peak flows changed minimally and inconsistently over the studied floods. Various lotic habitat conditions changed—namely a reduction in floodplain access and deepening of certain pools—but the overall flow-type diversity of the system was not largely impacted. The resilience of the active channel of Little Beaver Creek to the fire and flood disturbances while retaining key services demonstrates the importance of river beads for enhancing river-floodplain resilience to large disturbance events and highlights river beads as key areas for preservation and restoration.

ABSTRACT:

Low-tech river habitat restoration techniques, including Stage 0 treatments, are increasingly applied but often lack robust evaluation of their effects and benefits. In 2018, one kilometer of the South Fork McKenzie (SFMK) River OR was modified to Stage 0 conditions for the benefit of ESA-listed Chinook salmon by raising the incised channel to the geomorphic grade line, reconnecting relic side channels, increasing floodplain connectivity, and distributing large wood throughout the reach. Field observations indicated depths and velocities were lower at the treated site than at the untreated site. To understand how these changes in physical habitat may translate into changes in juvenile Chinook length, abundance, and emigration timing, this study combined field observations with the individual-based model inSALMO. Model results indicated the new habitat conditions can produce longer outmigrants, a result of rearing longer in the treated reach than the untreated reach. The treated reach also produced more outmigrants at the end of the season in dry and typical water years, a result of greater higher quality habitat conditions (lower velocities and depths, more cover) that favored fish remaining in the reach. No evidence of the treatment on spawning or incubation success was found. Numerical experimentation indicated that, under both treated and untreated conditions, outmigrant abundance was more sensitive to changes in temperature than to food resources, due to the particularly low temperatures released from an upstream dam impacting hatching success within this site. Mean outmigrant length was more sensitive to changes in food availability than temperature. Collectively, the model results demonstrated that the Stage 0 activities at SFMK may increase juvenile length and number of reared individuals, though results should be evaluated across the diverse styles of Stage 0 projects. Further, the methods reflect the utility of moving beyond species-habitat relationships as a tool in evaluating restoration practices.