Effective Hydrological Events in an Evolving Mid‐latitude Mountain River System Following Cataclysmic Disturbance—A Saga of Multiple Influences

Abstract

Cataclysmic eruption of Mount St. Helens (USA) in 1980 reset 30 km of upper North Fork Toutle River (NFTR) valley to a zero‐state fluvial condition. Consequently, a new channel system evolved. Initially, a range of streamflows eroded channels (tens of meters incision, hundreds of meters widening) and transported immense sediment loads. Now, single, large‐magnitude, or multiple moderate‐magnitude events are needed to accomplish substantial channel modification. Three large floods (two ≥100‐year events; one ∼10–25‐year event along lower Toutle River) from 1996 to 2015 indicate flood effectiveness in this environment is affected by both geomorphic and environmental factors. The largest and smallest of these floods (February 1996, November 2006) transported the most sediment by single floods since 1982; erosion and sediment transport by an ∼100‐year flood in December 2015 was not exceptional. Strong coupling between NFTR and its tall corridor banks, local geologic and hydraulic conditions promoting threshold erosion, event sequencing, and possibly a longitudinal gradient in stream power are important factors affecting event effectiveness on channel modification. In addition, environmental factors have also been influential, as variations in snowpack, storm trajectories and rainfall distributions, and episodic mobilization of debris flows have also influenced geomorphic response. Other factors such as vegetation anchoring, strong channel–hillside coupling, disparities between flood frequencies and perturbation relaxation times, and large variations in flood duration do not appear to be critical influences. Climate forecasts for warmer temperatures and a shift from snowfall to rainfall at high elevations may promote further acute geomorphic responses.

Stream power indices correspond poorly with observations of alluvial river channel adjustment

“…Further, it is not just the magnitude of peak flows that is important for channel adjustment during a flood event. The duration (Costa & O'Connor, 2011; Gervasi et al, 2021) and sequencing (Eagle et al, 2021; Major et al, 2021) of flood events are also of critical importance. Not accounting for the impact of either bed material characteristics or the full flow regime limits the ability of stream power indices to predict sediment output.…”

Channel Response and Reservoir Delta Evolution From Source to Sink Following an Extreme Flood

“…Large sedimentation events in fluvial systems occur naturally through disturbances including floods (e.g., Lisle, 1982; Magilligan et al., 2015), wildfires (e.g., Meyer et al., 2001; Moody & Martin, 2001), mass movements (e.g., Madej & Ozaki, 2009; Rathburn et al., 2017), and volcanic eruptions (e.g., Gran & Montgomery, 2005; Major et al., 2021). Extreme sediment influx can also be anthropogenically induced via land cover changes (Church & Ferguson, 2015), gravel mining (Church & Ferguson, 2015; Simon & Rinaldi, 2006), sediment releases from dams (Scott & Gravlee, 1968; Wohl & Cenderelli, 2000), and dam removal (East et al., 2015; Magilligan et al., 2016).…”

“…Second, narratives or conceptual models of post‐disturbance rivers are common and include pathways of response controlled by hydraulic variables such as shear stress relative to incipient motion (Rathburn & Wohl, 2003), bankfull Shields criteria compared to Shields criteria (Fields et al., 2021), a response progression from lahar to fluvial to ecological response (Gran & Montgomery, 2005), response that leads to long‐term sediment persistence (Grimsley et al., 2016; Moody & Martin, 2001), or revegetation of flood sediments by native species (Costa, 1974) or encroachment of upland species (Schook et al., 2017) that stabilize the landscape. Finally, most post‐disturbance studies span 1–5 years (Dadson et al., 2004; Moody & Martin, 2001; Nelson & Dubé, 2016; Pitlick, 1993), with many fewer studies over an intermediate time scale of 6–10 years (e.g., 8 years in Gran & Montgomery, 2005), and even fewer multi‐decadal, post‐disturbance data sets (Moody & Meade, 2018; Major et al., 2000, 2021). As a result, we are even less certain of the longer‐term response of fluvial systems to disturbances, and the need for broadly applicable potential pathways of response (Phillips & Van Dyke, 2016) is ever more salient.…”

Multi‐decadal changes in the relationships between rainfall characteristics and debris‐flow occurrences in response to gully evolution after the 1990–1995 Mount Unzen eruptions

“…The range and direction of volcanic flows and tephra falls vary depending on both the magnitude of eruption activity and the prevailing weather conditions, leading to differences in the areal extent and severity of disturbance (e.g., Ayris & Delmelle, 2012). Consequently, the associated sediment discharge can persist with different duration and rate in disturbed watersheds (e.g., Gran et al, 2011; Gran & Montgomery, 2005; Lavigne, 2004; Major, 2004; Major et al, 2016, 2021). Thus, it is important to analyze long‐term sediment discharge following eruptions to enable prediction of post‐eruptive sediment dynamics.…”

“…Following the cessation of eruptions, the evolution of landscapes consisting of incised gullies can persist over long timescales (e.g., Lavigne & Thouret, 2003;Zheng et al, 2014). Typical geometric changes in channels such as volcanic gullies include vertical adjustment (in response to channel incision and aggradation resulting from fluvial processes around the thalweg) and lateral adjustment (due to lateral channel migration and erosional processes including mass slumping of banks) (e.g., Malatesta et al, 2017;Major et al, 2019Major et al, , 2021. Lateral adjustment progresses with a nonlinear pattern over time (Major et al, 2019).…”