Post-Disturbance Sediment Recovery: Implications for Watershed Resilience
Total Page：16
File Type：pdf, Size：1020Kb
Geomorphology 305 (2018) 61–75 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Post-disturbance sediment recovery: Implications for watershed resilience Sara L. Rathburn a,⁎, Scott M. Shahverdian b, Sandra E. Ryan c a Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA b Ecogeomorphology and Topographic Analysis Lab, Utah State University, Logan, UT 84322, USA c US Forest Service, Rocky Mountain Research Station, Fort Collins, CO 80526, USA article info abstract Article history: Sediment recovery following disturbances is a measure of the time required to attain pre-disturbance sediment Received 31 March 2017 fluxes. Insight into the controls on recovery processes and pathways builds understanding of geomorphic Received in revised form 15 August 2017 resilience. We assess post-disturbance sediment recovery in three small (1.5–100 km2), largely unaltered water- Accepted 15 August 2017 sheds within the northern Colorado Rocky Mountains affected by wildfires, floods, and debris flows. Disturbance Available online 25 August 2017 regimes span 102 (floods, debris flows) to 103 years (wildfires). For all case studies, event sediment recovery followed a nonlinear pattern: initial high sediment flux during single precipitation events or high annual snow- Keywords: fl Sediment recovery melt runoff followed by decreasing sediment uxes over time. Disturbance interactions were evaluated after a Disturbance high-severity fire within the South Fork Cache la Poudre basin was followed by an extreme flood one year Resilience post-fire. This compound disturbance hastened suspended sediment recovery to pre-fire concentrations Post-disturbance recovery 3 years after the fire. Wildfires over the last 1900 YBP in the South Fork basin indicate fire recurrence intervals of ~600 years. Debris flows within the upper Colorado River basin over the last two centuries have shifted the baseline of sediment recovery caused by anthropogenic activities that increased debris flow frequency. An ex- treme flood on North St. Vrain Creek with an impounding reservoir resulted in extreme sedimentation that led to a physical state change. We introduce an index of resilience as sediment recovery/disturbance recurrence inter- val, providing a relative comparison between sites. Sediment recovery and channel form resilience may be in- versely related because of high or low physical complexity in streams. We propose management guidelines to enhance geomorphic resilience by promoting natural processes that maintain physical complexity. Finally, sedi- ment connectivity within watersheds is an additional factor to consider when establishing restoration treatment priorities. © 2017 Elsevier B.V. All rights reserved. 1. Introduction delivery from hillslopes to channels because of the effects of increased sedimentation on aquatic ecosystems (Newcombe and Macdonald, Assessing watershed response and recovery to extreme events pro- 2011), channel and floodplain morphology and hence flooding, water re- vides insight into landscape resilience, a concept that has gained much in- source management including water quality (Moody and Martin, 2009), terest as human activities continue to fundamentally alter Earth's surface and reservoir storage (Rathburn et al., 2017). In fluvial systems, resilience (Hooke, 2000; Sanderson et al., 2002; Hooke et al., 2012). The predicted refers to the ability to tolerate or absorb perturbations without changing increase in extreme events (disturbances) accompanying climate change to a qualitatively different state that is controlled by a different set of pro- (Coumou and Rahmstorf, 2012) is anticipated to alter the resilience of cesses (Brierley and Fryirs, 2005; Tabacchi et al., 2009). As an example, a geomorphic systems and their ability to recover after disturbance. A resilient river recovers channel geometry and sediment fluxes following a working definition of disturbance is any force that tends to move a system large flood and in a broader context reflects the persistence of a river eco- far from a given equilibrium or steady state (Tabacchi et al., 2009). Distur- system and its ability to return to pre-disturbance conditions (Webster bances to rivers may include large hydrological events that promote the et al., 1975; Corenblit et al., 2009). Resilience is based on how fast a sys- erosion, transport, and deposition of large amounts of sediment or distur- tem returns to its equilibrium state after a disturbance (Holling, 1996), bances that directly affect vegetation, such as wildfire, insect/pathogen or the recovery time (sometimes referred to as relaxation time). outbreak, or drought-related dieoff that reduces hillslope stability, lowers Gaps exist in our understanding of response to disturbances because substrate cohesiveness, and increases sediment loads (Corenblit et al., of limited temporal and spatial extent of data. As such, we often misun- 2007). Of particular concern are disturbances that enhance sediment derstand the dominant processes of recovery. Assessing the historical range of variability (HRV) of systems (Wohl and Rathburn, 2013) ex- ⁎ Corresponding author. pands the temporal scale of disturbance recovery analysis. Coupling E-mail address: [email protected] (S.L. Rathburn). HRV and contemporary process measurements of recovery trajectories http://dx.doi.org/10.1016/j.geomorph.2017.08.039 0169-555X/© 2017 Elsevier B.V. All rights reserved. 62 S.L. Rathburn et al. / Geomorphology 305 (2018) 61–75 from a variety of disturbances, along with recurrence intervals of the Understanding the sensitivity of earth materials to internal and external disturbance, gives a broad understanding of recovery. Here we focus processes and the resulting landscape forms is of major research impor- on post-disturbance sediment fluxes following fires, floods, and debris tance within the surface process community. flows to quantify the sediment component of recovery. This is defined as the time required to achieve pre-disturbance sediment magnitudes 2.2. River resilience and connectivity within fluvial systems, as well as understanding the complex pathways of recovery. We examine the geomorphic response and sediment recov- Evaluating resilience, according to Tabacchi et al. (2009), assumes ery of three small watersheds (1.5–100 km2) to varying disturbances that a system exists near a single equilibrium condition and that it is (fire, flood, and anthropogenically induced debris flow) in the southern possible to quantify how far a system has moved from equilibrium part of the Rocky Mountains in Colorado. In all cases, while we recognize and the time required to return. As applied to river systems, similar as- response to disturbances is highly context specific and spatially and sumptions of equilibrium are reasonable. Furthermore, a resilient river temporally contingent, our data augments existing studies on site- is able to adjust to and absorb perturbations, such that disturbances specific types of disturbance recovery. This has application to other do not impose a morphologic response. This ability is also referred to catchments where comparable geomorphic conditions and process do- as the system buffering capacity (Brierley and Fryirs, 2005). On highly mains occur (Montgomery, 1999; Wohl, 2010). Our approach is also ap- regulated systems, resilience has been indirectly addressed through en- plicable to other systems to determine whether a set of processes and vironmental flows that support discharges and sediment transport to responses are part of the expected behavior of a river or whether the re- maintain the physical channel (Ryan, 1997) or riparian vegetation sponse is anomalous. In all three basins examined, one or more extreme downstream from a dam (Rathburn et al., 2009). More commonly, resil- events altered runoff and abruptly increased sediment supply to chan- ience studies address recovery following natural disturbances that en- nels either as a single input of excess sediment (debris flow), or as a compass the trajectory of landform response over time, with full widespread sediment supply perturbation (wildfire or high magnitude recovery indicated by return to pre-disturbance conditions. Our empha- rainfall event). We evaluated post-disturbance sediment flux (sediment sis is on sediment recovery following disturbance, defined as the time mass per unit time or per storm) to assess recovery. Recovery time required to attain background sediment yields, concentrations, or frames span 100–103 years, allowing different pathways of response rates and volumes of aggradation and degradation. We should note, and recovery to be identified. While the focus is on sediment recovery however, that pre-disturbance conditions are not the only indicator following disturbances, we also address the spatial and historical varia- of recovery and resilience, nor may it be possible to return to pre- tions in sediment supply as influenced by antecedent conditions and the disturbance conditions, as in the case of a complete state shift or be- sequencing of events. We note that geomorphic response to various cause of long-term climate change. In such situations, ensuring that flu- disturbances may be recovery to pre-disturbance conditions or not vial processes maintain a desired channel form or that the system (disturbance pushed the system beyond the capacity to return to base- maintains a functional position in the landscape is a more appropriate line), and recovery may manifest as response toward a suite