Debris Flow Occurrence and Sediment Persistence, Upper Colorado River Valley, CO

Environmental Management DOI 10.1007/s00267-016-0695-1 Debris Flow Occurrence and Sediment Persistence, Upper Colorado River Valley, CO 1 1 2 3 K. J. Grimsley • S. L. Rathburn • J. M. Friedman • J. F. Mangano Received: 16 September 2015 / Accepted: 23 March 2016 Ó Springer Science+Business Media New York 2016 Abstract Debris flow magnitudes and frequencies are debris flows, exceeding HRV, create persistent effects due compared across the Upper Colorado River valley to assess to valley geometry and geomorphic setting conducive to influences on debris flow occurrence and to evaluate valley sediment storage that are easily delineated by valley con- geometry effects on sediment persistence. Den- finement ratios which are useful to land managers. drochronology, field mapping, and aerial photographic analysis are used to evaluate whether a 19th century Keywords Debris flow Á Sediment persistence Á earthen, water-conveyance ditch has altered the regime of Dendrochronology Á Valley confinement Á Colorado River debris flow occurrence in the Colorado River headwaters. Identifying any shifts in disturbance processes or changes in magnitudes and frequencies of occurrence is funda- Introduction mental to establishing the historical range of variability (HRV) at the site. We found no substantial difference in Debris flows are important processes of sediment transport frequency of debris flows cataloged at eleven sites of in montane ecosystems (Benda 1990; Korup et al. 2004; deposition between the east (8) and west (11) sides of the Stock and Dietrich 2006; Savi et al. 2013) that intermit- Colorado River valley over the last century, but four of the tently deliver material to the fluvial system (Dietrich and five largest debris flows originated on the west side of the Dunne 1978). Often initiated as slope failures in poorly valley in association with the earthen ditch, while the fifth sorted colluvium on sparsely or unvegetated hillslopes is on a steep hillslope of hydrothermally altered rock on the above tree line (Costa and Jarrett 1981) or as shallow east side. These results suggest that the ditch has altered the landslides in unchannelized hillslope hollows (Iverson regime of debris flow activity in the Colorado River et al. 1997), debris flows entrain additional sediment as headwaters as compared to HRV by increasing the fre- they travel down channels and scour to bedrock (Stock and quency of debris flows large enough to reach the Colorado Dietrich 2006). Debris flows are part of the natural dis- River valley. Valley confinement is a dominant control on turbance regime of fluvial systems and may constitute the response to debris flows, influencing volumes of aggrada- dominate mode of coarse sediment (Reneau and Deitrich tion and persistence of debris flow deposits. Large, frequent 1991) and wood delivery to channels, structuring bed morphology, creating important aquatic habitat (Mont- gomery et al. 2003), producing high erosion rates that are & S. L. Rathburn important for landscape evolution (Bennett et al. 2012) and [email protected] as a sediment transfer mechanism that couples hillslopes 1 Department of Geosciences, Colorado State University, and channels (Savi et al. 2013). Debris flows may be Fort Collins, CO 80523-1482, USA triggered naturally when steep hillslopes become unsta- 2 US Geological Survey, Fort Collins Science Center, 2150 ble due to disturbances like heavy rain, flooding, earth- Centre Ave, Bldg. C, Fort Collins, CO 80525, USA quakes, wildfires, or insect outbreaks (Wieczorek 1996)as 3 US Geological Survey, Oregon Water Science Center, 2130 well as failure of landslide dams (Costa and Schuster 1988; SW 5th Ave, Portland, OR 97201, USA Schuster and Highland 2007). In the semiarid Rocky 123 Environmental Management Mountains, large naturally occurring debris flows are pri- counted to precisely date scarring events (McBride and marily associated with wildfire (Cannon et al. 2001; Laven 1976). The application of tree scars to the study of Wondzell and King 2003), and extreme rainfall (Shroba debris flows has become increasingly prevalent in previous et al. 1979; Menounos 2000; Godt and Coe 2007; Coe et al. research (Hupp et al. 1987; Benda 1990; Baumann and 2014) or rapid snowmelt (Brabb et al. 1989) that saturates Kaiser 1999; Fantucci and Sorriso-Valvo 1999; D’Agos- unstable hillslopes. Anthropogenic causes of debris flows tino and Marchi 2001; Grau et al. 2003; Rubino and include overtopping or piping through earthen dams and/or McCarthy 2004; Bollschweiler et al. 2008; Stoffel and ditches (Jarrett and Costa 1984; McDonald 1999; Clayton Bollschweiler 2008; Arbellay et al. 2010). and Westbrook 2008), failures associated with road cuts Although frequency-magnitude relations are crucial for (Swanson and Dyrness 1975; Wemple et al. 2001), or understanding recurrence intervals and volumes of sedi- clear-cutting and deforestation (Swanston and Swanson ment transfer in mountain watersheds, another useful 1976; Barnard et al. 2001). parameter is the persistence of debris flow deposits, espe- Associated hazards from all debris flows, both natural cially for management and restoration issues of natural and anthropogenic, have important management implica- environments. Sediment that is episodically delivered to tions because of the ever-mounting costs of repairing channels via debris flows is temporarily stored and grad- damaged transportation infrastructure and personal prop- ually released by fluvial erosion, and the storage sites of erty. Over US$670 million in federal funds were obtained debris flow sediment are strongly influenced by valley for recovery efforts after the 2013 devastating floods and geometry whereby, flat, wide valley bottoms effectively debris flows in the Front Range of Colorado (Coe et al. limit the export of coarse-grained material from alpine 2014). As such, research into the controls on debris flow environments to downstream main stem rivers (Barsch and occurrence, both natural and human–induced events, is Caine 1984; Caine 1986). A considerable body of research vital to an improved process-level understanding of debris is devoted to quantifying the lag time between delivery of flows to improve hazard prediction. Determining the rela- sediment and subsequent erosion to address transient ver- tive occurrence of natural and human-induced causes of sus persistent landforms (Brunsden and Thornes 1979), to debris flows in a particular watershed, and any shifts in the use as input into sediment routing models (Benda and controls on occurrence over time, requires an assessment of Dunne 1997; Lancaster et al. 2001), for use in habitat magnitude and frequency of both types. management (Phillips 1995), to evaluate channel response When minimal to no documentation exists on the timing to extreme sedimentation events in the context of restora- of historical debris flows, analysis of trees at the debris tion (Madej and Ozaki 2009; Rathburn et al. 2013), and to flow site using dendrochronological techniques is typically quantify long-term landscape evolution (Stock and Dietrich an effective approach (Bollschweiler and Stoffel 2010). 2006). Trees may preserve a remarkable amount of information on The abrupt and pronounced changes in valley and the timing and frequency of debris flows, as well as indi- channel geometry present along many Colorado rivers cators of relative flow magnitude (Bollschweiler and (Wohl 2001) control how geomorphic setting influences Stoffel 2010). Dendrogeomorphic evidence of debris flows response to debris flows and persistence of debris flow can be preserved either through the age of an entire stand or deposits. An ideal setting exists along the Upper Colorado through the characteristics of individual trees (Butler et al. River in Rocky Mountain National Park (RMNP) to assess 1987). When no trees survive a debris flow, the age of trees debris flow occurrence over the last 91 years, and to on the disturbed surface corresponds to a minimum age of evaluate valley geometry influences on sediment persis- the most recent disturbance (Bollschweiler et al. 2008). tence. In this case, we first ask: What is the influence of Assuming the oldest trees have been sampled, the age of anthropogenic activities on debris flow occurrence relative the surface can be accurately calculated by adding in the to natural processes inherent in a steep-gradient mountain colonization time gap (Pierson 2007). Tree rings also system? To address this question, we compared natural and provide more precise dates of debris flow occurrence over anthropogenic influences on debris flows to evaluate the other records such as aerial photographs which only role of a 19th century earthen ditch on debris flow activity. bracket events between years of repeat coverage. A second research question is: How large and frequent do When survivor trees are present in the disturbed area, debris flows have to be to create persistent effects given the they can preserve evidence of impacts or other stresses existing valley geometry along the Upper Colorado River? through a variety of mechanisms including (1) growth We contend that stream channel and valley geometry are decrease, (2) callus tissue, (3) reaction wood, and (4) the dominant controls on sediment persistence within the traumatic resin ducts (Bollschweiler and Stoffel 2010). By Upper Colorado River Valley, and possibly many other sawing out a thin wedge through the discontinuity in the fluvial systems in the semiarid Rocky Mountains. This cambium at the edge of a scar, warped tree rings can be paper explicitly examines how geomorphic

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