1 Andrews Glacier in August 2016 Tyndall Glacier Lidar Scan in May

Final Report for “Glacier and perennial snowfield mass balance of Rocky Mountain National Park (ROMO): Past, Present, and Future” Task Agreement Number P16AC00826 PI Dr. Daniel McGrath June 28, 2019 Department of Geosciences Colorado State University [email protected] Andrews Glacier in August 2016 Tyndall Glacier LiDAR scan in May 2016 1 Summary of Key Project Outcomes • Over the past ~50 years, geodetic glacier mass balances for four glaciers along the Front Range have been highly variable; for example, Tyndall Glacier thickened slightly, while Arapaho Glacier thinned by >20 m. These glaciers are closely located in space (~30 km) and hence the regional climate forcing is comparable. This variability points to the important role of local topographic/climatological controls (such as wind-blown snow redistribution and topographic shading) on the mass balance of these very small glaciers (~0.1-0.2 km2). • Since 2001, glacier area (for 11 glaciers on the Front Range) has varied ± 40%, with changes most commonly driven by interannual variability in seasonal snow. However, between 2001 and 2017, the glaciers exhibited limited net change in area. Previous work (Hoffman et al., 2007) found that glacier area had started to decline starting in ~2000. • Seasonal mass turnover is very high for Andrews and Tyndall glaciers. On average, the glaciers gain and lose ~9 m of elevation each year. Such extraordinary amounts of snow accumulation is primarily the result of wind-blown snow redistribution into these basins (and to a certain degree, avalanching at Tyndall Glacier) and exceeds observed peak snow water equivalent at a nearby SNOTEL station by 5.5 times. • Ground-penetrating radar surveys of Andrews Glacier revealed that portions of the glacier exceed 45 m in thickness, likely making this the thickest glacier in Colorado. • In the upper reaches of Andrews Creek, between 50-75% of the streamflow in the late summer is likely sourced from glacial meltwater. • Streams fed by glacial meltwater have temperatures between 1 and 5° C, although lakes (e.g., Emerald Lake or The Loch) within these drainages result in significant warming. The loss of glaciers/perennial ice patches could result in significant changes to downstream ecosystems that are dependent on this cold water input. Introduction and Background Small alpine glaciers and perennial snowfields are intrinsically connected to both their immediate alpine and broader downstream ecosystems through the physical and chemical characteristics of the runoff delivered to them. Glacier derived runoff provides crucial water resources in certain regions (Immerzeel et al., 2013), modifies ocean currents (Royer and Grosch, 2006), influences biogeochemical cycling of nutrients, organic matter and contaminants (Anesio and Laybourn-Parry, 2012), and impacts freshwater ecology far downstream (Jacobsen et al., 2012). Moreover, glaciers are iconic features of 2 mountain landscapes, representing an important tourist attraction and their loss can reduce park visitations (O’Neel et al., 2014). In general, glacierized catchments modulate typical non-glacierized annual hydrographs by shifting peak flows later in the summer and producing higher specific discharge (Fountain and Tangborn, 1986; Jansson et al., 2003; O’Neel et al., 2014). This influence can be significant even when glacierized area accounts for <5% of the basin, as the resultant discharge pattern reflects the surface energy balance (i.e., snow and ice melt) rather than precipitation (Jansson et al., 2003; Moore et al., 2009). Glacier- derived runoff also has a unique geochemical signature, and thus, changes in the relative contribution of this runoff can modify downstream environments (Baron et al., 2009; Hood et al., 2015). Peak snow water equivalents (SWE) and runoff peaks have shifted earlier in the mountains of the western United States in recent decades due to increase rain-snow fraction and warmer spring temperatures (Stewert et al., 2004; Knowles et al., 2006; Mote et al., 2006) producing a significant stress on these alpine ecosystems (van Mantgem et al., 2009). Glaciers may initially help moderate this environmental stress through increased runoff production (Kaser et al., 2010), although as this comes as a deficit to glacial mass, it can, if sustained for years to decades, deplete the reservoir (Jansson et al., 2003). Glaciers are sensitive indicators of climate change, responding on annual to decadal time scales to changes in climate forcings (i.e., temperature and precipitation). Globally, glaciers have been retreating from highstands at the end of the Little Ice Age (circa 1880), although rates have accelerated rapidly in the past two decades due to anthropogenic forced warming (Marzeion et al., 2014), producing a significant contribution to global sea level rise (Zemp et al., 2019). In the short-term, glacier mass loss can lead to increased runoff, although as glacierized area retreats over the long-term, runoff will dwindle (Immerzeel et al., 2013; Bliss et al., 2014). Rocky Mountain National Park (ROMO) contains ~30 perennial ice patches/glacierets, ranging in size from 0.006 to 0.13 sq. km1, which are predominantly found in north- to east- facing high elevation cirques (Hoffman et al., 2007). Seven of these features (Andrews, Mills, Moomaw, Rowe, Sprague, Taylor and Tyndall) are labeled as “glaciers” on the USGS topographic map, although as this report details, only Andrews Glacier exhibits characteristics consistent with being classified as a glacier or glacieret (Lliboutry, 1965). The primary distinction between a glacier and an ice patch is the presence of internal motion or deformation (Serrano et al., 2011), which requires ~30 m of ice. Throughout this report, the term glacier and glacieret will be used interchangeably and these named features will be referred to as glaciers (primarily because historical precedence), despite evidence suggesting that they no longer actively deform. 1 In the analysis presented here, the largest feature in ROMO is Icefield snowfield at ~0.084 sq. km in 2017. 3 These glaciers are found below the regional equilibrium line altitude. Previous work has shown that they are sustained by extremely high rates of preferential snow accumulation due to both wind redeposition (4- 8 times regional accumulation) and avalanching, and reduced ablation through topographic shading (Outcalt and MacPhail, 1965; Hoffman et al., 2007; Moore et al., 2009). Although less common in ROMO, debris cover can play an important role in decoupling small glaciers from the regional climate, with Teton Glacier in Grand Teton National Park being an excellent example. Interannual net mass balance variability is typically large for these glaciers and glacier size/elevation ranges are relatively small, which produces the unusual situation where the entire glacier is either entirely in the accumulation or ablation zone (in contrast to most glaciers, which have distinct zones on the glacier, separated by the equilibrium line). Previous work suggested that this variability was predominantly tied to the summer energy balance (strongly correlated to air temperatures), and less dependent on winter snow accumulation variability, as direct snowfall is a minor component of the mass balance regime (Hoffman et al., 2007; Moore et al., 2009). Research Objectives This multi-faceted research project sought to develop a process-based understanding of modern glacier mass balance in Rocky Mountain National Park. This understanding was developed in the context of historic glacier response and is used to make informed predictions of future change for improved science- based resource management. Specific research objectives included: • Develop a broad process-based understanding of modern glacier mass balance in ROMO • Leverage historical climate data and photo archives to yield an improved understanding of historic glacier response • Make informed predictions of future change based on downscaled climate model projections and process understanding developed in the study 4 Figure 1. Key glacierets/perennial ice patches studied along Colorado’s Front Range, including six features in ROMO: Icefield, Rowe, Sprague, Tyndall, Andrews and Moomaw glaciers. 5 Section 1. Decadal Scale Glacier Area Change and Drivers Previous work by Hoffman et al. (2007) documented glacier area change over the past ~100 years for the many perennial ice patches/glacierets in ROMO. This work found that glaciers retreated from ~1900 to ~1940, expanded to the end of the century, at which point they noted that glacier area had started to decline. This progression can clearly be seen in historic photographs of Andrews Glacier (Figure 2) and in the Hoffman et al. (2007) timeseries (Figure 3). Of note, is that glacier area (and qualitatively, glacier volume) was smaller in 1940 than at present (see talus emerging near the terminus of Andrews Glacier in 1940). Photographs of Tyndall Glacier from 1940 suggest this glacier was comparable in size (or perhaps smaller) to present. The images below also illustrate that glacier volume can change rapidly (on the order of decades), as can be seen from 1916 to 1940. Figure 2. Photographic timeseries of Andrews Glacier. Photo credits: 1916: Lee Willis/NSIDC; 1940: Paul Nesbit/NSIDC; 1950: ROMO Glacier Report/NSIDC; 1979: Russell Allen/NSIDC; 2001: unknown/Rocky Mountain National Park Library; 2016: Daniel McGrath As part of this project, glacier area timeseries were established for 11 glaciers along the Front Range using imagery primarily

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