Calculating the peak discharge of the Glacial Lake East Fork outburst flo od, Big Lost River, Idaho

Glacial Lake East Fork (GLEF) was dammed by the Wildhorse Canyon glacier during the Pinedale Glaciation. Upon retreat from the Pinedale maximum, at approximately 16.9 ka ± 0.8 ka, the lake catastrophically drained releasing 1.3 km of ponded water. The ensuing flood transported thousands of huge boulders as far as 10 km downstream of the GLEF ice dam. This study estimates the peak discharge of that outburst flood using the size and location of the flood boulders found downstream of the ice dam. The peak discharge is calculated using Shield's shear stress criterion for depth, theoretical equations incorporating diameter for velocity, and valley cross-sections for area. The variables that have a known range within each equation are set to their range limits to produce the maximum possible peak discharge (546,366 m/s) and the minimum possible peak discharge (16,657 m/s). When the values of the '/ above variables are constrained by the geologic, hydrologic, and geomorphic limits applicable to the study area, a peak discharge between 27,644 and 32,839 mIs is obtained. A flow of this size is not large enough to cause the flood features documented 100 km downstream in Box Canyon, ID. Introduction Ice-dammed glacial lakes are common in all current or formerly glaciated landscapes (Herget 2005). The dynamic nature of ice-dams leads to frequent failure and the abrupt drainage of glacial lakes and associated downstream flooding (Herget 2005). As alpine glacial systems continue to melt in today's slowly warming climate, the number and size of glacial lakes is increasing. Adding meltwater increases the likelihood of catastrophic lake drainage and high-energy downstream flooding, endangering downstream communities. It is therefore vital to better understand the range of discharges resulting from past catastrophic glacial lake drainages in an effort to better constrain our understanding of the impacts of future outburst floods. Since the pioneering work of 1. Harlen Bretz (1925) on the Lake Missoula floods in Washington State, outburst floods from glacial lakes in the northwestern United States have intrigued numerous researchers (Malde 1968; Jarret and Malde 1987: Baker 1973; O'Connor and Baker 1992; O'Connor 1993), most of whom have investigated paleohydraulic flow conditions and attempted to calculate peak discharges of the floods. During the late Pleistocene Bull Lake (200,000 130,000 ka) and Pinedale (45,000 18,000 ka) glaciations, central Idaho hosted extensive alpine glacial systems. In the Pioneer Mountains, the Wildhorse Glacier advanced northwards across the mouth of the East Fork of the Big Lost River, blocking the westward flowing meltwater and forming GLEF (Evens<;m et al. 1982; Fig. 1). At its maximum extent Glacial Lake East Fork was 10 km long, 1 km wide at the ice dam, had a maximum depth -56 m, and impounded 1.3 km of water (Norton 2000). Numerous ice-rafted 2 boulders at an elevation of ~2240 m above sea level mark the stable shoreline of the lake. Upon retreat of the Wildhorse Canyon Glacier, GLEF drained catastrophically. The evidence for a catastrophic flood is the presence of very large flood-rafted boulders (0.5m to 5m in diameter) sitting on the glacial outwash terraces as far as 10 km downstream from the paleo ice dam (Evenson et al. 1979; 1982). Detailed mapping (Brugger 1983) demonstrates that no glacier ever advanced beyond the location of the Wildhorse Glacier terminus (Figure 1), thus eliminating the possibility of the boulders being transported glacially. A particularly large (5x3x2 m) known as the Swenson Butte boulder (Knudsen 2002) that sits on an outwash terrace surface about 10 km downstream of the ice-dam position has been cosmogenically dated at 16.9 ± 0.8 ka (Gosse, personal communication), suggesting a late Pinedale flood event. Similarly, Ceding et al. (1994) find average ages of 20.5 ka for flood boulders found further downstream at Box Canyon on the'1daho National Laboratory (INL). I A single 3 cm thick sample was collected from the top center of the 2 m high boulder. The surface sampled was horizontal and not shielded by local topography. No significant indications of erosion were evident. Although snowfall depths and densities in the past are uncertain, the semi-arid climate, elevation, and height make it unlikely that the was ever significantly covered by snow. The sample was processed for lOBe chemistry at Dalhousie University, using 36 g of quartz and 0.26 g of a 1000 ppm Be carrier prepared from beryI crystal recovered from the Homestake Gold Mine. AMS was conducted at Lawrence Livermore National Laboratory, using standards KNSTD311 0 and KNSTD9422 and lOBe t1l2==1.5 Ma. The process blank was < 1% ofthe sample. The age of the boulder, calculated according to Lal (1991) as modified by Stone (2000) is 16.9 ± 0.8 ka (20uncertainty is AMS precision). Calculations that treat geomagnetic paleointensity variation and atmospheric shielding differently than Stone (2000) range from 16.9 ka (Lifton et aI., 2005) to 17.4 ka. (Desilets and Zreda, 2003; Dunai, 2001). Therefore the total uncertainty in this age, including all internal and external errors is probably < 20% at 20confidence. 3 Previous estimates of the peak discharge of the GLEF flood utilized the estimated volume of the lake along with published r~gression relationships correlating lake volume and peak discharges of historic large outburst floods, resulting in discharges ranging from 26,000 to 66,000 m/s (Rathburn 1993; Norton 2000). However, lake volume is generally noted to be a poor indicator of peak paleodischarge because of the variation in which an ice-dam can fail (Costa 1988). For example, a catastrophic subaerial breach would release the lake water almost instantaneously (resulting in a larger discharge), while subglacial tunnel enlargement would allow only a gradual release of lake water (resulting in a vastly lower peak discharge). Other factors such as variability in outlet shape and lake dimension also complicate paleodischarge estimates based on lake volume alone. In this study, I estimate the peak discharge of the outburst flood from GLEF using the size and location of flood boulders deposited on terrace surfaces downstream of the lake. Using hydraulic calculations to reconstruct paleodischarge from flood deposited material provides a direct estimate of flow conditions, negating any uncertainty regarding specific dam failure mechanism. While previous researchers have reconstructed paleoflow conditions using computer modeling (the step-backwater method; O'Connor and Baker 1992; O'Connor 1993; Baker et al. 1993; Clarke et al. 1984), I was unable to do that for GLEF because of the lack of downstream paleostage indicators (Knudsen 2002). My calculations are based solely on the dimensions and location of the flood-transported boulders and the energy required to move them.

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