Postglacial adjustment of steep, low-order drainage basins, Canadian Rocky Mountains

Abstract

[1] It is generally argued that Pleistocene glaciation results in increased sediment flux in mountain systems. An important, but not well constrained, aspect of Pleistocene glacial erosion is the geomorphic decoupling of cirque basins from main river systems. This study provides a quantitative link between glacier-induced basin morphology, postglacial erosion, and sediment delivery for mountain headwaters (with basin area <10 km 2 ). We analyze the morphology of 57 headwater basins in the Canadian Rockies and establish postglacial sediment budgets for select basins. Notable differences in headwater morphology suggest different degrees of erosion by cirque glaciers, which we classify into headwater basins with either cirque or noncirque morphology. Despite steeper slope gradients in cirque basins, higher-mean postglacial erosion rates in basins with noncirque morphology (0.43-0.6 mm a À1 ) compared to those in cirques (0.19-0.39 mm a À1 ) suggest a more complex relationship between hillslope erosion and slope gradient in calcareous mountain environments than implied by the threshold hillslope concept. Higher values of channel profile concavity and lower channel gradients in cirques imply lower transport capacities and, thus, lower sediment delivery ratios (SDR). These results are supported by (i) postglacial SDR values for cirques and noncirque basins of <15% and >28%, respectively, and (ii) larger fan sizes at outlets of noncirque basins compared to cirques. Although small headwater basins represent the steepest part of mountain environments and erode significant postglacial sediment, the majority of sediment remains in storage under interglacial climatic conditions and does not affect large-scale mountain river systems.

Controls on Rockfall-Talus Process-Response Systems, Kananaskis, Canadian Rockies

“…Similarly, recent research in an unglaciated environment applied the concept of the analytic solution and found that frost-dominated erosional processes were prevalent during the Last Glacial Maximum (Marshall et al, 2015) Topographical structures, which consist of northwest-southeast aligned ridges, range from ~1 400 masl along the valleys, rising up to a peak of 3 100 masl (Jackson, 1980;Hoffmann et al, 2013). On average, Kananaskis experiences 105 days of precipitation, with 350 mm falling as rain and 270 mm as snow (Whitfield, 2014).…”

“…Climate System Model Version 3 (NCAR CCSM3) (Liu et al, 2009 Hoffmann et al (2013)). For each millennium between 0 and 12 ka BP, decadal averages of seasonal climate variables from the first 100 years for each millennium of the simulation were extracted and averaged (e.g.…”

“…Finally, erosion rates are adjusted to remove the effect of regolith porosity by following the procedure outlined by Hoffmann et al (2013); volume is adjusted by multiplying this value by the ratio of the regolith density To allow for comparison with these other studies, erosion rates divided by the total area for each study area are also reported in Table 4.2; the erosion rates in these cases are much lower than when adjusted for the total area of steepland only. For example, Marmot basin has only 0.32% of its area greater than or equal to 45˚, with the erosion rate for steepland areas having a value of 3.30 mm yr -1 ; this is a relatively high erosion rate.…”

“…(ii) rockfalls represent a significant natural hazard; (iii) rockfalls and their locations of deposition affect sediment connectivity between hillslopes and channel networks (Hoffmann et al, 2013); and (iv) rockfalls have distinct morphological and sedimentological properties that affect the development of weathered layers in the landscape; (v) talus affects mountain hydrology, in particular the storage and transmission of water (Muir et al, 2011). For these reasons, it is important to obtain a better understanding of rockfall-talus process-response systems.…”

“…Multiple advancements of local valley and cirque glaciers occurred between 15 and 11 ka (Liverman et al, 1989;Menounos et al, 2009). Similarly, glacier expansion during the Holocene (beginning after ~ 11 ka) was punctuated by several advances and retreats (Menounos et al, 2009 Luckman, 1981;Gardner et al, 1983;Hoffmann et al, 2013). These rock slides and rockfall events are expected to have continued to operate throughout the Holocene, although it is generally thought that these processes would be most active directly after glaciation.…”