Glaciofluvial Processes and Sediment Yield
Glacier coverage is a well-known control of sediment yield (Hallet et al., 1996; Meade et al., 1990), and heavily glaciated catchments in Alaska have been found to produce an order of magnitude more sediment than other Alaskan catchments (Hallet et al., 1996). Against this trend, we found that specific sediment yields (SSYs) for Chamberlin Creek (23% glacier coverage) were similar to Carnivore Creek (10% glacier coverage) in 2015, and less in 2016 (Table 5). Our results are comparable with Gurnell et al. (1996), who also found an inverse relation between SSY and glacier coverage for Alaskan catchments. If the rates of glacier recession and glacier thermal regimes are consistent between sub-catchments, this signifies the importance of non-glacial processes. Non-glacial sediment sources (i.e. proglacial extra-channel and hillslope sources) eroded during rainfall events are likely significant, as reported for Matanuska Glacier in southern Alaska (O’Farrell et al., 2009). At Lake Peters, more exceptional sediment delivery from Carnivore Creek during high discharge events dominates the yield to Lake Peters, despite lower glacier coverage than Chamberlin Creek (Table 4; Figure 4). Conversely, at low flow, Chamberlin Creek’s steeper slopes result in persistently turbid water on the alluvial fan (63 NTU average for Q < 0.325 m3s-1), compared with relatively clear water in Carnivore Creek (30 NTU average for Q < 15 m3s-1), although low flow delivery is a meager proportion of the sediment yield.
Glacier processes may enhance sediment delivery in Carnivore Creek under the current hydrological regime. Ellerbrook (2018) report that old water (glacier melt and subsurface flow) contributed a higher proportion of the hydrograph than rainfall in the Carnivore sub-catchment, whereas rainfall dominated over old water in the Chamberlin sub-catchment. If the glaciers have surface-bed connections, it is possible that more intense rainfall over the Carnivore glaciers contributes to erosive glacier processes, compared with Chamberlin sub-catchment, which is a more isolated massif. Subglacial conduits may have melted by the time the most peaked rainfall events occur (mid-July to early-August), supporting subglacial erosion and enhancing hydrological response (e.g. Bogen & Bønses, 2003; Gurnell et al., 1996; Hodson & Ferguson, 1999; Hodson et al., 1997). The nearby polythermal McCall Glacier (~50 km west of Lake Peters) was found to have a zone of basal sliding, and moulins—likely transferring surface meltwater to the glacier’s base, but a complex subglacial drainage network was probably not active (Pattyn et al., 2009). Although the contemporary subglacial network at Lake Peters has not been studied, Benson et al. (2019) relates millennial-scale changes in sediment accumulation and other sediment properties in Lake Peters to large-scale glacial fluctuations and other hydro-climatic Holocene trends, and note that increased accumulation rates during the last century may reflect contemporary glacier retreat.