Meteorological Controls on Sediment Transfer
Precipitation was excluded from three of four best-fit models due to
significant correlations with Q and NTU, suggesting that precipitation
has a strong influence on discharge and turbidity. P12at 1750 m asl was a significant predictor in the Chamberlin Creek
NTU-based model (Table 3). P12 possibly represents a
coarse fraction of suspended sediment eroded and entrained during
precipitation-induced high discharges. This would be consistent with
nephelometric turbidimeter signals being more sensitive to fine
particles (clay and silt) due to their greater light scattering
efficiency and particle surface area, thus potentially
under-representing the coarse (sand) fraction (Orwin et al., 2010).
We estimated that substantial seasonal sediment delivery in both study
creeks was transported over 48 hours (Figure 5). The hydrological
response to rainfall during these events may have been enhanced by their
timing in mid-July to early-August, when channel-ice and snow were less
likely to impede erosion (e.g. Crawford & Stanley, 2014; Irvine-Fynn et
al., 2005), and glacier conduits are likely open to flush sediment into
the proglacial fluvial system. Additionally, the placement of
hydrological stations near stream outlets may have represented the
rainfall signal more completely than if they were placed farther
upstream (e.g. Irvine-Fynn et al., 2005; Orwin & Smart, 2004b; Willis
et al., 1996). The short duration over which the majority of sediment is
transferred to Lake Peters is comparable with other rivers that drain
the northern Brooks Range, including: the Kuparuk River near Prudhoe Bay
where 90% of the annual suspended sediment load was transferred over
three days in 2001 (Rember & Trefry, 2004); the Sagavanirktok River,
with 88% over 12 days in 2001 (Rember & Trefry, 2004); and the
Colville River, with 62% over 13 days in 1961 (Walker & Hudson, 2003),
although these studies emphasize melt processes rather than rainfall.
Similar to precipitation variables, temperature variables were excluded
from three of four best-fit models due to significant correlations with
other predictors. We note that such correlations have not been reported
by authors of similar models (Irvine-Fynn et al., 2005; Schiefer et al.,
2017). Albeit model limitations, field observations and diurnal signals
suggest that temperature-driven melt processes influence sediment
transfer in both Carnivore and Chamberlin Creeks. The inclusion of
GT2 (1750 m asl) in our Carnivore NTU-based model
suggests that melt-processes mobilize a sediment supply less discernible
to the turbidity sensor. We note that this model (Table 3) is the first
to incorporate ground temperature as a supplementary predictor of SSC,
although ground conditions have previously been related to sediment
transfer in the Arctic (Favaro & Lamoureux, 2014; Irvine-Fynn et al.,
2005; Syvitski, 2002). Supplementing this NTU-based model with ground
temperature (using GT2 at 1750 m asl, R2 = 0.68)
explains slightly more variability in SSCs than air temperature (using
T2 at 1750 m asl, R2 = 0.63), which
may relate to ground temperatures better reflecting thaw-related
sediment mobilization and/or improved model relations with
high-frequency filtering in the ground temperature record (Figure 4).
Although both rainfall and temperature affect discharge and sediment
transfer, they do not fully explain the disparities between the 2015
versus 2016 hydrographs (Figure 4). In 2016, Q was elevated for an
extended period between mid-June and mid-July in both creeks, but
neither precipitation nor temperature were notably higher in 2016.
End-of-winter snow water equivalence has been positively related to
discharge and sediment transfer in the Arctic (Bogen & Bønses, 2003;
Cockburn & Lamoureux, 2008; Forbes & Lamoureux, 2005; Lewkowicz &
Wolfe, 1994); however, simple DEM differencing of repeat photogrammetric
surfaces calibrated for catchment snow measurement at Lake Peters
(Broadman et al., 2019; Nolan, Larsen, & Sturm, 2015) suggested a
greater overall snowpack in 2015, thus snow water equivalence does not
explain the elevated 2016 Q. Limitations associated with developing
continuous discharge time-series from stage could contribute to the
seasonal hydrograph differences, although such limitations do not appear
great enough to affect average Q, nor the magnitude of sediment yield
results (Thurston, 2017). An earlier study on Chamberlin Creek reports
an average discharge of 0.65 m3 s-1for 44 days in late summer (between 07/01/1958 and 08/13/1958)
(Rainwater & Guy, 1961), which is similar to the mean daily discharges
of 0.62 and 0.77 m3 s-1 (2015 and
2016, respectively) for the same days of the year in this study.
We interpret temperature-driven melt processes as secondary to rainfall
in controlling sediment yield, because most of the sediment load is
transported to Lake Peters during rainfall-induced flood events.
Although the same intensity and volume of rainfall does not necessarily
equate to the same magnitude of sediment load, rainfall is clearly
associated with events that transport the majority of the annual
sediment yield to Lake Peters. Rainfall and sediment transfer are
strongly correlated from the onset of the open-channel seasons, with no
distinct shift between temperature-driven spring snowmelt and late
summer rainfall-induced sediment transfer during the period of record in
2015 and 2016, as has been observed in Arctic Canada (Dugan et al.,
2009; Lewis et al., 2012; McLaren, 1981). In Arctic Alaska, literature
reporting sediment yields (Table 5) has not established the metrological
processes driving water and sediment discharge, although hydrograph
research provides inference. At Lake Peters, the dual snowmelt and
rainfall early in the open-channel-season, and more significant rainfall
in July and August, is inconsistent with a regional hydrograph developed
for the coastal plain of the Arctic National Wildlife Refuge, which
showed most of the total water discharge during the spring freshet
(Lyons & Trawicki, 1994) and later work that applies this as an
assumption (e.g. Rember & Trefry, 2004). Following observation of an
extreme flood event in the Upper Kuparuk River, Kane et al. (2003)
suggest that most floods of record in Arctic Alaska are
rainfall-generated, especially in smaller catchments; however, our
research suggests that seasonal floods are also rainfall-generated. At
Lake Peters, significantly greater, and primarily rainfall-driven,
discharge volume in 2016 explains the higher annual sediment yield that
year than in 2015. In Svalbard and Arctic Canada, metrological processes
driving sediment discharge have been researched, in addition to
hydrographs. Dominantly rainfall-induced sediment transfer (Bogen &
Bønses, 2003; Lamoureux, 2000; Lewis et al., 2012), similar to Lake
Peters, and temperature-dominated melt regimes of sediment transfer
(Braun, Hardy, Bradley, & Retelle, 2001; Favaro & Lamoureux, 2014;
Hardy, 1996; Irvine-Fynn et al., 2005; Moore, Hughen, Miller, &
Overpeck, 2001; Smith, Bradley, & Abbott, 2004), have both been
reported. Comparison with arctic forecasting studies suggests that
relatively small, mountainous arctic catchments (e.g. Lake Peters; and
Lewis & Lamoureux, 2010) are more responsive to rainfall than larger,
coastal arctic catchments (e.g. Syvitski, 2002).