Arctic Science
Impact of permafrost thaw on the turbidity regime of a subarctic river: the Sheldrake River, Nunavik, Quebec
Journal: Arctic Science
Manuscript ID AS-2016-0006.R3
Manuscript Type: Article
Date Submitted by the Author: 26-Apr-2017
Complete List of Authors: Jolivel, Maxime; centre d'études nordiques, géographie Allard, Michel; Université Laval, Centre d'études nordiques
Keyword: permafrost,Draft Northern Quebec, thermokarst, turbidity, subarctic river
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1 Impact of permafrost thaw on the turbidity regime of a subarctic river: the
2 Sheldrake River, Nunavik, Quebec.
3 Maxime Jolivel and Michel Allard
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5 M. Jolivel and M. Allard, Centre d’études nordiques (CEN) and Département de
6 Géographie, Université Laval, Québec QC, G1V 0A6 Canada.
7 Corresponding author: [email protected]
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22 Abstract
23 In order to assess the impact of seasonal active layer thaw and thermokarst on
24 river flow and turbidity, a gauging station was installed near the mouth of the Sheldrake
25 River in the discontinuous permafrost zone of Northern Quebec. The station provided
26 five years of water level data and three years of turbidity data. The hydrological data for
27 the river showed the usual high water stage occurring at spring snow melt, with smaller
28 peaks related to rain events in summer. Larger and longer turbidity peaks also occurred in
29 summer in response to warm air temperature spells suggesting that a large part of the
30 annual suspension load was carried during mid summer turbidity peaks. Supported by 31 geomorphological observations acrossDraft the catchment area, the most plausible 32 interpretation is that the rapid thawing of the active layer during warm conditions in July
33 led to the activation of frostboils and triggered landslides throughout the river catchment,
34 thus increasing soil erosion and raising sediment delivery into the hydrological network.
35 These results indicate that maximum sediment discharge in a thermokarst affected region
36 may be predominantly driven by the rate of summer thawing and associated activation of
37 erosion features in the catchment.
38 Keywords: permafrost, Northern Quebec, thermokarst, turbidity, subarctic river
39 Introduction
40 Rivers are natural pathways from land to sea that carry sediments and other matter
41 eroded from their catchments. Their behavior reflects different geomorphic and
42 biogeochemical processes in the landscape with cascading effects downstream to the
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43 coastal marine environment. In the context of climate change, river studies are essential
44 to quantify the pace and intensity of geosystem changes (Prowse et al. 2015). In subarctic
45 regions, the riverine hydrological regime is strongly linked to seasonal climate variations,
46 which generate a large annual range in water discharge (Déry et al. 2005). Sediment loads
47 can also be extremely variable even if they are sometimes low in comparison with rivers
48 from temperate and tropical regions (Syvitski 2002).
49 The hydrological cycle of high latitude rivers is regulated by snow storage and
50 melting and by the freezing of soil water. Permafrost is a major factor that restricts
51 infiltration and percolation at depth; a perched water table is maintained in the active
52 layer near the surface in summer (Carey and Woo 2001; Carey and Quinton 2005;
53 Quinton and Carey 2008). BaseDraft flow may cease in winter since sub permafrost
54 groundwater may be non existent or too deep to discharge in the catchment and because
55 taliks can be only poorly connected with springs on the river beds. Soil warming,
56 thinning and decay of permafrost, earlier breakups, decline of snow cover duration and
57 increase in shrub, forest and peatland covers are factors affecting the hydrology of high
58 latitude rivers under ongoing climate change (Magnuson et al. 2000; Sturm et al. 2001;
59 Payette et al. 2004; Brown and Romanovsky 2008; Jolivel and Allard 2013; Lesack et al.
60 2014). For example, it is broadly expected that the sediment load of high latitude rivers
61 would increase by 30% for every 2 °C of warming of the averaged catchment temperature
62 (Syvitski 2002).
63 Thawing of permafrost is known to release large volumes of sediments through
64 thermokarst processes such as thaw slumping and thermal erosion (Jolivel and Allard
65 2013; Kokelj et al. 2013). The released sediments are mobilized by soil erosion, in
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66 overland flow and in water courses; they feed sedimentation (Guo et al. 2004; Goni et al.
67 2005; Jolivel et al. 2015) and get involved in biogeochemical processes (Emmerton et al.
68 2008; Galand et al. 2008; Vonk et al. 2015) in lakes, deltas and coastal seas.
69 Evidence of general permafrost decay has been observed throughout all high
70 latitude regions (e.g. Sollid and Sorbel 1998; Luoto and Seppälä 2003; Jorgenson et al.
71 2006). In northern Québec, near the southern limit of permafrost distribution, thawing of
72 permafrost in large areas of palsas, lithalsas, peat plateaus and permafrost plateaus has
73 led to the reduction of permafrost extent by roughly 40% over the last 50 years (Payette
74 et al. 2004; Marchildon, 2007; Vallée and Payette 2007; Fortier and Aubé Maurice 2008; 75 Jolivel and Allard, 2013). ContinuedDraft warming will lead to further degradation, releasing 76 sediments and making previously frozen organic matter available for bacterial
77 decomposition and recycling into bio available carbon and greenhouse gases (Schuur et
78 al. 2008; Deshpande et al. 2015; Vonk et al. 2015).
79 Thermokarst and associated landslides generate large sediment loads in rivers.
80 This is particularly evident in the case of retrogressive thaw slumps and large active layer
81 detachment slides (Kokelj et al. 2002; Lewis et al. 2005; Lewkowicz and Harris 2005a;
82 Jorgenson et al. 2006; Lantuit and Pollard 2008; Lantz and Kokelj 2008; Lamoureux and
83 Lafrenière 2009; Lacelle et al. 2010; Kokelj et al. 2013). These inputs can alter terrestrial
84 and aquatic ecosystems and affect food webs as well as primary and secondary
85 production (Kokelj et al. 2002, 2009; Bowden et al. 2008; Mesquita et al. 2010).
86 Ultimately, a significant fraction of the organic carbon released by thermokarst may
87 reach the marine environment (Jolivel et al. 2015; Vonk et al. 2015).
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88 More gauging of rivers and monitoring of processes are necessary to understand
89 the sedimentary regime of rivers in regions affected by thermokarst. This is particularly
90 true during periods of significant temporal change in fluvial fluxes resulting from
91 seasonal variations of thermokarst processes in response to climate forcing (Prowse et al.
92 2015). There are few measurements of the impacts of eroding permafrost catchments on
93 fluvial sedimentary regimes (Bowden et al. 2008), and more data are required to better
94 understand geomorphological processes in these regions in transition.
95 The main objectives of this study were to (1) document the annual and seasonal
96 hydrologic fluctuations of a Subarctic river; (2) describe the dynamics of turbidity and
97 sediment fluxes during the thawing season; and (3) assess the relative impacts of
98 precipitation and thawing on dischargeDraft and sediment transport. Because the rate of soil
99 thawing influences the rate of thermokarst which releases sediments in the drainage
100 network, we raised the hypothesis that variations in air temperature can influence
101 turbidity of surface water, and so the amount and timing of sediment fluxes in the
102 collector river.
103
104 Methods
105 Study area
106 The 25 km long Sheldrake River flows to the eastern coast of Hudson Bay. It
107 drains a 76 km 2 watershed (Fig. 1). Its catchment is typical of the area of decaying
108 sporadic/discontinuous permafrost in the Tyrrell sea fine sediments of Eastern Hudson
109 Bay. The Sheldrake is among many rivers of the east Hudson Bay watershed, including
110 large fluvial systems such as the Nastapoka River that transport sediment resulting from
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111 erosional, thermokarst and periglacial processes (Fig. 2).Typically, in summer conditions,
112 the river varies in width from 25 to 50 m. Its depth varies between 3 m in water pools to
113 50 cm in rapids. In the upper part of the river, the river bed is generally composed of a
114 thin veneer of sand and gravel covering thick marine silty clay. From its passage in the
115 coastal hills to the shore, the bed and the banks are essentially composed of exposed
116 bedrock and boulders, which greatly limits bed load transport.
117 The river originates from Sheldrake Lake, on the Archean sector of the Canadian
118 Shield. Near the coast, the river valley runs across a range of coastal hills in Late
119 Proterozoic bedrock and the river flows into Hudson Bay at 56°37’N; 76°32’W (Fig. 1).
120 On a topographic 1:50,000 map, the low gradient Sheldrake River is a third order stream.
121 However, the rapid and recent permafrostDraft decay increased the hydrologic connectivity
122 between thermokarst ponds, hollows and gullies thereby increasing stream density
123 (Jolivel and Allard 2013).
124 The east west elongated shape of the catchment (Gravelius index: 1.9; Gravelius
125 1914) is principally due to the carving activity of the Pleistocene glaciers that flowed to
126 the west. Inland, the topography is dominated by flat valley floors, scattered with lakes
127 and small hills with a general elevation range of 200 to 250 m a.s.l. The mean
128 longitudinal river slope is 0.6%. It is <0.5% inland and increases to 3% once crossing the
129 coastal hills near the Hudson Bay.
130 The entire watershed was submerged after deglaciation by the postglacial Tyrrell
131 Sea from 8000 BP to about 6000 BP. Therefore, 85% of the surficial deposits of the area
132 are marine silty clays (Fig. 3). Inland, sand and gravel deposits are associated with ice
133 contact glacio fluvial deltas which mark the postglacial marine limit, whereas glacio
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134 marine sand and gravel anchored on the slopes of the coastal hills have been reworked
135 into beaches and terraces during the postglacial fall of the relative sea level (Fig. 3)
136 (Lavoie et al. 2012; Lajeunesse and Allard 2003). Because of permafrost, dispersed forest
137 and tundra cover, presence of impermeable clay and bedrock, water infiltration and
138 percolation rates are very low, therefore, most of the water input from rainfalls and
139 snowmelt flows as surface run off. However, numerous wetlands, topographic
140 depressions, thermokarst ponds and lakes in the Sheldrake River catchment act as surface
141 water storage areas.
142 The regional climate is subarctic, characterized by cold winters (monthly average 143 air temperature of 24°C in January),Draft cool summers (monthly average air temperature of 144 10°C in August) and mean annual temperatures varying between 4°C and 5°C. The area
145 is covered by snow for ~ 8 months every year. Rain accounts for 60% of total
146 precipitation and snow for 40%. The average annual precipitation is 550 mm
147 (Environment Canada 2013) while the average annual evapotranspiration is ~ 200 mm
148 yr 1 (Payette and Rochefort 2001). The eastern sector of Hudson Bay is generally ice
149 covered from early December to the end of May or beginning of June. However, during
150 the warm winter of 2010 2011, freeze up did not occur before mid January. The
151 Sheldrake River has a ~1 m thick ice cover from early November onwards. The breakup
152 occurs a few weeks before the melt of the Hudson Bay ice cover. Snowmelt generally
153 occurs in late May and early June but some thicker snow banks can last until mid
154 summer in the landscape. Between 2009 and 2014, the average date of the beginning of
155 the Sheldrake River ice breakup was 6 May (σ= 6 days) and the level of the river
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156 remained high during nearly two months before reaching its summer flow regime (Table
157 1).
158 The basin is located at the transition between the sporadic permafrost zone and
159 the widespread discontinuous permafrost zone (Allard and Seguin 1987). Permafrost is
160 present in 20% of the surficial deposits in the form of lithalsas, palsas, permafrost
161 plateaus and peat plateaus (Fig. 1) (Jolivel and Allard 2013). Lithalsas are mineral
162 permafrost mounds, while palsas are peaty permafrost mounds or mineral cored
163 permafrost mounds with a peat cover. Permafrost plateaus and peat plateaus are
164 equivalent but wider landforms. These heaved landforms are generally 3 to 5 m high. 165 This is due to the development of Draftice segregation lenses formed by cryosuction in the 166 frost sensitive marine silt (Pissart 1985, 2002). Permafrost thickness in these frozen
167 landforms varies typically from 10 to 15 m (Lévesque et al. 1988).
168 The Sheldrake River catchment is actually an area of intensive permafrost decay.
169 Between 1957 and 2009, 21% of the area covered by permafrost disappeared.
170 Widespread thermokarst ponds, landslides, active layer failures, and expanding gullies
171 are the main features of permafrost degradation (Jolivel and Allard 2013). Thermokarst
172 pond coverage has nearly doubled between 1957 and 2009 allowing an increase of the
173 stream density and better connections between water tracks. The number of active
174 erosional landforms counted in the landscape has increased by 46 to 217%,
175 corresponding to an increase in the volume of eroded fine grained sediments of 12 to
176 38% potentially released in the fluvial system (Jolivel and Allard 2013). This degradation
177 is more important inland to the extent that some sub catchment areas are now devoid of
178 permafrost.
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179 The tree line runs across the basin and is oriented from south to north due to the
180 cooling influence of the Hudson Bay; the landscape close to the bay is dominated by the
181 shrub tundra while the forest tundra extends further inland (Fig. 3). In the western part of
182 the basin (shrub tundra area), the river and its tributaries flow through an area of
183 permafrost plateaus, with incising meanders and deep tributary gullies (Fig. 1). In the
184 forest tundra in the eastern part, some peat deposits over clay or sand extend over flat and
185 poorly drained valley bottoms. Most of the palsas in the catchment are located in those
186 bogs and fens.
187 Field instrumentation and laboratory analyses Draft 188 A gauging station was installed 2 km upstream from the river mouth in late summer 2008
189 (see Fig. 3 for location). As the river bed is rocky and the current is strong in the lower
190 reach of the river, the instrumentation had to be installed between two rapids in a
191 convenient pool of calmer water. This automated station continually records water
192 temperature, water level and turbidity. The mooring consists of a dead weight (20 kg)
193 attached to a buoy that rests on the river bed and is tied to the shoreline with a steel cable
194 (Fig. 4). The buoy was submerged under ~ 1.2 m water at a distance of 4 m from the river
195 bank in order to avoid being swept away during ice breakup. The instrumentation
196 attached to the mooring, 20 cm from the river bottom, consists of a Levellogger (Solinst)
197 and an OBS 3+ (Optical Backscatter Sensor, Campbell Scientific, Inc.). As the depth
198 sensor is close to the river bed, possible changes in the verticality of the mooring cable at
199 higher flow speed can introduce only a very slight error in stage measurement. Given the
200 swift current (maximum measured velocity in summer: 0.5 m/s) that favors mixing in the
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201 pool, we consider that the measured turbidity at this point represents the average turbidity
202 of the whole river cross sectional area, despite the possibility of a gradient with depth or
203 along cross bank distance. Finally, the bedrock riverbed prevents change in the stream
204 cross section and bed scour that could locally affect turbidity values at the measurement
205 site during high stages.
206 The levelogger was calibrated for barometric compensation with a barologger
207 (Solinst). The accuracy of the water level sensor is ±0.3 cm. The turbidity sensor has a
208 range of 0–1000 nephelometric turbidity units (NTU) with an accuracy reading of 2% or
209 0.5 NTU (whichever is larger). Both the levelloger and the turbidimeter have a 1 hour 210 time step output which was then convertedDraft to daily average data to facilitate the reading 211 of the graphs. No noise effect was recorded during ice free conditions. The mooring is
212 situated several meters upstream of a 20 m waterfall. Thus, at spring, breakup at the
213 gauging site is facilitated and backwater effects are prevented even if the river upstream
214 breaks up several days later. Data presented in this paper reflect hydrological conditions
215 at the gauging station only.
216 An attempt of conversion of water level into discharge was done but the
217 difficulties to measure current velocity during extreme high flow due to remote access
218 prevented the calculation of a robust relationship. Thus, in this study, we have settled for
219 water level data only. However, stage and discharge are closely linked in river hydrology:
220 variations in water stage give indications of variations in discharge, i.e. the lower the
221 river stage the lower is the discharge and conversely (Herschy, 1995). Thus, in this study,
222 we considered that the highest water level recorded in spring during snowmelt
223 corresponds to the annual peak flow.
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224 The OBS 3+ uses its sideways facing optics to emit a near infrared light beam
225 into the water. It then measures the light that bounces back from the suspended particles
226 in the water. The sensor head was cleaned at every site visit while servicing the station to
227 prevent accumulation of algae that could have invalidated the data (Schoellhamer 1993).
228 In fact, we never observed any algae on the sensor. It is assumed that sediment particle
229 colour and reflectivity are not significant factors affecting sensor accuracy (Schoellhamer
230 and Wright 2003). However, besides the choice of the turbidity range by the operator, the
231 color and reflectivity of sediment are unique for each river and consequently, no absolute
232 comparison with NTU curves in other rivers is possible. 233 When there is an ice cover,Draft pressure and roughness of the ice and snow cover 234 make the sensors readings inaccurate. Some sections of the Sheldrake River are less than
235 1 m deep and should be frozen to the bed in winter time. Thus, in this study, the winter
236 flow is considered as negligible. Curves of water stage and turbidity start with the break
237 up and end with the first signs of ice on the river when air temperature drops under 0 ⁰C
238 for several consecutive days.
239 Water analyses were made by the Laboratoire de l’INRS, Centre Eau, Terre,
240 Envionnement and by the Environex laboratory in Quebec City. Total suspended solids
241 (TSS) were measured from filtration through a 0.45 m filter and weighting after drying.
242 Air temperature and precipitation data were provided by an automated regional
243 meteorological station, operated by the Centre d’études nordiques, located in Tasiapik
244 valley, at 125 m above sea level (a.s.l.), near the village of Umiujaq 8 km south of the
245 Sheldrake River (See Fig. 1 for location) (CEN 2014). Through the area, the altitudinal
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246 gradient is small (coastal range: < 400 m a.s.l.; inland Canadian Shield: ̴ 200 to 250 m
247 a.s.l.) whereas vegetation cover types and variable snow cover conditions maintain soil
248 surface temperatures within a limited range around 0 °C (Ménard et al. 1997). Similarly,
249 convective precipitation are highly improbable within such a small catchment with low
250 relief amplitude.
251 Sheldrake Lake, where the Sheldrake River originates, has not been included in
252 the delineation of the Sheldrake River catchment area for this study. The lake is a
253 sedimentation basin which receives sediments from its surrounding catchment; which has
254 an estimated area of 91 km 2. We found no visible signs of erosion on the lake catchment, 255 as it is surrounded by wetlands and Draftbedrock outcrops (Jolivel and Allard 2013). Thus, the 256 suspended sediment released by the lake as a potential cause of turbidity of the Sheldrake
257 River is considered as negligible. This is confirmed by the clarity of the water of the lake
258 outlet over which we flew each summer. However, water input coming from the
259 Sheldrake Lake accounts for an unmeasured fraction of the Sheldrake River flow.
260 Spring flood
261 The freshet flood event is clearly visible on hydrographs (Fig. 5). We estimated its
262 starting date as the period during snowmelt time (several consecutive days) when the
263 water level curve starts to rise after being stable and low during winter. The end of
264 snowmelt time is associated with the resumption of river flow to its summer stage and
265 when air temperature remains above 0°C. For each year separately, we estimated this
266 summer stage by calculating average water stage in July and August. In 2014, water level
267 data ended on 4 October, the date of last visit on site for downloading the datalogger.
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268 Turbidity/Total Suspended Solids calibration
269 Our interpretations of turbidity events over time and with climate parameters use the
270 NTU values. Nevertheless an attempt to approximately assess the concentration of
271 suspended sediment was made by calculating a NTU TSS relationship:
272 TSS = 0.46NTU + 2.457 (1)
273 with r2= 0.88 , p < 0.001 , n =22. Due to the remoteness of the site, only three samples in
274 the calibration data were obtained under conditions of relatively high TSS concentration
275 (≥14 mg.L 1) (Fig. 6). The NTU/TSS relationship was used here only to provide a crude
276 estimate of sediment concentration since no TSS samples were recovered at very high
277 NTU and no robust discharge data areDraft available. However, the correlation NTU/TSS with
278 this instrument is reported to be linear up to 4000 NTU (Downing 2006). This is
279 particularly true for fine grained sediments, such as glaciomarine silt and clay (Lewis
280 1996). In this study, the spectrum of calibration NTU/TSS only covers the range 3 15
281 NTU; maximum turbidity peaked at 160 NTU in summer 2010. Thus, we assume that the
282 correlation NTU/TSS is linear all along the range covered by this study (3 160 NTU).
283 The cable linking the submerged instrument and the datalogger onshore was twice
284 severed by ice, and therefore complete summer coverage of turbidity was obtained only
285 in 2010, 2013 and 2014.
286 As 85% of the catchment is covered by silty clay and numerous pools in bedrock and
287 reaches on boulder beds very likely prevent or limit bed load transport, turbidity of the
288 Sheldrake River is considered as a reliable indicator of sediment transport.
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289 Both water level and turbidity data are expressed as mean daily values. This time unit
290 also corresponds to the time unit used to calculate the soil thaw regime, i.e. degree days.
291 Active layer depth
292 The rate of active layer thawing during a summer is key to many erosional and
293 sediment releasing processes. Downward migration of the thaw front is rapid at the
294 beginning of the season, following Stefan’s equation of heat transfer with phase change.
295 Typically, the pace of thawing is fast in June and July and it slows down towards the end
296 of summer. Cryoturbation during thaw brings loose sediments to the soil surface in the
297 center of frostboils (Mackay and Mackay 1976; Egginton and Dyke 1982), which are 298 extensive in the studied catchmentDraft on lithalsas and permafrost plateaus. Generation of 299 high pore pressures at the thawing front when the thawing rate is fast is the main trigger
300 mechanism of active layer detachment slides (Lewkowicz and Harris 2005b). Melting of
301 residual snowbanks in gullies is faster during warm summer spells, generating erosion
302 (Jolivel and Allard 2013). Therefore, we considered the air temperature curve, the
303 cumulative curve of thawing degree days and the derived curve of thaw front progression
304 as the best integration of controlling variables of the geomorphic processes that
305 potentially release sediments in summer.
306 To evaluate the thermal regime of the active layer in 2010, 2013 and 2014, a one
307 dimensional heat transfer model, TONE was used (Goodrich 1978). This numerical
308 model is widely used for active layer and permafrost modelling with good results and
309 allows researchers to simulate the evolution of the active layer under different
310 environmental conditions (Riseborough et al. 2008; Bouchard 1990; Zhang et al. 2008;
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311 Barrette 2010; Larouche 2010). The thermal model is driven by cumulative thawing
312 degree days gathered from the meteorological station. Clay and silt thermal parameters of
313 the model’s PRAM routine with a water content of 20% and only one stratigraphic layer
314 were applied in the simulations as in other studies in the region (Buteau et al. 2004).
315 Grain size composition is rather homogenous throughout the clay soils of the region
316 (Calmels 2005). The sediment is composed of fine silt and clay deposited in the Tyrrell
317 sea during deglaciation. N factors measured by Ménard et al. (1998) in the region were
318 applied to account for the buffer effect of snow cover and vegetation between the
319 atmosphere and the soil surface. They measured a thawing n factor of 1.17 and a freezing
320 n factor of 0.64 for a typical lithalsa composed of postglacial silt with a lichen cover.
321 Those values are therefore generallyDraft applicable to most of the permafrost patches and
322 landforms across the Sheldrake River catchment. Soil temperatures profiles were
323 simulated at 10 cm intervals (model node spacing).
324 Once the soil temperatures for 2010, 2013 and 2014 were calculated with the
325 model, a piecewise cubic hermite interpolating polynomial (PCHIP) allowed estimations
326 of the depth of the thaw front according to the calculated depth of the 0 ⁰C isotherm over
327 the thawing period (L'Hérault 2009). The downward thaw of the active layer was
328 simulated for the summers of 2010, 2013 and 2014, i.e. from 1 May to 1 October, which
329 is considered for this study as the day of the maximum depth reached by the 0°C
330 isotherm.
331 Results
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332 Seasonal variations of water stage
333 The breakup generally occurs in late April or early May. The freshet high stages,
334 i.e. between break up and drawdown to summer stage, generally last about 8 weeks,
335 except in 2014 (5 weeks) which had also the earliest hydrological summer condition
336 recovery date of the six years of the study period (Table 1). The highest water stage,
337 corresponding to the annual discharge peak, generally occurs between end of May and
338 mid June, except in 2013 where the river flow was maximum on 3 May (Table 1, Fig. 5).
339 The Sheldrake River reaches its summer flow regime at the end of June or at the
340 beginning of July. Starting in late August, successive rainfall events generate an increase 341 in water flow until the river freezes.Draft 342 Turbidity regime over the thawing season
343 Turbidity variations during the snowmelt period
344 In 2010, the snowmelt period lasted from 25 April to 17 June (Table 1, Fig. 7).
345 During this period, daily NTU values varied between 5 and 15 and peaked at 22 on 6
346 May (Fig. 7). This value was reached after five days with average daily air temperatures
347 fluctuating around 0°C. No significant rain event occurred and the active layer began to
348 thaw only after this date, i.e. from 8 May onwards (Fig. 8). During the spring flood,
349 turbidity values did not increase significantly. However, in early June, five days after the
350 maximum water stage of the freshet flood, turbidity increased and stayed at a level of ~7
351 NTU, compared with a mean 4 NTU in May. From 25 April to 17 June, only two days of
352 light rain were recorded (19 May: 4 mm ; 29 May: 2 mm) (Fig. 7).
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353 In 2013, the period of high water levels associated with the snowmelt period
354 lasted 59 days and ended at the end of June (1 May to 28 June) (Table 1, Fig. 9). Over
355 this period, only 6 days of light rain totaled less than 40 mm. Daily NTU values generally
356 varied between 3 and 20 NTU. Contrary to 2010, the highest water stages were associated
357 with a higher turbidity, ranging from 14 to 20 NTU from 1 May to 5 May. During the
358 first half of May 2013, two major peaks of turbidity occurred. The first one (average 18
359 NTU from 1 to 5 May) was correlated with the peak of the freshet flood and followed the
360 first rain of the year on 28 29 May (25 mm of rain). The second peak of turbidity was of
361 higher amplitude (average 80 NTU from 11 to 14 May, with a peak of 135 NTU reached
362 on 12 May) and was independent of rain and river stage. This peak coincides in time with
363 the fast initial thawing of soils to a depthDraft of about 15 cm (Fig. 10).
364 From 8 to 12 June 2013, turbidity averaged at 59 NTU, with a peak at 80 NTU on
365 10 June. Before and during this period, no significant rain was recorded (4 mm on 10
366 June). However, it occurred simultaneously with a significant increase in air temperature
367 (+15 °C) and a rapid deepening of the active layer (Figs. 9 and 10). It was also correlated
368 with the onset of the period when air temperature maintains itself continuously above
369 0°C, as shown by the increase of the slope of the degree days curve (Fig. 10). No change
370 in precipitation and thus, in water stage was noticed.
371 In 2014, the snowmelt period lasted 42 days, from 4 May to 14 June, which is
372 considerably shorter than in the other years (Table 1, Fig. 11). During this period,
373 turbidity averaged 8 NTU and again, no significant rain occurred. However, two peaks of
374 turbidity were recorded on 20 May (17 NTU) and on 24, 25 and 26 May (respectively 32,
375 29 and 21 NTU). As during the 2013 spring high water levels, pulses of high turbidity
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376 corresponded with relative high river stage and coincided with rapid and short increases
377 of the active layer depth (Figs. 11 and 12). Meanwhile, air temperatures increased from
378 1°C on 19 May to 11°C on 22 and 23 May, accelerating snowmelt, and the thaw front
379 quickly reached a depth of ̴ 30 cm.
380 Turbidity variations during summer
381 In 2010, the Sheldrake River water level lowered to reach its mean summer level
382 on 17 June (Table 1, Fig. 7). Before that date, as seen above, snowmelt driven high levels
383 did not strongly influence the turbidity. Whereas the period of higher river stage and thus 384 maximum discharge occurred duringDraft the spring freshet, the turbidity in the river reached 385 its summer maximum in July. Indeed, between 30 June and 23 July 2010 (24 days), NTU
386 values averaged at 83 with a peak at 160 on 10 July, while the rest of the 2010 thawing
387 period registered an average turbidity of only 4.2 NTU (Fig. 7, Table 2). The 2010
388 turbidity maximum was reached on 10 July following several days of light rain (Fig. 7).
389 On the other hand, the NTU curve rises three days following the increase of the average
390 daily air temperatures from 0 5 °C up to above 10 °C for the rest of the turbidity period
391 (Fig. 7). The turbidity peak also occurred at a time when the thaw rate was the fastest in
392 the soil (Fig. 8).
393 In 2013, summer river stage was reached on 28 June (Table 1, Fig. 9). An
394 increase in turbidity similar to 2010 occurred in midsummer but with a lesser magnitude.
395 The general period of high turbidity lasted 19 days, i.e. 15 July to 2 August, with a daily
396 average NTU of 33, while the rest of the thawing period registered an average turbidity of
397 only 8.2 NTU (Table 2). During this period, 10 days with light to moderate rain were
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398 registered (4 to 13 mm/day). The two main peaks of turbidity, on 17 July and 29 July,
399 were associated with two (10 mm) and three (25 mm) rainy days, respectively. As a
400 consequence the higher water levels, and thus discharge, were also correlated with
401 turbidity and generated small peak events (Fig. 9).
402 In 2014, several pulses of turbidity were recorded in June, e.g. 5 June with 26
403 NTU, 12 June with 18 NTU and from 23 to 27 June, with an average NTU of 31 and a
404 peak of 64 NTU reached on 26 June. On 12 June, the turbidity pulse was associated with
405 an increase in air temperature from 9 to 21°C and very light rain (2 mm). The two other
406 episodes were not associated with rain or significant changes in air temperatures. 407 However two days with air temperaturesDraft above 15°C preceded the turbidity pulse which 408 occurred from 23 to 27 June (Fig. 11).
409 In early July 2014, a turbidity increase, with a maximum of 19 NTU on 4 July,
410 was recorded during a rainy and warm week (74 mm of rain between 28 June and 5 July)
411 (Figs 11 and 12). Then, as in 2010 and 2013, a period of high turbidity, distinctive from
412 the rest of the thawing season, occurred in midsummer, from 12 July to 28 July 2014
413 (Fig. 11). During these 17 days, average turbidity was 72 NTU, with a peak of 141 NTU
414 on 20 July, discharge was low and stable and air temperatures fluctuated between 7 and
415 21°C (Table 2). The onset of this event was associated with an increase in air temperature
416 from 8 to 13°C and light rain (5 mm) two days before the increase in NTU. However,
417 from 20 to 24 July, 55 mm of rain were recorded and coincided with the maximum of
418 turbidity on 20 July (Fig. 9).
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419 In 2010, 2013 and 2014, small peaks of turbidity that occurred in August were
420 directly correlated with precipitation and river levels (Figs. 7, 9 and 11). In autumn, air
421 temperatures decreased until freeze up and no turbidity event like those recorded in
422 spring and midsummer occurred despite an elevation of water stage and thus an increase
423 of discharge. several peaks of turbidity unrelated to high flows are noticeable on the
424 hydrographs.
425 Fig. 13 shows relationship between turbidity and water level and between
426 turbidity and air temperature. We selected NTU values <10 in order to remove small
427 peaks caused by light rains. Thus, there exists no correlation between turbidity and water 428 stage. In 2013 and 2014, there appearsDraft to be a positive relation between turbidity and air 429 temperature whereas 2010 showed no trend (Fig. 13).
430 The major turbidity period occurred when the thaw front reached 40% of its
431 maximum end of summer depth in 2010, 60% in 2013 and 64% in 2014 (Figs. 8, 10 and
432 12). In July 2010, the high turbidity period coincided with the steepest part of the
433 cumulative thawing degree days curve, i.e. when the thaw front was still rapidly
434 penetrating in the ground (Fig. 8). In 2013, except for two short but intense pulses of
435 suspended sediment in May and June, the major period of high turbidity occurred in mid
436 July early August as the thaw front was progressing at a sustained high rate (Fig. 10). In
437 2014, during the second part of July, turbidity increased significantly and dominated the
438 thawing season, also coinciding with an increase of thaw rate and a period of faster rise in
439 the cumulative degree days (Fig. 12). Except for some small peaks of turbidity induced
440 by rain events, NTU values generally stayed low in September and October as
441 temperatures fell and the thaw front tended to stabilize before freeze back.
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442 Discussion
443 Hydrological regime of a typical subarctic river
444 The flow regime of the Sheldrake River is typical of those of the subarctic
445 environment and is closely linked to precipitation and air temperature (Déry et al. 2005).
446 Water stages and relative water flow are high at the end of spring (snowmelt); they rise
447 again in autumn as heavy rains precede the coming of winter, contrasting with low flow
448 in summer (July and August) and practically no flow in winter. Snowmelt runoff is
449 clearly the dominant hydrologic event, surpassing peaks generated by rain event, as 450 observed in other subarctic catchmentsDraft (Déry et al. 2005; Jing and Chen 2011). In details, 451 the magnitude and timing of water level fluctuations varied significantly each year and
452 from one year to another. For example, the highest water stage reached 366 cm in 2012
453 but only 313 cm in 2010 and 2011. Peak flow recession at the recording station does not
454 show any irregularity that could have arisen due to some disturbing event such as a
455 temporary ice jam up river. Moreover, during our early summer visits, we never
456 observed any signs of ice jams (remaining ice blocks and boulder barricades, wood debris
457 on banks) along the river.
458 The Great Whale River (GWR), located 150 km south of the Sheldrake River, is
459 the closest river where a similar study was done previously, allowing some broad
460 qualitative comparisons on the spring high water stage (Hudon et al. 1996). GWR is the
461 only other river with a functioning gauging station in the region (operated by the Centre
462 d'Expertise Hydrique du Québec). Comparisons are only indicative since the hydro
463 geomorphological regime of the GWR is not studied and its catchment is considerable.
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464 GWR basin is 43,200 km 2 and its mean annual discharge is 600 m 3 s 1 (Déry et al. 2005;
465 Hülse and Bentley 2012). Compared to the GWR, the highest flow in Sheldrake River
466 occurred generally one to two weeks later (Table 3), average annual air temperatures
467 being 1°C colder in the Sheldrake River area than in the GWR area, which delays the
468 freshet (Environment Canada 2013).
469 An exception occurred in 2013 when the highest water stage of the Sheldrake
470 River preceded the one of the GWR by 18 days. Five days before the peak flow of the
471 Sheldrake River, an early spring storm (25 mm of rain and daily air temperatures of 5°C)
472 occurred in the Umiujaq area. The meteorological station situated at the mouth of the 473 GWR, in Kuujjuarapik, registered Draft a coincident warm spell in late April 2013 but with 474 lighter rain than in the Sheldrake River area (14 mm of rain spread over 3 days)
475 (Environment Canada 2013). This comparison between the Sheldrake River and the
476 GWR should be interpreted with caution however.
477 In periglacial regions, runoff rates and discharge response time tend to wane over
478 the summer period as snow depletes, seasonally frozen soils thaw and the active layer
479 increases in thickness over permafrost areas, allowing water infiltration and soil water
480 storage (McNamara et al. 1998; Kane et al. 2000; Carey and Woo 2001; Hinzman et al.
481 2003; Woo et al. 2008). For the Sheldrake River, the snowmelt and the thawing depth in
482 soils (permafrost or not) control the seasonal trend of the river flow. In spring,
483 precipitation has only a limited and temporary influence on river stage and discharge.
484 Then, after the freshet high water stages, our hydrographs and field observations suggest
485 that summer river flow depends on both rain storm frequency and intensity and water
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486 flow to the main stream after a short delay by retention in soils, ponds, lakes and
487 wetlands, as observed in southern temperate Québec (James and Roulet 2009).
488 Rain generally stops at the end of October and is replaced by snow when air
489 temperature drops under 0 ⁰C. However, some variations of discharge still occur in the
490 first half of November. The presence of the Hudson Bay cools the coastal zone, whereas
491 temperature remains warmer inland (Payette and Rochefort 2001; Bhiry et al. 2011;
492 Jolivel and Allard 2013). Autumn rains inland likely raise the level of the Sheldrake Lake
493 therefore sustaining a relatively high flow in the river until freeze up. But turbidity does
494 not increase during that autumn period of high flow, suggesting that no erosion is 495 generated in soils and on slopes in theDraft catchment.
496 Turbidity and sediment transport
497 Turbidity and sediment transport during spring high water levels
498 In rivers flowing in permafrost free watersheds or in Arctic catchments with
499 permafrost, transport of sediment is generally a function of discharge and dependent on
500 rainfall and on the amount of snow accumulation during the previous winter (Kriet et al.
501 1992; Hudon et al. 1996; Braun et al. 2000; Forbes and Lamoureux 2005). However, in
502 the Sheldrake River, during the spring, several peaks of turbidity unrelated to high flows
503 are noticeable on the hydrographs. The absence of correlation between turbidity and
504 water stage (Fig. 13) clearly demonstrates that the hydrological regime is not the
505 dominant factor of suspended sediment transport in the catchment. However the rate of
506 soil thawing driven by cumulative warming (here expressed as degree days) stands out as
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507 a very probable driver of turbidity. Otherwise, there is considerable differences of water
508 level from one year to another, reflecting an important interannual variability. For
509 example, in 2010 and 2014, the spectrum of turbidity was broader than in 2013 which
510 showed larger variation of water level. Even if we can not totally exclude a slight
511 displacement of the mooring during precedent breakup, this further suggests that water
512 level of the Sheldrake River is linked to winter snow accumulation in the catchment
513 and/or the level of the Sheldrake Lake in Spring.
514 In winter, the frozen soils, the snow cover and the absence of runoff impede
515 overland flow and soil erosion as well as river flow and bank erosion (Hudon et al. 1996; 516 Burn et al. 2004). In spring, the riseDraft in solar radiation and above freezing air temperatures 517 rapidly melt the snow cover and result in an increase in water flow and transport (Hudon
518 et al. 1996). However, during the freshet flood, a relatively low turbidity indicates that
519 transport of TSS likely remains low, because some channels stay armored with ice and
520 snow and soils are still frozen, as observed in other catchments in the Arctic and
521 Subarctic (McDonald and Lamoureux 2009).
522 Inter annual fluctuations of turbidity are significant and can be partly explained
523 by the length of the snowmelt high flow period, which was ̴ 2 weeks shorter in 2014 than
524 in the five other years.
525 In May and June 2013 and 2014, i.e. during high water levels, 3 4 days pulses of
526 sediment were recorded independently from temporary increase in water stage. These
527 short events are likely due to episodic landslide activity, as observed in Arctic catchments
528 (Lewis et al. 2005; Lamoureux and Lafrenière 2009). The increase in air temperature
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529 causes rapid deepening of the active layer and trigger shallow active layer landslides
530 along the river banks. Such mass movements are widespread in the Sheldrake catchment
531 (Jolivel and Allard 2013). Such an event occurred in spring 2010 (Fig. 14) before 15 June
532 (date of a visit in the field). This landslide was likely responsible of the turbidity peak
533 recorded on 6 May 2010, even if no direct correlation was made in the field, the precise
534 date of the landslide occurrence being unknown.
535 The mid-summer turbidity period
536 Mid summer is the time of the year when frostboils on clay and silt rich soils are
537 reactivated by excess pore water pressure caused by soil thawing, bringing to the surface 538 new sediments that can be washed Draftaway on the slopes of the lithalsas by surface runoff 539 (Shilts 1978; French 2007) (Fig. 15A). In the Sheldrake River catchment, where 1100
540 lithalsas covering a cumulated area of 3 km 2 were inventoried in 2009 (Jolivel and Allard
541 2013), this runoff is shown by the milky colour of thermokarst ponds (Fig. 15B). During
542 rain events, thermokarst ponds overflow into channels and feed the fluvial system (Jolivel
543 and Allard 2013). Moreover, rapid thawing of the active layer causes small active layer
544 detachment failures on the side slopes of palsas and lithalsas releasing silt and clay into
545 tributary streams (Jolivel and Allard 2013). Larger slides occur along gullies and
546 immediately along river banks (Fig. 15C and 15D); activity and contribution of gullies to
547 the fluvial load are confirmed by the presence of small deltas at the confluence with
548 streams or with the Sheldrake River during low stages (Fig. 15E). Simple observations of
549 rivers during helicopter flights in the silt rich permafrost dominated region east of
550 Hudson Bay also reveals the summer high turbidity of small rivers flowing into Hudson
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551 Bay or as tributaries of major rivers such as the Nastapoka River, suggesting that the
552 processes may be similarly over the wider region.
553 Precipitation recorded during the three mid summer turbidity periods generate
554 surface runoff and facilitate the washout of the thaw released sediments as shown by the
555 time correspondence of small turbidity peaks with rain events. Indeed, hot summer
556 temperature with moderate rains can trigger earth flows, active layer detachments slides
557 and gully erosion, delivering considerable volume of sediment into Subarctic fluvial
558 systems, while overland flow can also simply wash out sediments released by soil
559 thawing and thermokarst (Lamoureux 2000; Lewkowicz and Harris 2005a; Lamoureux 560 and Lafrenière 2009; Lewis et al. 2012).Draft
561 After the mid summer maximum, turbidity rapidly comes back to lower values.
562 Sediment releasing and periglacial erosion processes slow down as the thaw front nearly
563 stabilizes near the base of the active layer over permafrost areas and when soils are
564 finally thawed in the permafrost free areas of the catchment. Finally, in October, when air
565 temperatures get closer to 0°C, turbidity comes back to very low values despite higher
566 discharge than in midsummer. Furthermore, the well defined turbidity concentration
567 relationship suggests that particle size does not change significantly with variable flow
568 rates. This confirms that the eroded soils are well sorted and fine grained, i.e., typically
569 belonging to the ice rich marine silty clays, characterized by their homogeneity through
570 the catchment (Calmels 2005) and currently affected by thermokarst.
571 Implications for sediment transport
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572 Daily turbidity peaks recorded during the three summers corresponded to
573 sediment concentrations of 76 g m 3 in 2010, 29 g m 3 in 2013 and 67 g m 3 in 2014. SSC
574 reported in the Sheldrake River is far less than SSC measurements made in small Arctic
575 catchments immediately downstream from large mass movements. For example, hourly
576 SSC reached 60,800 g m 3 in a small creek on Ellesmere Island (Lewis et al. 2005).
577 Short lived peaks up to 800 g m 3 were recorded in a small creek on Melville Island
578 (Lamoureux and Lafrenière 2009) and in rivers on the Peel Plateau (Kokelj et al. 2013)
579 downstream of large active layer slides and retrogressive thaw slumps. Pulses of TTS
580 generally occur within two or three days of landslides (Lewis et al. 2005; Lamoureux and
581 Lafrenière 2009). However, those conspicuous processes also occur in summer and are
582 triggered by soil thawing under warmingDraft temperature. In the low relief and discontinuous
583 permafrost of the Sheldrake River catchment, the thawing processes are smaller in scale
584 and more diffusively scattered in the landscape. A dilution effect associated with the
585 contribution of low SSC water from the outlet of Sheldrake Lake should also be
586 mentioned.
587 The 24 days period in 2010, the 19 days period in 2013 and the 17 day periods of
588 sustained high turbidity recorded in 2014 in the Sheldrake River catchment are likely
589 related to frostboil activity, many small soil instabilities, landslides on permafrost slopes
590 and along riverbanks, and to erosion of gullies in the catchment. In this periglacial
591 environment, all those mass wasting processes are driven by the rate of summer warming
592 and thaw of the active layer especially in the ice rich permafrost in marine silty clays.
593 Conclusion
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594 Our results indicate that the sedimentary regime of a river that drains geosystems and
595 ecosystems on fine grained permafrost, such as the Sheldrake, can in large part be
596 affected by summer thaw rates in the active layer and associated thermokarst processes.
597 The annual peak flow of the Subarctic Sheldrake River shown by the annual highest
598 water stage typically occurs during the snowmelt period. However large pulses of
599 suspended sediments of a few weeks of duration occur in mid summer, during the
600 steepest part of the cumulative thawing degree days curve. Light rains in summer also
601 drive the flow regime that assumes transport of the suspended sediment load during the
602 high turbidity period. Indeed, in 2010, 2013 and 2014 the period of high turbidity lasted
603 for ̴ 3 weeks and occurred in July when warm summer air temperatures transferred heat
604 into the active layer and provoked theDraft rapid penetration of the thaw front. This is likely a
605 time of the year when frost boils on permafrost and active layer detachment failures on
606 the slopes of palsas and lithalsas are the most active, releasing fine grained sediments that
607 are rain washed at the soil surface and transported to the ponds, gullies and stream
608 system. Thaw front penetration in steep river banks likely also favors slumping and
609 sediment release. We suggest that these mid summer episodes of sediment transport that
610 correspond with warm days and light rains carry a large part of the annual suspended
611 sediment load although further more investigations are needed to quantify these summer
612 exports. This is important because it would imply that maximum sediment transport in an
613 active thermokarst region dominated by fine grained soils may be primarily thermally
614 driven rather than hydrologically driven. More intensive observations within the
615 catchment will help better appraise the scale and extent of the thaw related sediment
616 delivery processes that are diffusely distributed over the territory.
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617
618 Acknowledgements
619 Climate data to support this article are from the Centre d'études nordiques. This
620 work received financial support from grants to M. Allard from ArcticNet and the Natural
621 Science and Engineering Research Council of Canada. The Centre d’études nordiques of
622 Université Laval provided important logistical support. We thank Denis Sarrazin for the
623 installation and maintenance of the gauging station, Marc André Ducharme for field
624 assistance, and Sarah Aubé Michaud for help with the Tone model. The comments of 625 Mickael Lemay, Daniel Fortier, PatrickDraft Lajeunesse and Guillaume St Onge greatly 626 improved the manuscript. We are also grateful to the Inuit community of Umiujaq for its
627 generous hospitality. We finally thank the two anonymous reviewers.
628
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788 Lantz, T.C., and Kokelj S.V. 2008. Increasing rates of retrogressive thaw slump activity 789 in the Mackensie Delta region, N.W.T.. Geophysical Research Letters. 35 : L06502. 790 doi:10.1029/2007GL032433.
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794 Lavoie, C., Allard M., and Duhammel, D. 2012. Deglaciation landforms and C 14 795 chronology of the Lac Guillaume Delisle area, eastern Hudson Bay: A report on field 796 evidence. Geomorphology. 159-160 : 142 155. 797 798 Lesack, L.F.W., Marsh, P., Hicks F.E., and Forbes D.L. 2014. Local spring warming 799 drives earlier river ice breakup in a large Arctic delta. Geophysical Research Letters. 41 : 800 1560–1566. doi: 10.1002/2013GL058761. 801 802 Lévesque, R., Allard M., and Seguin M.K. 1988. Le pergélisol dans les formations 803 quaternaires de la région des Rivières Nastapoka et Sheldrake, Québec Nordique. 804 Collection Nordicana Centre d'Études Nordiques 51 . 805 806 Lewis, J. 1996. Turbidity controlled suspended sediment sampling for runoff event load 807 estimation. Water Resources Research. 32 : 2299 2310. 808 809 Lewis, T., Braun, C., Hardy, D.R., Francus, P., and Bradley R.S. 2005. An extreme 810 sediment transfer event in a Canadian high arctic stream. Arctic, Antarctic and Alpine 811 Research. 37 : 477 482.
812 Lewis, T., Lafrenière, M.J., and Lamoureux, S.F. 2012. Hydrochemical and sedimentary 813 responses of paired High ArcticDraft watersheds to unusual climate and permafrost 814 disturbance, Cape Bounty, Melville Island, Canada. Hydrological Processes. 26 : 2003 815 2018. 816 817 Lewkowicz, A.G., and Harris, C. 2005a. Frequency and magnitude of active layer 818 detachment failures in discontinuous and continuous permafrost, Northern Canada. 819 Permafrost and Periglacial Processes. 16 : 115 130. doi: 10.1002/ppp.522.
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851 Ménard, É., Allard, M., and Michaud, Y. 1998. Monitoring of ground surface 852 temperatures in various biophysical micro environments near Umiujaq, eastern Hudson 853 Bay, Canada. Seventh International Conference on Permafrost, Collection Nordicana, 854 Yellowknife, Canada, 723 729. 855 856 Mesquita, P.S., Wrona F.J., and Prowse,Draft T.D. 2010. Effects of retrogressive permafrost 857 thaw slumping on sediment chemistry and submerged macrophytes in Arctic tundra 858 lakes. Freshwater Biology. 55 : 2347 2358. doi:10.1111/j.1365 2427.2010.02450.x.
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864 Pissart, A. 1985. Pingos et palses: un essai de synthèse des connaissances actuelles. Inter 865 Nord. 17 : 21–32. 866 867 Pissart, A. 2002. Palsas, lithalsas and remnants of these periglacial mounds. A progress 868 report. Progress in Physical Geography. 26 : 605–621. 869 870 Prowse, T. , Bring , A., Mård, J., Carmack, E., Holland , M., Instanes , A., Vihma T., and 871 Wrona, F.J. 2015 . Arctic Freshwater Synthesis: Summary of key emerging issues . Journal 872 of Geophysical Research Biogeosciences. 120 : 1887 –1893. 873 874 Quinton, W.L., and Carey, S. K. 2008. Towards an energy based runoff generation theory 875 for tundra landscapes. Hydrological Processes. 22 : 4649–4653. doi: 10.1002/hyp.7164. 876 877 Riseborough, D., Shiklomanov, N., Etzelmüller, B., Gruber, S., and Marchenko, S. 878 2008. Recent Advances in Permafrost Modelling. Permafrost and Periglacial Processes. 879 19 : 137 156. 880
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881 Schuur, E.A.G., Bockheim, J., Canadell, J.G., Euskirchen, E., Field, C.B., Goryachkin, 882 S.V., Hagemann, S., Kuhry, P., Lafleur, P.M., Mazhitova, G., Nelson, F.E., Rinke, A., 883 Romanovsky, V.E., Shiklomanov, N., Tarnocai, C., Venevsky, S., Vogel J.G., and 884 Zimov, S.A. 2008. Vulnerability of Permafrost Carbon to Climate Change: Implications 885 for the Global Carbon Cycle. BioScience. 58 : 701 714. doi: 886 http://dx.doi.org/10.1641/B580807. 887 888 Schoellhamer, D.H. 1993. Biological interference of optical backscatterance sensors in 889 Tampa Bay, Florida. Marine Geology. 110 : 303 313. 890 891 Schoellhamer, D.H., and Wright, S.A. 2003. Continuous measurement of suspended 892 sediment discharge in rivers by use of optical backscatterance sensors, in, Bogen, J., 893 Fergus, T., and Walling, D.E. (eds), Erosion and Sediment Transport Measurement in 894 Rivers, Technological and Methodological Advances: International Association of 895 Hydrological Sciences Publication 283, pp. 28 36. 896 897 Shilts, W.W. 1978. Nature and genesis of mudboils, central Keewatin, Canada. Canadian 898 Journal of Earth Sciences. 15 : 1053 1068. 899 900 Sollid, J.L., and Sorbel, L. 1998. Palsa bogs as a climate indicator Examples from 901 Dovrefjell, Southern Norway. Ambio.Draft 27 : 287 291.
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Table legends
Table 1 Features of the period of spring high water stage of the Sheldrake River from 2009 to 2014.
Table 2 Comparison between the summer periods of turbidity in 2010, 2013 and in 2014.
Table 3. Comparison of the date of the peak flow in the Great Whale River and in the Sheldrake River, from 2009 to 2014. For the Sheldrake River, the date of the peak flow corresponds to the day where the annual highest water stage was recorded.
Figure legends
Figure 1 Location of the study area and photograph of the Sheldrake River flowing through an area of permafrost plateaus (foreground), and finally flowing to Hudson Bay across rock outcrops. VDT indicates the location of the meteorological station, in the Vallée des Trois (also called Tasiapik) Figure 2 Two tributaries of the Nastapoka River. Note the high turbidity of the streams reflecting high thermokarst activity in catchments. Draft Figure 3 The Sheldrake River catchment with the distribution of surficial deposits, the position of the tree line and the organization of the drainage network. Sheldrake Lake does not appear on this map (see Fig.1 for location).
Figure 4 Schematic representation of the positioning of the gauging station and photograph of its location on a bank of the Sheldrake River. Figure 5 Hydrographs for 2009, 2010, 2011, 2012, 2013 and 2014. Water level record starts with the first signs of the break up (see in text) and end at mid November (except for 2013 and 2014, due to lack of data).
Figure 6 Relationship between turbidity (NTU) and total suspended sediment (TSS) (g m3).
Figure 7 Water level and turbidity regime of the Sheldrake River with air temperature and rainfall events in 2010: rain (mm d 1), air temperature (°C), water level (cm) and turbidity (NTU). Data are presented on a daily basis.
Figure 8 Turbidity (NTU), thaw front depth (cm), rain (mm d 1) and cumulative degree days ≥ 0 ⁰C from 1 May to 1 October 2010.
Figure 9 Water level and turbidity regime of the Sheldrake River with air temperature and rainfall events in 2013: rain (mm d 1), air temperature (°C), water level (cm) and turbidity (NTU). Data are presented on a daily basis.
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Figure 10 Turbidity (NTU), thaw front depth (cm), rain (mm d 1) and cumulative degree days ≥ 0 ⁰C from 1 May to 1 October 2013.
Figure 11 Water level and turbidity regime of the Sheldrake River with air temperature and rainfall events in 2014: rain (mm d 1), air temperature (°C), water level (cm) and turbidity (NTU). Data are presented on a daily basis.
Figure 12 Turbidity (NTU), thaw front depth (cm), rain (mm d 1) and cumulative degree days ≥ 0 ⁰C from 1 May to 1 October 2014.
Figure 13 (A) Relationship between turbidity and air temperature; (B) Relationship between turbidity and water stage. One point represents one day during the thawing season of 2010, 2013 and 2014. Days when turbidity was less than 10 NTU were excluded to limit the effects of light rain. The regression line and the coefficient of correlation represent only turbidity values ≥10 NTU. Figure 14 Typical landslide on the bench of the Sheldrake River. The landslide probably triggered on 6 May 2010 and caused a peak of turbidity. Figure 15 Thermokarst as source of suspended sediment in the Sheldrake River catchment (A): active and flowing frostboilDraft on the slope of a permafrost plateau (1 meter length) (photo: Denis Sarrazin); (B) : Typical thermokarst ponds, the strong turbidity is caused by frostboils activity and surface runoff; (C) : Network of gullies between permafrost plateaus (Jolivel and Allard, 2013); (D): typical landslide on a permafrost bank along the Sheldrake River; ( E) : small delta at the confluence between a thermokarst gully and the Sheldrake River.
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Table 1. Features of the period of spring high water stage of the Sheldrake River from 2009 to 2014.
Start/end Length Peak of the Highest snowmelt high snowmelt freshet water water stage (date) (number of days) flood (date) stage (cm)
2009 16 May/10 July 56 14 June 333 2010 25 April/17 June 54 27 May 313 2011 12 May/2 July 52 5 June 313 2012 4 May/1 July 59 7 June 366 2013 1 May/28 June 59 3 May 331 2014 4 May/14 June 42 28 May 338 Average 6 May/27 June 53 30 May 332
Draft
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Table 2. Features of the mid-summer turbidity periods
Context fitting with the onset of the period of turbidity Daily turbidity Average turbidity for the Water Period of high Number of min./max./av. rest of the thawing season Rain Air temperature stage turbidity days (NTU) (NTU)
Increase from 0-5 30 June - 23 July 2010 24 24/160/43 4.2 No preceding rain event Decrease to 13 ⁰C
2 days of light rain (8 and 15 July - 2 August Generally rising 19 15/57/33 8.2 6 mm) preceding the onset Stable 2013 above 10 ⁰C of the turbidity period
Light rain (5 mm) 2 days Increase from 8-9 12 July - 28 July 2014 17 11/141/72 8.2 before the onset of the Stable to 13 ⁰C turbidity period
Draft
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Table 3. Comparison of the date of the peak flow in the Great Whale River and in the Sheldrake River, from 2009 to 2014. For the Sheldrake River, the date of the peak flow corresponds to the day where the annual highest water stage was recorded.
Great Whale River Sheldrake River Date Date 2009 8 June 14 June 2010 14 May 27 May 2011 20 May 5 June 2012 26 May 7 June 2013 21 May 3 May 2014 23 May 28 May Average 24 May 30 May
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Figures
Draft
Fig. 1. Location of the study area and photograph of the Sheldrake River flowing through an area of permafrost plateaus (foreground), and finally flowing to Hudson Bay across rock outcrops. VDT indicates the location of the meteorological station, in the Vallée des Trois (also called Tasiapik).
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Fig. 2. Two tributaries of the Nastapoka River. Note the high turbidity of the streams reflecting high thermokarst activity in catchments. .
Draft
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Fig. 3. The Sheldrake River catchment with the distribution of surficial deposits, the position of the tree line and the main drainage network. Sheldrake Lake does not appear on this map (see Fig.1 for location).
Draft
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Draft
Fig. 4. Schematic representation of the positioning of the gauging station and photograph of its location on a bank of the Sheldrake River.
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Draft
Fig. 5. Hydrographs for 2009, 2010, 2011, 2012, 2013 and 2014. Water level record starts with the first signs of the break up (see in text) and end at mid November (except for 2013 and 2014, due to lack of data).
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Fig. 6. Relationship between turbidity (NTU) and total suspended sediment (TSS) (g m3).
Draft
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Draft
Fig. 7. Water level and turbidity regime of the Sheldrake River with air temperature and rainfall events in 2010: rain (mm d 1), air temperature (°C), water level (cm) and turbidity (NTU). Data are presented on a daily basis.
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Fig. 8. Turbidity (NTU), thaw frontDraft depth (cm), rain (mm d 1) and cumulative degree days ≥ 0 ⁰C from 1 May to 1 October 2010.
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Draft
Fig. 9. Water level and turbidity regime of the Sheldrake River with air temperature and rainfall events in 2013: rain (mm d 1), air temperature (°C), water level (cm) and turbidity (NTU). Data are presented on a daily basis.
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Fig. 10. Turbidity (NTU), thaw front depth (cm), rain (mm d 1) and cumulative degree days ≥ 0 ⁰C from 1 May to 1 October 2013. Draft
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Draft
Fig. 11. Water level and turbidity regime of the Sheldrake River with air temperature and rainfall events in 2014: rain (mm d 1), air temperature (°C), water level (cm) and turbidity (NTU). Data are presented on a daily basis.
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Fig. 12. Turbidity (NTU), thaw frontDraft depth (cm), rain (mm d 1) and cumulative degree days ≥ 0 ⁰C from 1 May to 1 October 2014.
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Draft
Fig. 13. (A) Relationship between turbidity and air temperature; (B) Relationship between turbidity and water stage. One point represents one day during the thawing season of 2010, 2013 and 2014. Days when turbidity was less than 10 NTU were excluded to limit the effects of light rain. The regression line and the coefficient of correlation represent only turbidity values ≥10 NTU.
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Fig. 14. Typical landslide on the bench of the Sheldrake River. The landslide probably occurred on 6 May 2010 and caused a peak of turbidity. Draft
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Draft
Fig. 15. Thermokarst as source of suspended sediment in the Sheldrake River catchment (A): active and flowing frostboil on the slope of a permafrost plateau (1 meter length) (photo: Denis Sarrazin); (B) : Typical thermokarst ponds, the strong turbidity is caused by frostboils activity and surface runoff; ( C) : Network of gullies between permafrost plateaus (Jolivel and Allard, 2013); (D): typical landslide on a permafrost bank along the Sheldrake River; ( E) : small delta at the confluence between a thermokarst gully and the Sheldrake River.
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