Erwan ; Roussel

Philip M Marren

Etienne Cossart

Jean-Pierre Toumazet

Marie Chenet

Delphine Grancher

Vincent Jomelli

Grancher

Incision and aggradation in proglacial rivers: Post-Little Ice Age long-profile adjustments of Southern Iceland outwash plains

Keywords: proglacial river, glacial retreat, proglacial landforms, fluvial geomorphology, paraglacial processes proglacial river, glacial retreat, proglacial landforms, fluvial geomorphology, paraglacial processes 29 30

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INTRODUCTION

32

Since the end of the Little Ice Age, rising temperatures have elevated the equilibrium lines altitude of many 33 glacial systems and intensified ice melting (Barry, 2006;Vaughan et al., 2013). This implies an adjustment of 34 glacier geometry, which is usually reflected by the retreat of the glacier front. Glacial retreat leads to the proglacial river (proximal outwash plain) and the formation of a single deep channel [START_REF] Fahnestock | Morphology of the Slims River[END_REF] which are not in equilibrium with the water and sedimentary flux (Germanoski & Schumm, 1993). The incision 81 of the proximal river results in the establishment of glacio-fluvial terraces whose morphosedimentary features 82 reflect the former hydrological conditions before the incision (Marren, 2005). Flume experiments show that the 83 long profile of an incising braiding river is usually characterized by an inflection point, downstream of which 84 aggradation is dominant (Germanoski & Schumm 1993). Indeed, the reworked material of the incising upper 85 reach is transported and deposited downstream induces an increase of the braiding intensity of the lower 86 reach of the river. When the sediment delivery deficit continues, flume experiments have shown that the 87 inflection point, which separates the incision from the aggradation reach, tends to migrate downstream 88 (Germanoski & Harvey, 1993). At the same time, glacial retreat promotes the formation of proglacial lakes that 89 trap the coarser fraction of the sediment flux and may strengthen the proximal river incision and the 90 aggradation of the distal river reach [START_REF] Embleton | Glacial and Periglacial Geomorphology[END_REF]Chew & Ashmore, 2001;Benn et al., 2003; 91 Gardarsson & Eliasson, 2006;Schomacker, 2010;Carrivick & Tweed, 2013;Bogen et al., 2015). Nevertheless, 92 this pattern of fluvial response to glacier retreat can vary over space and time due to topographical constrains 93 induced by marginal landforms: (1) the spatial and temporal evolution of the overall sediment connectivity 94 within the proglacial margin can lead to damming and localised river aggradation [START_REF] Cossart | Landform connectivity and waves of negative feedbacks during the 603 paraglacial period, a case study: the Tabuc subcatchment since the end of the Little Ice Age 604 (massif des Écrins, France). Géomorphologie : relief, processus[END_REF]Cossart & 95 Fort, 2008); (2) Channel pattern changes can be controlled by moraine ridges between which the river is forced 96 to find a course (Marren & Toomath, 2013, 2014).

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Field observations report that most proglacial rivers associated with retreating glaciers in Iceland exhibit 98 proximal incision which decreases in magnitude downstream and stops at an inflection point where 99 aggradation begins (Marren, 2005 ;Roussel et al., 2008 ;Marren & Toomath, 2013). However, no studies have 100 measured longitudinal profile adjustments of this type on a large sample of proglacial rivers and over a 101 sufficiently long time period to determine whether this behaviour is a representative response to glacial 102 retreat. Furthermore, there is a lack of information about the rates and magnitude of this fluvial adjustment Iceland to date the LIA moraines of the south Vatnajökull (Chenet et al., 2010). This method provides a more 158 statistically robust estimate of the age of the deposits because it takes into account the bias induced by the 159 strategy of data collection (i.e. the selection of the largest thalli whose distribution is similar to the model GEV).

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In addition, this method provides confidence intervals to assess the quality of the estimated age.

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In the field, measurement of lichen long axes was carried out according to the procedure recommended by the statistical treatments applied to the lichenometric database are available in several papers (Cooley et al., 172 2006;Naveau et al., 2007;Rabatel et al., 2008).

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Topographic survey

174

Longitudinal topographic profiles of active outwash and outwash terraces were surveyed in the field.

175

Immediately downstream of LIA moraines of each investigated glacier, valley transverse profiles were also 176 undertaken in order to capture the nesting of outwash terraces (Figure 2-C). Following the main direction of 177 flow, longitudinal profiles of the active outwash and terraces were surveyed from the LIA moraines to the were carried out in order to systematically check that the difference between measurements were below the 186 telemeter accuracy (0.03m) and to limit error propagation. As the objective was to assess the topography of 187 the outwash plains, measurements were carried out between topographic stations located on the heads of 188 fluvial bars. Therefore, fine topography at the channel scale was not captured. Each topographic station was 189 located using a handheld GPS (Trimble Geoexplorer XH).

214

Proglacial landform assemblages

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The proportion of each landform unit within the post-LIA proglacial margin has been quantified (Table 2 and216 Figure 3). Glacigenic deposits, combining moraine ridges and fresh proximal till deposits, are the most affected the proximal outwash of the proglacial rivers 1, 2, 5, 8, 9, 11, 13 and 14. The duration of this 259 aggradation or steady state stage (computed as the difference between the T1 lichenometric dating and the 260 age of LIA moraines) is highly variable and ranges between 5 (±14) and 147 (±15) years for the outwash 11 and 261 5 respectively (Figure 4 and Table 3). The outwash 7 is the only one which seems to persist in the aggradation 262 stage (Figure 4). Nevertheless, as its proximal reach is affected by an artificial embankment, its fluvial response 263 is not fully controlled by natural drivers. Based on the other investigated outwash surfaces, aggradation may be 264 considered as a transient stage of post-LIA outwash response.

265

Contraction and incision of the proximal outwash 266

The post-LIA contraction of the proximal outwash width is detectable in the successive abandonment of 267 outwash terraces (T2 to T5 on Figure 3) by proglacial rivers. These fluvial deposits are interpreted as nested cut 268 terraces and reveal the incising dynamic affecting proximal outwash. This glaciofluvial evolution follows the 269 transient aggradation stage and is still currently the dominant behaviour in most of the rivers. The outwash 270 plain 7 is the only one which appears to still be aggrading or remaining in a steady state, likely due to the 271 artificial constraints described above (Figure 4). On the other outwash plains, the proximal fluvial activity was 272 gradually concentrated downstream of a single breach in the LIA moraines. Currently, the active outwash plain 273 is framed by multiple paired or unpaired terraces whose preservation is dependent on the lateral mobility 274 and/or the rate of proximal river incision (Charlton, 2008).

275

Stream capture and shift in the outwash plain location

276

The mapping of glacier forelands (Figure 3) and the sequence of outwash terrace abandonment (Figure 4) also where two ice lobes were coalescent at the LIA glacial maximum (Figure 3). This is the case for the glaciers 3, 11 281 and 13 (Figure 3) whose glacial fronts were respectively coalescing with those of 4, 12 and the western arm of 3 and Figure 4). Thus, proximal incision, or at least lateral contraction, of the outwash of the 300 rivers 3 and 4 began before the glacier reached its maximum LIA extent. Early incision also affected the 301 outwash 6 whose T1 terrace (Figure 3) was built by the 1727 jökulhlaup. The incision of this deposit may be 302 independent of the fluctuations of the front of the glacier 6 and is instead related to a progressive return of the 303 proglacial river to pre-jökulhlaup conditions (Thompson & Jones, 1986).

305

Outwash long profile adjustment 306 Outwash longitudinal profiles reveals a dominant pattern of fluvial adjustment characterized by a proximal 307 incision often accompanied by distal aggradation (Figure 5). The inflection point (IP) connecting the incising 308 upper reach and the aggrading lower reach tends to migrate downstream through time.

309

Proximal incision, slope decrease and distal aggradation 310 Post-LIA proximal incision can be quantitatively assessed for the rivers 1, 2, 8, 9 and 14, which were not 311 affected by stream captures or early incision prior to the LIA glacial maximum. These proglacial rivers present 312 several nested outwash terraces, the oldest of which, (T1) is always backed by the LIA moraine ridges (Figure 313 5). More recent outwash terraces can be disconnected from these moraines. This is the case for the rivers 8, 9 314 and 14. This observation reflects either a downstream migration of the proximal outwash contraction or the 315 destruction of the upstream part of recent terraces due lateral erosion and avulsion of the active river channel.

316

Following the transient aggradation stage (or steady state stage), the initiation of proximal incision implies the 317 abandonment of the oldest outwash terrace (T1) by river flows. As proximal degradation continues, proglacial 318 rivers abandon younger terraces. This is particularly the case for the rivers 2 and 14 where six and four 319 generations of terraces are distinguishable, respectively. The rate of proximal outwash incision, computed over 320 the period from the abandonment of the oldest terrace until 2007, averages 0.073 ± 0.03 m.a -1 and ranges from 321 0.045 ± 0.006 m.a -1 to 0.131 ± 0.012 m.a -1 for the rivers 2 and 14 respectively (Table 3).

322

The slope of the proximal rivers has decreased almost systematically since the LIA maximum. This is 323 qualitatively detectable on Figure 5

334

Consequently, the slope measurements on this outwash plain reflect more closely the proximal concavities and 335 are difficult to compare with the other rivers.

336

Decrease of the proglacial rivers slopes is mainly due to the proximal incision of the outwash plains. However,

337

proximal incision is often associated with a distal aggradation of the outwash plain, which contributes to the 338 overall decrease of the proglacial river slope. Indeed, nine of the twelve studied outwash plains present this 339 type of post-LIA adjustment of their longitudinal profile: these include the outwash plains of glaciers 2, 3, 4, 5, 340 8, 9, 11, 13 and 14 (Figure 5).

341

Downstream migration of the inflection point (IP)

342

Post-LIA changes in the location of the inflection point (IP) were reconstructed for the proglacial rivers 2, 3, 4, 343 5, 8, 9, 11, 13 and 14 (Figure 5). The longitudinal profiles of terraces and active outwash of the proglacial rivers 344 1, 6 and 7 do not exhibit inflection points. Results demonstrate the post-LIA downstream migration of the 345 inflection points, indicating a longitudinal increase of the extent of proximal outwash area affected by incision

346

(Figure 4). This evolution may indicate a perennial sediment deficit in the marginal area leading to a 347 downstream progression of the proximal incision. According to these results, the sediment recharge of the 348 proglacial rivers is primarily carried out by the reworking of bed material (proximal incision) rather than lateral 349 erosion of the former proximal outwash terraces (proximal contraction).

350

Average rates of longitudinal migration, elevation lowering and downstream migration of the inflection points 351 were also computed for the proglacial rivers 2, 3, 4, 5, 8, 9, 11, 13 and 14 (Table 3). Rates of IP longitudinal 377

Atypical outwash adjustments

378

The outwash plains of the glaciers 1, 6 and 7 exhibit different post-LIA adjustments of their longitudinal profile 379 (Figure 5). The outwash plain of glacier 1 has experienced an incision of its proximal part since the LIA 380 maximum. However, no evidence of distal aggradation was found in the field. In addition, the post-LIA decrease 381 of the proximal slope remains extremely low (0.013 m.m -1 for T1 terrace versus 0.012 m.m -1 for the river in 382 2007). The deep incising efficiency of recurrent volcanic jökulhlaups which affect the Glacier 1 glacier foreland 383 may produce this uniform degradation of the entire outwash plain and inhibits post-LIA distal aggradation. The 384 glacier 6 is also affected by volcanic jökulhlaups. Moreover, its outwash terrace T1 (Figure 5) constitutes the 385 deposit of the historical jökulhlaup of 1727. The incision of this deposit may be independent of the fluctuations 386 of the glacier front and is rather related to a progressive return of the river 6 to the pre-jökulhlaup conditions 387 (Thompson & Jones, 1986). Alternatively, the case of the outwash plain 7 is more surprising: no incision of the 388 abandoned and vegetated outwash plain was observed on the field. As mentioned above, the vegetated 389 outwash plain of glacier 7 seems extremely recent (absence of lichens). Its abandonment by proglacial flows 390 seems artificial and related to the embankment structure located on the right bank of the upstream reach of 391 the river 7 (Figure 3). As a consequence, the abandoned outwash plain of the river 7 and the decrease in the sediment providers for the proglacial fluvial system. On the proglacial margin of the Small River Glacier in 436 Canada, marginal deposits and mostly abandoned intra-morainic plains have been shown to cease to function 437 as sediment sources after several decades (Orwin and Smart, 2004). Our results tends to support this 438 hypothesis of a stabilization of morainic ridges and intra-morainic deposits within half a century or even a 439 century. This period of time is too short to encompass all of what is commonly referred as the so-called 440 paraglacial period [START_REF] Ballantyne | Paraglacial geomorphology[END_REF], Mercier, 2010[START_REF] Cossart | Evolution géomorphologique du haut bassin durancien depuis la dernière 601 glaciation (Briançonnais, Alpes française du Sud)[END_REF]. More likely, the duration of stage I is only 441 the time period during which the effectiveness of paraglacial processes is sufficient to, at least, keep the 442 outwash plain in balance. As the marginal landscape differs widely from one glacier to another, the duration of the transient aggrading period also varies accordingly. This variability may be related to the amount of the 444 morainic sediment stock in the proglacial margin and its sensitivity to potential reworking processes.

445

In particular, the presence of dead ice in the moraine deposits may play a major role in the initiation of debris 446 flows eroding moraine ridges inducing the recharge of the proglacial fluvial system with paraglacial sediments

The incision stage of the proximal outwash plain is initiated when the sediment stock of the proglacial margin is 450 deficient and / or disconnected and the efficiency of paraglacial remobilization of the morainic and intra-451 morainic deposits is no longer sufficient to maintain the outwash plain in balance (Figure 7

456

This post-LIA response of the marginal channel network was also observed in Spitsbergen on the Lovén glacier 457 margins (Griselin 1982, Mercier, 2001, Mercier & Laffly, 2005). The response of south-Icelandic outwash plains 458 to post-LIA glacial retreat is the consequence of this marginal sediment deficit and the concentration of water [START_REF] Cossart | Landform connectivity and waves of negative feedbacks during the 603 paraglacial period, a case study: the Tabuc subcatchment since the end of the Little Ice Age 604 (massif des Écrins, France). Géomorphologie : relief, processus[END_REF][START_REF] Cossart | Assessment of structural sediment connectivity within 611 catchments: insights from graph theory[END_REF].

469

Proglacial lakes within the marginal landscape also play a key role as they interrupt the delivery of water and 470 sediment to the outwash plain (Schomacker, 2010;Carrivick & Tweed, 2013;Geilhausen, 2013;Bogen et al., 471 2015, Staines et al., 2015). According to our results, the abundance of proglacial lakes within the proglacial 472 margin speed up the downstream progression of the incision wave affecting proximal outwash. They clearly 473 reduce the marginal sediment connectivity in trapping the coarsest part of the glacigenic sediment production.

474

Thus, in a similar way to artificial dams, proglacial lakes deliver at their outlets "hungry water" prone to channel 475 incision (Kondolf, 1997) and bed armoring (Vericat et al., 2006). High-magnitude/low-frequency events 476 emerging from proglacial lakes as outburst floods (Westoby et al., 2014;Worni et al., 2014), rainstorm floods 477 (Marren & Toomath, 2013) and iceberg jam floods (Roussel et al., 2016) may be able to temporarily reconnect 478 the sediment connectivity between the proglacial lake and the outwash plain. results suggest that the proximal sediment deficit of the rivers 4, 5, 8 and 9 is less severe than for the rivers 2, 541 11 and 14, whose inflection points, and the spatial extent of proximal incision, rapidly migrated downstream.

542

(3) The downstream migration rate of the inflection point is positively correlated with the proportion of 543 proglacial lakes within the glacier foreland.

  35emergence and growth of proglacial margin landforms at the interface between the glacier and the fluvial 36

  103 and the effects of the glacial retreat rate and the proglacial margin configuration on the temperance or the 104 aggravation of the fluvial response to glacier retreat. 105 Therefore, this paper investigates the proglacial landscapes and the outwash plains of 14 retreating glaciers in 106 southeast Iceland over a post-Little Ice Age timescale. It combines quantitative assessment of the glacier 107 retreat, the landform assemblage of the proglacial margin and the post-LIA outwash plains changes through 108 lichenometric dating and longitudinal profile measurements of active outwash and fluvial terraces. The aims of 109 this paper are threefold: (1) to test the representativeness of the outwash plains response to glacier retreat 110 described above over the time-scale of centuries; (2) to assess the rates of this fluvial response through the 111 within ArcGIS software (10.4), the landform assemblages in the proglacial margin, 148 the outwash terraces, and the active outwash were mapped. The proglacial margin areas provide a quantitative 149 assessment of the deglaciated surface since the LIA. An average linear retreat of the glacier fronts was 150 computed by dividing the proglacial margin area by the glacier front length for each glacier. A manual 151 measurement of the linear retreat of glacier fronts was also performed, along the centerline of the glacier 152 tongues, between the maximum LIA moraines and the glacier front in 2009. 153 Lichenometric dating 154 Lichenometric dating of outwash terraces (Figure 2-B) was performed using the generalized extreme value 155 approach (GEV) described in Jomelli et al. (2007) for the statistical treatment of the measurements of the 156 longest axis of the thalli of Rhizocarpon Geographicum. This procedure has already been successfully applied in 157

162

  Jomelli et al. (2007): (i) a random selection of 50 boulders distributed over the whole surface of the deposit; (ii) 163 on each boulder, the long axis of the largest thallus of Rhizocarpon Geographicum was measured with an 164 accuracy of 0.5 mm. Coalescing lichens and thalli with irregular shape are systematically rejected. Consistent 165 with all lichenometric methods, two datasets of thalli long axis were collected in the field: on already dated 166 surfaces and on the deposits requiring dating. Eight well-known dated surfaces in an area of ca. 45 km2 in 167 south foreland of Vatnajökull were used to ensure that climatic conditions do not vary excessively (Chenet et 168 al., 2010). Dated surfaces included jökulhlaup deposits, lava flows, rockfalls, dams, and moraines; all made of 169 basalt to ensure lithological homogeneity. Surfaces were dated using historical descriptions, aerial photographs 170 (available since 1945), and personal communications from the Skaftafell Park staff. Additional consideration of 171

  178 downstream edge of the fluvial deposit (confluence with a tributary or the shoreline). Transverse topographic 179 profiles located just downstream of LIA moraines enable the computation of the relative altitude of each 180 outwash terraces in respect to the active outwash. They are essential to align longitudinal profiles between 181 them and to capture the downstream migration of the inflection points separating the upstream incising reach 182 from the downstream aggrading reach of the proglacial rivers. These profiles were achieved using a laser 183 telemeter (Impulse 200LR from Laser Technology) with a centimeter accuracy and a tripod target. The average 184 measurements was 20 meters. Two measurements by stations (foresight and backsight) 185

190

  Reconstruction of the downstream migration of the inflection point (IP) 191 Inflection points (sensu Germanoski & Harvey, 1993) separate upstream incising reaches from downstream 192 aggrading reaches of a river. The reconstruction of changes in the location of the inflection point (IP) 193 characterizes the evolution of the extent of proximal outwash affected by incision. IP changes were 194 reconstructed based on the identification of intersection points of the longitudinal profiles of two successive 195 (dated) terrace generations. These intersections are the former IP location separating the incision and the 196 aggradation reach of the outwash plain at the date of the youngest of the intersecting terrace profiles (Figure 197 2-D). For each reconstructed IP, its location (distance from LIA moraines and elevation) was validated in the 198 field and recorded with the handheld GPS (Trimble Geoexplorer XH). The initial IP location (distance = 0 m) at 199 the LIA glacial maximum coincides with the contact between the LIA moraines and the oldest and highest 200 outwash terrace (T1), if lichenometric dating of the latter is younger than LIA moraine ridges. In other words, at 201 the LIA glacial maximum, aggradation of the outwash is likely predominant and there is no upstream incision 202 because there is no outwash upstream to the LIA moraines. The reconstruction of the former location of the 203 inflection point when T1 was incised and the IP location observed in 2007 enabled the computation of average 204 rates of longitudinal migration (increasing distance of the IP from the LIA moraines) and altitudinal lowering 205 (decrease in the IP elevation) of the IP through time. A synthetic metric of average rate of downstream 206 migration of the inflection point was also computed as the Euclidean norm of the vector (square root of the 207 sum of the square of coordinate values) defined by the two values of longitudinal migration and altitudinal 208 lowering of the inflection points. Note that this average rate of IP downstream migration does not necessarily 209 reflect a gradual progression of the inflection point as IP migration may occur suddenly during extreme events.210Finally, a stepwise multiple linear regression was performed in order to test the effect of proglacial landform 211 assemblage on the downstream migration rates of the inflection point.

  217abundant proglacial landforms (respectively 44.8 % and 5.2% on average of the investigated proglacial 218 margins). The marginal landscape is obviously dominated by glacial deposits associated with seasonal 219 fluctuations of glacier fronts and the continuous re-advances of the 1970s and 1980s. Proglacial margins of the 220 steepest, Öraefajökull glaciers (glaciers 2 to 10), show a higher proportion of moraine deposits. Steep proximal with a large amounts of moraine deposits that may be related to large production 222 rates of glacigenic sediments and/or limited fluvial reworking. Abandoned and active intra-morainic plains form 223 respectively around 16.8 % and 8.9 % on average of the proglacial landforms. The abundance of these two 224 types of landforms testify the involvement of fluvial processes in the morphogenesis of the proglacial margin. 225 Rocky outcrops (11.4% on average) are only present in the proglacial margins of piedmont glaciers (Glaciers 1, 226 11, 12 and 13) whose tendency to overdeepening would generally exhume more resistant areas of the glacial 227 bed, and in the foreland of glacier 6 where steep proximal relief tends to rapidly clean the glacial bed and 228 remove till layers via gravity and runoff processes. Proglacial and intra-morainic lakes (respectively 10.4% and 229 1.5% on average) are totally absent from the proglacial margins of the steepest glaciers of the Öraefajökull 230 sector (glaciers 6 and 7), most likely due to the extreme steepness of the ablation zone and the glacier 231 forefield. Alternatively, proglacial lakes are common on the margin of piedmont glaciers of the southeastern 232 sector of Vatnajökull where their large frontal lobes probably favored overdeepening of the marginal areas 233 during the LIA glacier advance. Alluvial fans (0.6 % on average) are only present in steep marginal settings 234 (glaciers 7 and 9). This association seems logical since development of alluvial fans is strongly dependent on the 235 presence of steep marginal slopes. Embankments (0.3 % on average) are only present on the margins of two 236 piedmont glaciers (glaciers 3 and 13). Artificial embankments are associated either with tourist trails running 237 through the proglacial margin (in the case of glacier 13), or with consolidation structures of the main Icelandic 238 road (road number 1) built through the LIA moraines (glacier 3). 239 Outwash terrace mapping and dating 240 Outwash terrace mapping and lichenometric dating (Figure 3) emphasizes four types of fluvial processes 241 affecting the proglacial river responses to post-LIA glacier retreat: (1) post-LIA transient aggradation; (2) 242 incision and contraction of the proximal outwash; (3) shifts in the location of the fluvial activity due to stream 243 capture occurring within the proglacial margin and (4) outwash incision prior to LIA glacial maximum. Based on 244 the analysis of glacier foreland mapping and lichenometric dating of outwash terraces, an overview of 245 successive aggrading and incising stages of proximal outwash response to post-LIA glacial retreat has been 246 produced (Figure 4). The outwash terraces of the proglacial rivers 6 and 7 could not be dated due to the 247 absence of lichens: the oldest terrace (T1) of the outwash plain 6 is the historic jökulhlaup deposit of 1727 and 248 is exclusively composed of fine materials devoid of lichens. The terrace (T1) of the outwash plain 7 is lightly 249 vegetated and its abandonment by proglacial flows might be recent and artificial, relating to the upstream 250 embankment structure (Figure 3). The youth of this fluvial deposit may explain the absence of lichens on the 251 proximal surfaces. 252 Post-LIA transient aggradation of proximal outwash 253 First generation outwash terraces (T1 on Figure 3) are adjacent to the moraines ridges marking the maximal 254 glacier extent during the LIA. They are interpreted as fill terraces built up by the aggrading proglacial river 255 during the LIA glacier advances. Dating reveals that proximal aggradation, or at least the maintenance of the 256 outwash plain in a steady state, has persisted after the glaciers reached their maximum LIA extent and their 257 fronts likely initiated their retreat or stagnation phase. This post-LIA aggradation stage (or steady state phase)

  277 shows a typical marginal fluvial process: stream capture within the proglacial margin leading to a shift of the 278 outwash plain location. This process affected the outwash plain 3 in 1939 (±3), 5 in 1891 (±3), 11 in 1914 (±8) 279 and 1959 (±5), and 13 in 1918 (±7). Stream capture seems to preferentially occur within proglacial margins 280

  282 13 at the LIA glacial maximum. Stream capture and location shift of the proximal outwash plain can occur 283 slowly, and follows a sequence of proximal outwash narrowing: In the foreland of glaciers 11 and 12(Figure 3), 284 three outwash terraces (T1, T2 and T3) were formed due to the contraction of the proximal braiding belt of the 285 outwash plain shared by the two coalescing glaciers. In 1914, about 50 years after the LIA glacial maximum, a 286 first stream capture occurred within the glacier 11 proglacial margin causing the abandonment of the outwash 287 plain T3 and the development of the outwash plain T4. After 45 years of activity (during which T4 may have 288 eroded the outwash terrace T3), a new stream capture occurred in the proglacial margin of glacier 11. T4 is 289 abandoned in 1959 in favour of glacier 11 intra-morainic plains and the outwash plain which is still currently 290 active. Ultimately, proximal incision and stream capture processes do not exclude each other. Moreover, 291 according to our results, stream captures occur exclusively in the incising stages of the proximal outwash.

292

  Proximal outwash incision prior to the LIA maximum293The lichenometric dating of the first terrace generation (T1) of the proglacial rivers 3 and 4 predates the LIA 294 glacial maximum (M1). According to these lichenometric ages, the abandonment of outwash T1 downstream of have occurred 16 years (±25 yr.) before the LIA maximum. But, as the absolute uncertainty 296 exceed the difference between the ages of M1 and T1, proximal outwash incision and LIA maximum could also 297 have occurred synchronously between 1864 and 1873. Dating results are much more surprising for the river 4, 298 since the abandonment of the oldest terrace occurred almost two centuries (179 ±21 years) before the LIA 299 maximum (Figure

  by a visual comparison of the long profiles of the first generation of post-324 LIA outwash terraces (T1) with those active in 2007 (A). The mean slope of the proximal rivers (the first 500 325 meters from the maximum LIA moraines) were calculated using a simple linear regression: results indicate that 326 proximal slope decreased by 0.011 m.m -1 (SD =0.009 m.m -1 ) on average between the first-generation of post-327LIA outwash terraces and the active rivers in 2007. However, there is a high variability in the post-LIA lowering 328 of the proximal slope. For example, the proximal slope values of the active and the outwash terraces of the 329 proglacial river 7 are identical, in agreement with the absence of proximal incision (Table3). Conversely, the 330 proximal slope of the river 9 decreased by 0.0311 m.m -1 , dropping from 0.0339 m.m -1 in 1891 (79 years after 331 the LIA maximum) to 0.0028 m.m -1 in 2007 (195 years after the LIA maximum PAG). For the river 9, the 332 old (terrace T1) and current (2007) proximal slopes is only based on the first 200 meters 333 from the LIA maximum moraines due to the narrowness of the conserved fragment of T1 terrace.

  9 to 0.5 m.a -1 for the river 14. The synthetic rates of IP downstream migration are similar to 355 longitudinal migration rates (average = 20.4 m.a -1 , SD=13.6 m.a -1 and values range from 5.2 m.a -1 for the Virkisá 356 to 46. 4 for the Hoffellsá).

357A

  category of proglacial rivers in which the downstream migration of the inflection point is rather slow can be 358 identified. These are rivers 4, 5, 8, and 9, whose post-LIA average rates of IP longitudinal migrationThe stepwise multiple linear regression procedure test the dependency of the IP downstream migration rate on 371 the landform assemblage of the proglacial margin and the post-LIA glacial retreat. The linear stepwise 372 procedure only selected the proportion of proglacial lakes within the marginal landscape as a significant model 373 parameter (t=3.527, p=0.01). The downstream migration rate of the inflection point is positively and 374 significantly correlated with the percentage of proglacial lakes within the proglacial margin (R²=0.640, P=0.010).375According to this result (Figure6-B), the development of large proglacial lakes in the glacier foreland increases 376 the rate of that the IP migrates downstream.

  392 lateral extent of the fluvioglacial dynamic cannot be considered as a purely natural response of the proglacial 393 river to glacial retreat. Alternatively, large alluvial fans located within the proglacial margin of this steep glacier 394 may provide an abundant sediment supply that mitigates the marginal sediment deficit and inhibits proximal 395 incision. 396 Local controls on the long profile of active outwash in 2007 397 Long profiles of the active outwash of the glacier 4, 8, 10 and 13 (in 2007) reveal the local controls exerted by 398 embankment structures or rocky outcrops on the efficiency of sediment transport. Indeed, these long profiles 399 display significant decrease of local river slope values in the upstream vicinity of bridgeworks (related to the 400 Icelandic Ring Road) or rocky outcrops. These zones force sediment deposition, related to dam effects and 401 lateral constraints exerted by embankment and outcrops, and can hinder the downstream migration of 402 sedimentary waves and mitigate the standard pattern of post-LIA adjustment of the outwash long profile by 403 moderating proximal incision. Alternatively, the large alluvial fan located on the left bank of the river 14 seems 404 to exert a control over the river long profile. A local increase in the slope value can be observed in the vicinity 405 of the fan toe, probably due to lateral sediment contribution coming from the alluvial fan (Knighton, 1998). 406 Conversely, the alluvial fan located on the left bank of the distal part of the outwash plain 2 does not seem to 407 over the slope values of the active river in 2007. Nevertheless, it appears that presence of 408 large active alluvial fans can also modify the post-LIA adjustment of the outwash long profile by results, the dominant response of Type I (sensu Maizels, 1993) Icelandic proglacial rivers to 414 post-LIA glacier retreat follow two successive stages: (1) a first stage of transient aggradation, or steady state, 415 of the proximal outwash, (2) a second stage characterising by proximal incision, distal aggradation and the 416 downstream migration of the inflection point (IP) which separates the upstream incising reach and the 417 downstream aggrading reach of proglacial rivers. 418 Stage I: Outwash transient aggradation (or in balance), a paraglacial signature? 419 During the first stage, even if the post-LIA glacial retreat is initiated, the outwash plain is still aggrading or, at 420 least, kept in a steady state (Figure 7-A). Two explanations of this transient aggrading stage are possible: (1) 421 The glacial front may have registered a slight retreat during stage I and experienced a steady period with minor 422 seasonal fluctuations that maintain high glacigenic sediment supply to proglacial rivers in which aggradation 423 continues (or remains in its LIA maximum state); (2) Alternatively, if the ice front registered a pronounced and 424 continuous post-LIA retreat, the transient aggradation stage can only be related to efficient recharge of the425marginal sediment stock by paraglacial processes and / or greater rates of subglacial sediment evacuation. In 426 particular, in the early stages of deglaciation, thermokarstic processes, related to the dismantling of dead-ice 427 moraines, may be able to compensate for the decrease of glacigenic sediment supply induced by glacial retreat 428 for a time(Mercier et al., 2009, Irvine-Fynn et al., 2011). When the efficiency of subglacial sediment evacuation 429 decrease, thermokarstic processes cease and the moraine ridges are stabilized, the paraglacial recharge of the 430 proglacial fluvial system is no longer sufficient to maintain high sediment supply and stage I ends.431According to lichenometric dating of the first generation outwash terraces (T1), the duration of the transient 432 aggrading stage is highly variable: it averages 61.8 years(SD= 40.48 years) and ranges between 5 (±14) and 147 433 (±15) years. However, the range of duration of post-LIA transient aggradation stage of the South Icelandic 434 fluvioglacial plains is in agreement with previous studies on the duration of paraglacial processes as effective 435

  -A). The erodibility of 452 the marginal sediment stock and the continuation of glacial retreat results in the lowering of the local base 453 level which promotes stream captures within the proglacial margin. The occurrence of stream captures implies 454 the abandonment by proglacial flows of former intra-morainic plains, often to the benefit of a single moraine 455 breach which constitutes the favoured outlet of water and sediment flows to the outwash plain.

  459 and sediment flux at a single moraine breach: The sediment recharge of the outwash plain is primarily carried 460 out by the incision of the proximal fluvioglacial plain, resulting in the abandonment of the previous aggrading 461 outwash plain and the contraction of the proximal fluvial activity width. The production of glaciofluvial 462 sediments in the upstream incising reach caused further aggradation in the downstream reach. As the marginal 463 sediment deficit continues, the degradation of the proximal outwash progresses downstream as an incision 464 wave (Germanosky & Harvey, 1993). Rather than a uniform degradation of the outwash plain, the successive 465 formation of different terrace generations reflect the crossing of geomorphological thresholds and the 466 nonlinearity of the fluvioglacial response to fluctuations of water an sediment fluxes. These crossing of 467 geomorphological thresholds may be linked to abrupt variations in the efficiency of the marginal sediment 468 connectivity caused especially by moraines dams and their failures

  to pre-jökulhlaup conditions. In more general terms, and as Nicholas & Sambrook-Smith 518 (1998) suggest, in terms of morphogenesis and reworking of the fluvioglacial sediment stock, the outwash 519 plains impacted by jökulhlaups (Type III) are insensitive to the range of meltwater discharge associated with the 520 "normal" glacial ablation. The size of the material deposited during these catastrophic floods is beyond the 521 scope of seasonal proglacial flows. Consequently, the re-shaping of fluvioglacial forms of type III outwash is 522 totally independent of glacial fluctuations. Their fluvial morphogenesis seems to depend exclusively on dating and long-profiles surveys of Icelandic proglacial margins, glacio-fluvial terraces 527 and active proglacial rivers permit to address the threefold aims of this study: (1) Proximal incision, associated 528 with distal aggradation and downstream migration of the inflection point, is the dominant pattern of proglacial 529 river response to post-LIA glacier retreat in Iceland. The post-LIA contraction and incision of Icelandic proximal 530 outwash plains usually occurs after a first transient aggradation or steady state stage of proximal outwash 531 (duration ranges between 5 ±14 and 147 ±15 years) which may be related to efficient recharge of the marginal 532 sediment stock by paraglacial processes and / or greater rates of subglacial sediment evacuation. Proximal 533 incision, distal aggradation and downstream migration of the inflection point can precede or be preceded by 534 stream capture phenomena which particularly affects marginal landscapes where two glacier fronts were 535 coalescent at the LIA maximum. Outwash plain incision prior to the LIA maximum have been observed and may 536 be related to damming effect of proglacial moraine ridges or transient glacial retreat during the LIA. Two

  537 proglacial rivers affected by historical jökulhlaups exhibit an alternative adjustment rather controlled by 538 extremes events than by glacier retreat. (2) The mean rates of downstream migration of the inflection point 539 average 20.4 m.a -1 (SD=13.6 m.a -1 ) and range from 5.2 m.a -1 for the river 5 to 46. 4 for the river 14. IP migration 540

Figure 1 :

 1 Figure 1: Location of investigated glaciers and associated proglacial rivers

Figure 2 :Figure 3 :Figure 4 :Figure 5 :

 2345 Figure 2: Methodological workflow: A -Field prospection and GIS mapping of proglacial and glaciofluvial

Figure 1 :Figure 2 :Figure 4 :Figure 5 :Figure 6 Figure 7 :

 124567 Figure 1: Location of investigated glaciers and associated proglacial rivers 110x69mm (300 x 300 DPI)

  These findings suggest that proglacial margins dominated by 544 proglacial lakes intensify the rates of the fluvial adjustment to glacial retreat. Barry RG. 2006. The status of research on glaciers and global glacier recession: a review. Jansson P, Hock R, Schneider T. 2003. The concept of glacier storage: a review. Journal of Radic V, Bliss A, Beedlow AC, Hock R, Miles E, Cogley JG. 2014. Regional and global

		Church M, Slaymaker O. 1989. Disequilibrium of Holocene sediment yield in glaciated Gardarsson SM, Eliasson J. 2006. Influence of climate warming on Halslon Reservoir
	714	to glacier retreat. Earth Surface Processes and Landforms 39: 943-951. DOI:
		10.1002/esp.3545
		F o r F o r F o r F o r F o r F o r
		P e P e P e P e P e P e
		e r e r e r e r e r e r
		R e R e R e R e R e R e
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	545
	832	Figures and Tables

Table 1 :

 1 Main characteristics of glaciers and associated proglacial rivers investigated in this study.

	845
	846

Table 2 :

 2 Post-LIA glacier retreat and landform assemblages of proglacial margins afterChenet et al. , 2010 and Roussel et al. , 2008 ; ** after Chenet et al. , 2010 and Hannesdóttir et al. , 2014 ; *** after Maizels, 1993 *** after Maizels, , 1997 ; "na" for not available

	847

*

Table 3 :

 3 Outwash responses to post-LIA glacial retreat

	850

Maizels, 1979Maizels, , 1983 ; ; Germanoski & Schumm, 1993; Gurnell et al., 1999; Marren, 2005 ; Wilkie & Clague, 2009,

Owczarek et al., 2014). In Iceland, this fluvial response has been observed on the proglacial margins of the 77 southern glacial outlets of the Vatnajökull icecap, especially on the proglacial rivers of Skaftafellsjökull and

Svínafellsjökull,(Thompson & Jones, 1986 ; Thompson, 1988 ; Marren, 2002; Roussel et al., 2008 ; Marren &

Toomath, 2013Toomath, , 2014)). During the incision phase, fluvial bars tend to merge into larger sedimentary units,

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(rivers 2, 11 and 14), exhibit the highest rates of IP longitudinal migration and elevation lowering, exceeding 28

(Church & Ryder, 1972, Church & Slaymaker 1989, Mercier, 2009).

Stage II: proximal incision, distal aggradation and downstream progression of incision wave

Acknowledgements

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546

Research was supported by the GEOLAB-CNRS 6042, and the GDR "Mutations Polaires" (Coord. M. Griselin).

547

The authors thank the Skaftafell National Park which provided assistance and Pr. Johannes Steiger for his 548 helpful suggestions. The contribution to the manuscript has to be shared between authors as follows: the