Available results for five studied valleys of the Khibiny
Mountains, Kola Peninsula, suggest that slush flows and, possibly for some
valleys, typical debris flows with lower frequency, are a leading mechanism
for downstream sediment delivery and valley floor topographical formation.
Typical fluvial topography in slush flow-affected basins is extremely
suppressed or nonexistent, since under such conditions, stream channels are
unable to rework slush flow deposits. The recovery phase of fluvial
topography can serve as an indicator of the magnitude and time passed since
the last extreme event. A combination of grain size analysis, radionuclide
fingerprinting with the
The largest mountain massif of the Kola Peninsula – Khibiny – is comprised of Devonian alkaline intrusions (Pozhilenko et al., 2002). Compact plateau-shaped mountains (up to 1201 m a.s.l.), with relatively steep slopes, are dissected by numerous deep (100–400 m) erosional valleys, glacial troughs and cirques, and tectonic lineaments. Extremely rich in minerals, especially in apatite and nepheline ores and rare earth elements (Fig. 1), Khibiny have been intensely explored since the 1920s. Recently, the area has become a focus of a rapidly growing recreational industry. At the same time as being an arena of widespread natural hazards, its industrial and civil infrastructure (ski resorts, newly established National park, quarries, mines, plants, roads, etc.) is strongly affected by snow avalanches, rockfalls, screes, and debris flows (Belyaev et al., 2015). The most frequently observed types of the latter are snowmelt period slush flows and rainfall-induced medium magnitude debris flows (Perov et al., 2017). Slush flow represents a specific gravitational flow of a water-saturated mixture of snow and a limited amount of clastic sediment (up to 12 %, clasts size up to 1–2 m) occurring in low-order stream channels (Perov, 1996; Bozhinsky and Myagkov, 2001; Fleishman, 1978). Slush flows are classified as subtypes of wet snow avalanches or debris flows or as independent phenomena between the latter two (Hestnes, 1998; Eckerstorfer and Christiansen, 2012).
Location of the Khibiny Mountains on the Kola Peninsula,
NW Russia
Resembling sets of hazardous episodes, some with fatal consequences, are reported for other mountainous Arctic and Subarctic environments in Russia, Scandinavia (Nyberg, 1989; Hestnes and Kristensen, 2010; Jonsson and Gauer, 2014) and Northern America (André, 1995; Larocque et al., 2001; Relf et al., 2015, etc.). In Japan, more than 90 casualties due to slush flows have been registered since 1945 (Kobayashi et al., 1994). To estimate potential risks of these hazards, a thorough understanding of their nature and dynamics is required.
The Khibiny Mountains provide a unique dataset of more than 200 catchments affected by slush flows for the last centennial (Bozhinsky and Myagkov, 2001). Fifty years of survey mainly concentrated on monitoring the observed events and developing protection recommendations for mining infrastructure (Chernous, 2006) whilst the distribution, magnitude and frequency of such hazardous events in the past remained beyond the scope of exploration. However, large-scale bottom and piedmont fans found in the majority of small valleys do not always correspond to a glacial origin suggesting previous periods of much higher debris flow intensity. Given the lack of spatial data and absolute age determinations, improved understanding of correlations between such deposits and landforms is essential for reliable paleogeographic reconstructions. In this article, we therefore try to evaluate dynamics of debris and slush flows during the Holocene in the Khibiny mountain valleys.
Comprehensive field investigations including detailed descriptions of several geological sections were accomplished in five valleys of the Kuna River, Malaya Belaya River, and Bolshaya Belaya River basins (Fig. 1). Geomorphic interpretation of high-resolution satellite imagery from public services, aerial photography both archived and obtained by UAV DJI Phantom III, and 1 : 50 000 scale topographic maps assembled evidence for relic and active debris flows. The relative ages of these phenomena were estimated based on both morphological indications and vegetative cover (Rudinskaya et al., 2018).
Grain size analysis, radionuclide fingerprinting, and
We applied a standard procedure for the finer-grained sediment matrix (size
A limited geochronological framework was established by
In 2015, we started investigations of the frequency and age of debris and
slush flow events in the Khibiny (Garankina et al., 2018) and Lovozerskiye
Tundry (Rudinskaya et al., 2018). Field observations revealed both modern
and older corresponding landforms: valley floor and stream bank incisions,
ridges, lobes and hummocks, debris covers, terraces, and fans. The surfaces
of relatively fresh landforms have no or poor vegetation cover (fragmentary
mosses, lichens, herbs, shrubs) and do not exceed 1 to 2 m in height and
0.02 km
Most fresh slush flow deposits were found days to months after their accumulation in valleys of the Alyavumjok, Northern Lyavojok, and Mannepahkuaj rivers. In the latter, they formed thin debris covers, individual hummocks up to 1.5 m high and ridges 7–8 m long during the 2016 early spring snowmelt (fresh silty gravel and boulder loads at the confluence of main headwaters were registered at the end of June). Analysis of freely available remote sensing data allowed determination of the associated slush flow discharge periods in the spring of 2014 and at least once before, in 2012–2013. A slush flow from the main Northern Lyavojok tributary constituted a larger debris fan protruding into the forested valley bottom. Based on the sparse vegetative cover, this slush flow occurred less than 10–15 years ago.
Several generations of debris flow fans distinguished by relative elevation and vegetation succession stages below the V-shaped cut into moraine under the southern slope of the Vudjavrchorr Mountain.
At the footslope of the Vudjavrchorr Mountain, Southern Khibiny, a deep
V-shaped erosional incision cuts into the 150 m thick glacial drifts. Here,
the fan has several prominent age generations represented by a series of
inclined lobes with
Vegetation characteristics for indicating the age of debris or slush flow deposit, established for the forest belt in the Khibiny Mountains (after Bozhinsky and Myagkov, 2001).
Evidence of relatively young slush flows of substantial volume is observed
in the Malaya Belaya River basin. Two small catchments (Alyavumjok and
Eljok) on its left side have large unvegetated fans. The valley floor of the
former is almost devoid of loose deposits, and the latter has a deep (up to
35 m) V-shaped incision in the middle reaches. Debris fans 200–250 m in
radius with boulder paving are shown on the first topographic maps from the
1930s. Thus, they are at least 90 years old, being superimposed on forested
and even more expansive older fans, which displays repetitive activity of
extreme slush flows. The older Alyavumjok fan overlapped the Malaya Belaya
trough floor and forced the main river channel to shift its position
significantly and, thus, a meander up to 350 m in radius to form. The
frequency of such disastrous events can be estimated by
The distribution of slush flow patterns and landforms is strongly controlled by tectonic structure. For example, the feather-shaped drainage network of the Mannepahkuaj Basin induces tributary slush flow runout zones in the main valley bottom. Such coarse debris dams occasionally block the main channel forcing the stream to filtrate through and leading to further outbursts. Downstream, such “wavy” structure of deposition repeatedly induced normal debris flows with unequal transportation distances of the sediment load. This caused a series of debris fan lobes of different sizes to form at the forested piedmont (up to 6 km from the sources). Ages of embryonic paleosols (Fig. 5a) in two successive deposition zones, 0.5 km apart, reflect high-magnitude debris flows at least twice per millennia and probably much more often (Garankina et al., 2018).
It has been determined that slush flow fans or other landforms in their deposition zones under certain conditions become an important control of the evolution of fluvial topography. In the higher order river basins, it is often challenging to recognize zones of dominant slush flow origin, transport or deposition. Smaller and less complex catchments (mostly of the 1st order streams) present distinctly confined slush flow morphodynamic zones. For more detailed investigations of the slush flow–fluvial interactions, several basins were chosen for grain size and radionuclide content evaluation.
Grain size data of slush flow deposits in the Mannepahkuaj
Basin:
In fresh slush flow and alluvial deposits at the Mannepahkuaj streambed
(Fig. 3), sandy and silty fractions gradually increase downstream. In the
lower reaches, inhomogeneity is caused by a combination of variable
transportation capacities of different magnitude debris and slush flows and
the amount of fluvial reworking of their deposits. Older fans and terraces
in the middle reaches show much coarser composition, probably, due to the
higher transportation capacity of slush flows in the past. This is in
agreement with the much larger extent of correlated landforms (up to 10 m
relative height). At the relic piedmont fan, the sand content increases,
however, the grain size composition is generally finer than in modern slush
flow deposits. This may be explained by the greater area of the relic fan
(approximately 8 km
Recent publications report the use of
Redistribution of
The Northern Lyavojok Valley – with confirmed but scattered
Holocene geochronology for the valleys in the Khibiny
Mountains. New
Magnitude and frequency analysis and dating of key activity periods for slope processes (screes, rockfalls), snow avalanches, debris and slush flows represent important areas of research for improving understanding of morphodynamics in the Khibiny. Work has already been undertaken to date phases of decreased activity of catastrophic processes (Perov, 1971; Vashchalova, 1987; Bozhinsky and Myagkov, 2001; Vladychensky et al., 2007; Kosareva, 2007; Romanenko et al., 2011; Nikolaeva, 2014; Shilova and Romanenko, 2016; Nikolaeva et al., 2016). These studies employed dendrochronology, airborne photograph interpretation, and landform analysis. Unfortunately, all these approaches are limited in terms of both temporal resolution and maximum possible dated age.
Sets of geobotanic landscape indicators for slush flow fan and colluvial (rockfall or scree) cone ages were distinguished (Table 1). In general, the longer the interval without catastrophic geomorphologic events, the denser the vegetation cover on a fan. However, it must be borne in mind that such indicators are inapplicable for the tundra landscapes occupying a large part of the mountain slopes and summits with slush flow-affected catchments.
Perov (1966, 1971) first used a dendrochronological approach in the Kunijok River basin, and this was later improved by Olga I. Budarina and Galina G. Sapunova (Bozhinsky and Myagkov, 2001). This approach is seriously limited by dependency on local hydrothermal conditions, soil, and substrate, poorly readable tree rings, and dramatic loss of information with increasing age. The most reliable dates can be obtained by measuring young shoots from the bases of trees fallen or deformed by flows. On this basis, massive activations of slush flows were established for 1943, 1946, 1950–1952, 1960, 1966, 1969, 1977, and 1987 (Perov, 1966; Sapunov, 1991; Bozhinsky and Myagkov, 2001). Ananiev (1998) reported massive slush flow traces in the Hackman Valley that had almost damaged the infrastructure of the Rasvumchorr mines in May 1995.
Avalanches in the Kola Mountains typically occur annually – snow bodies are
regularly found in valleys and can remain unthawed till the following
winter. Rockfall, scree, and debris flow processes normally take place much
less frequently. However, huge block fields – seismic rockfall bodies –
are widespread on the foothills. In several basins of the Northern and
Western Khibiny, dramatic imprints of extreme events represented by
substantial clastic volume but which was transported for relatively short
distances (0.5–2 km) are regarded as traces of seismically triggered
catastrophic events. Combinations of poorly sorted large clastic bodies
(block sizes
The introduction of the radiocarbon method became a turning point in progressing understanding of the age of all of the above-mentioned processes. However, dating of the catastrophic events themselves is still rather difficult. Their age limits are most reliably obtained by testing organic compounds formed during periods of relative stability, which separate the phases of extreme activities. Such materials include peat and primitive soil layers or even lenses of mineral substrate with little content of humified matter. Such layers form on stable surfaces and later become buried by younger clastic deposits. For the events themselves, such dates determine lower (prior to the event) time marks or, alternatively, both lower (prior) and upper (post-event) marks in sections with several datable organic horizons separating several clastic layers.
Vashchalova (1987) assembled the first series of
The current insufficient number and spatial coverage of the available dates point to the need for collecting more data on debris flow phenomena over the entire Late Glacial–Early Holocene. Nevertheless, the extensive occurrence of distinctive large relic landforms and thick bottom deposits without any detectable organic matter indicates a substantially higher magnitude of debris flow activity in the distant past. Most likely, these debris flows functioned in colder and snowier environments during the last deglaciation stages. Here, it is probable that the remaining mountain glaciation in the affected valley headwaters caused periodical moraine-dammed lake outbursts and associated glaciogenic debris flows.
Interpretation of the evidence can be substantially improved by comparing
data from other natural paleoarchives reflecting the general tendencies of
landscape evolution. Traces of catastrophic events fixed in lake bottom
sediment records can be revealed by supplementary approaches. For instance,
recent work by a research group from the Kola Scientific Center RAS
(Nikolaeva et al., 2016) revealed a catastrophic outburst into the Imandra
Lake from the surrounding mountainous catchments as indicated by a coarse
clastic breccia-type layer within the gyttja deposits. The authors associate
this outburst with the seismic event between 6.5 and 5.6
A prominent 6 cm thick layer of clear grey silt was found within organic deposits southward of the Goltsovoye Lake. As reported by Shilova and Romanenko (2016), this layer is younger than 5.5 ka and contains a substantially lower amount and taxonomic variability of diatoms, although without significant differentiation of their species. In addition, higher amounts of terrigenous material are characteristic for the underlying Early Holocene sandy facies. All of the above can be taken as evidence of some catastrophic (probably, debris flow) events within the basin, which caused fast and large-scale inputs of mineral sediment into the lake mainly at the beginning and occasionally in the second part of the Holocene.
The erosional potential of slush and debris flows can be estimated based on geomorphic footprints. Deep V-shaped incisions widespread in the upper reaches of the 1st order streams, which are almost dry during most of the warm season, attest to an origin resulting from an agent other than fluvial. In places, these features incise even bedrock, revealing the substantial intensities of the eroding flows. Normally, the catchment areas above them (cirques, niches, etc.) provide abundant snow and water supply and serve as modern slush flow sources. Downstream in the major valleys, several incisive cycles are imprinted in the bottom topography. Based on the relatively low discharge rates and no geomorphic indication of active erosion by fluvial processes, those features should also be associated with slush flow episodes, probably caused by climatic oscillations, or sometimes, local base level decrease.
A trend of reduction in the magnitude of debris flow phenomena since the last deglaciation is observed. During the early stage of the Holocene widespread debris flow deposition was dominant, leaving large geomorphic footprints inconsistent with modern runoff conditions. Most likely the largest of those were caused by proglacial lake outbursts. Later, debris flow transportation capacity and frequency decreased markedly, probably due to the reduction of both water and sediment sources. As a result, debris flows were largely replaced by slush flows with much lower clastic content, which resulted in much smaller modern accumulations. However, the higher erosional potential of slush flows has been causing ongoing incision into relic glacial and debris flow landforms.
Available results for the Khibiny Mountains suggest slush flows and typical
debris flows with lower frequency are both leading mechanisms and
underestimated agents of downstream sediment delivery and valley floor
topographical formation during the Holocene.
In the 1st order streams, fluvial topography is extremely suppressed
or nonexistent as stream channels are unable to rework slush flow deposits
and are forced to adjust passively. In typical erosional valleys of the
2nd order, fluvial processes are also almost completely paralyzed by
even minor deposition as a result of high-frequency slush flows. Here,
streams are limited to reworking the finer fractions of slush flow fans,
forming secondary alluvial features downstream. Small valleys of the
3rd and higher stream orders with glacial topography, are usually
devoid of slush flows except for low-magnitude events of the wet avalanche
type in riverbeds. Only rare major slush flow ejections from tributaries
producing large superimposed fans in the main valley floor can influence
their fluvial cycles. Such rare catastrophic events can lead to major river
channel shifts and deep fresh-looking incisions. The recovery phase of normal fluvial topography can be used to indicate
the magnitude and time of the last extreme slush flow events by assessing
the amount of fluvial reworking of associated deposits, transportation
distances and capacities using grain size and radionuclide analyses. The recurrence interval of medium magnitude slush flows does not exceed
10–30 years, which is in agreement with previously published monitoring
data. Low magnitude events are even more frequent (up to every 1–2 years)
in headwaters and tributaries of certain valleys. The frequency of extreme
events is much lower and can be estimated as at least twice per millennia,
according to radiocarbon dating, and is probably even higher. Application of the radiocarbon method to determine the ages of
stabilization periods for the Khibiny Mountains fans and valley bottoms
provided a basis for distinguishing stages of non-uniform activity of slush
flow phenomena during the second half of the Holocene, most likely
controlled by climatic oscillations and, at places, by the fluctuations of
local base levels. The intensity of catastrophic slope and slush flow
processes increased during BCE 3600–2800, 1500–1250, 850–700, BCE 150–CE 300, and since CE 1350, and an even higher frequency of associated events
can be speculated for the first half of the Holocene climatic optimum
(7.5–6 ka) though direct geochronological evidence for this is still
scarce. Large geomorphic footprints inconsistent with modern runoff conditions
and lacking any buried visible organic matter, in general, attest to a
leading role of debris flows in sediment transport during the late
deglaciation and the Early Holocene associated with repetitive breaches of
moraine-dammed lakes and subsequent powerful glaciogenic debris flows.
Later, a distinct decrease of debris flow transportation capacity and
frequency occurred due to the depletion of both water and sediment sources.
Here, debris flows were largely substituted by slush flows with much lower
clastic content, which explains the much smaller modern sediment
accumulations. However, the higher erosional potential of those phenomena
caused a direct incision of glacial and debris flow landforms marked by a
series of widespread deep superimposed cuts in valley bottoms. A transition
period from the dominance of typical debris flows to that of the slush flows
is yet to be determined more precisely by means of absolute geochronology
and this represents the most challenging problem for future research in the
area.
The majority of the data presented here are from an ongoing and yet incomplete project. All datasets will be made (and partially already is) available on completion of the project and the results will be published with a link probably hosted by Russian Foundation for Basic Research. Please contact Ekaterina V. Garankina for further information.
All co-authors took part in field working and sampling. MMI and NVK performed radionuclide content studies. EDT carried out grain size analysis. FAR summarized previously accomplished geochronological data. ALG, VRB, and EVG designed cartographic and artwork and the latter prepared the manuscript with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Land use and climate change impacts on erosion and sediment transport”. It is a result of the ICCE Symposium 2018 – Climate Change Impacts on Sediment Dynamics: Measurement, Modelling and Management, Moscow, Russia, 27–31 August 2018.
The authors are grateful to Andrey A. Lukashov, Olga S. Shilova and numerous MSU students for fieldwork assistance and data processing during the 2008 winter expedition and the 2015–2018 summer training courses of the Geomorphology and Paleogeography Dept.
This research has been supported by the Russian Foundation for Basic Research project no. 17-05-00630 in part of fieldwork organization and radiocarbon dating and by the Geomorphology and Paleogeography Dept., Faculty of Geography, MSU (Governmental Assignment of no. AAAA-A16-11632810089-5) for assisting with grain size and radioisotope measurements.