1. Introduction
Quantifying and understanding catchment sediment yields (SY , t
km−2 y−1; i.e., the amount of
sediments exported from a river system per unit of time and catchment
area) has been a central research theme for many decades. Over this
time, it became increasingly clear that humans have a strong and rapidly
growing impact on SY (Walling and Fang, 2003; Syvitski et
al. , 2005; Montgomery, 2007; Borrelli et al. , 2017). Quantifying
and understanding these impacts is not only a key research challenge in
hydrology and fluvial geomorphology (Hoffmann et al. , 2010;
Tarolli, 2016; Poesen, 2018) but also of great societal/economic
relevance and necessary to fully understand anthropogenic impacts on
carbon fluxes and the Earth System as a whole (e.g., Oost et al. ,
2007; Galy et al. , 2015).
From earlier work, it became evident that human impacts on sediment
fluxes are highly scale-dependent (Walling, 1988; Walling and Fang,
2003). Hillslopes and small catchments typically demonstrate a much
stronger and faster response to land-use changes than larger river
systems (Dearing et al. , 2006; Montgomery, 2007; Montanheret al. , 2018). This is also apparent at a global scale: estimates
of impacts of human land cover changes on hillslope erosion rates (Oostet al. , 2007; Borrelli et al. , 2017) are about ten times
larger than the corresponding impact on the sediment flux to the oceans
(Syvitski et al. , 2005). While the general mechanisms explaining
this scale-dependency are known (e.g. Trimble, 1999), its
characteristics and relation to other environmental factors remain
poorly quantified and comprehended (Dearing et al. , 2006;
Hoffmann et al. , 2010; Tarolli, 2016; Poesen, 2018).
Nevertheless, understanding the sensitivity and scale dependency of
river systems to human disturbances is crucial for effective catchment
and land management strategies (Trimble, 1999; Vanmaercke et al. ,
2011; de Vente et al. , 2013; Poesen, 2018) and strongly links to
ongoing debates about the role of (historic) land use as the primary
driver of soil erosion and land degradation in the Mediterranean and
other regions (Cox et al. , 2010; García-Ruiz, 2010; Dusaret al. , 2011; Vanacker et al. , 2014). Detailed catchment
sediment budget studies or lake sediment analyses can provide valuable
insights into the history and degree of human impacts on SY(Trimble, 1999; Dearing et al. , 2006; Hoffmann et al. ,
2010; Dusar et al. , 2011; Golosov et al. , 2021; Ivanovet al. , 2021).
However, the human factor is not the only one controlling the sediment
flux — variations in SY are driven by a wide range of natural factors,
including geomorphic, tectonic, climatic, and biotic factors (e.g.
Syvitski and Milliman, 2007). For example, it was shown that
interactions between lithology and seismicity could exert strong
controls on erosion and catchment denudation rates via the effect of
rock fracturation (Molnar et al. , 2007; Portenga and Bierman,
2011; Vanmaercke et al. , 2014b, 2017). While some recent works
revealed no significant climatic impact on natural SY (Vanmaerckeet al. , 2014a), this factor will likely be more relevant at a
temporal scale for mountain regions (Carretier et al. , 2013;
Jeffery et al. , 2014).
To deepen the understanding of the impacts of land use and climate
change on sediment load, we explore mechanisms of the suspended sediment
yield formation in the Northern Caucasus during the Anthropocene (Waterset al. , 2014, 2016).
The collapse of the Soviet Union in 1991 has led to significant land
reforms in Russia (Ioffe et al. , 2004; Golosov et al. ,
2018), including the Caucasus region (Hartvigsen, 2014). Agricultural
land previously owned by the State and used for large-scale farming was
privatized in the early 1990s by distributing ownership rights to large
state farms among former collective farm members (Hartvigsen, 2014).
Overall, the economy’s restructuring has led to agricultural land
abandonment in the former Soviet Union (Lesiv et al. , 2018).
Recent studies show that Northern Caucasus has cropland loss byca. 8% in 2015 compared to 1987 (Buchner et al. , 2020).
However, common for former Soviet Union forest recovery on abandoned
agricultural fields (Griffiths et al. , 2014) has resulted in
forest gain of 6% in the Northern Caucasus (Buchner et al. ,
2020).
The Northern Caucasus has experienced climate changes over the past
decades, with summer temperature increased by 0.5-0.7°C over the past 30
years (Toropov et al. , 2019). The recent warming over the
Caucasus Mountains has substantially impacted the glaciers, leading to
losses at an average of 0.46% of the glacierized area per year
(Tielidze and Wheate, 2018). However, the water cycle’s associated
intensification did not cause any significant change in the
precipitation regime (Toropov et al. , 2019). Recent studies on
the North Caucasus rivers (Rets et al. , 2020) show that June
runoff increased by 1.1-9.1% per decade for the last 70 years, while
August’s runoff from highly glacierized catchments has decreased by
1.0-6.3% per decade. In contrast, the August runoff of non-glacierized
catchments has increased by 1.5–11.5% per decade. With the growth of
interest in environmental change over the Caucasus Mountains (Tielidze
and Wheate, 2018; Toropov et al. , 2019; Rets et al. ,
2020), it is crucial to consider the extent to which sediment flux is
changing, as an important index of the functioning of the earth system
mainly in response to landuse and climate changes (Walling and Fang,
2003).
Based on previous studies relating to sediment flux response to climate
change (Walling and Fang, 2003; Li et al. , 2020; Zhang et
al. , 2020), we hypothesized that in mountain and high-mountain
catchments of the Northern Caucasus, the suspended sediment discharge
values (SSD , [kg s−1]) have been
decreasing since the beginning of the Anthropocene in ca. 1945.
To test this hypothesis, we analyzed catchment suspended sediment yields
calculated using observed hydrological data. We explore how sediment
flux of various river basins with different land-use/landcover and
glacier cover changes over time. We mainly focused on small catchments
(A < 103 km2) as they are
typically more sensitive to human impacts (Walling, 1983; Dearing and
Jones, 2003; Vanmaercke et al. , 2015) and climatic change (Oswoodet al. , 1992; Moore et al. , 2009).