5. Proglacial Hydrogeology
5.2. Dominant geomorphic processes and hydrological environments at the
Skaftafellsjökull foreland
This section describes the dominant geomorphic processes at the Skaftafellsjökull
foreland, notably glaciofluvial and glacial processes. These varied processes are the main cause for the high spatial variability in hydraulic conductivity at the site. In contrast to Skeiðarársandur, the Skaftafellsjökull foreland is mainly impacted by glacial processes,
109 such as moraine development, glaciofluvial processes (Marren, 2002b) rather than glacial surges and jökulhlaups.
5.2.1. Glaciofluvial deposits
Glaciofluvial deposits are deposited by meltwater streams which originate at the glacier margin. These deposits are generally composed of coarse sand and gravel size, which then becomes finer downstream (e.g. Anderson, 1989). Glaciofluvial deposits have high hydraulic conductivity, of around 1.00x100 to 1.00x102 m/day (Brassington, 2007). Therefore, coarse-grained glaciofluvial sediments form extensive aquifers in areas that were previously glaciated in North America (e.g. Burns et al., 2010; Bajc et al., 2014) and Europe (e.g. Bayer et al., 2011).
The largest extent of glaciofluvial deposits at the Skaftafellsjökull foreland is associated with the active braid plain of the Skaftafellsá. This area consists of outwash and is
characterised by active and relict braided channels, that reflect the lateral migration of the Skaftafellsá (Figure 5.1). The majority of the glaciofluvial deposits at the Skaftafellsjökull foreland originated from low-magnitude, high-frequency events, which are mainly
controlled by ablation (e.g. Marren, 2002b). This is in stark contrast to Skeiðarársandur, where high magnitude, low frequency events play a major role in the deposition of sandur glaciofluvial deposits (e.g. Robinson et al., 2008). Fluctuations of the glacier margin are an important control on channel development and spacing at both Skeiðarársandur
(Robinson et al., 2008) and the Skaftafellsjökull foreland, with drainage patterns and channel positions continuously changing in response to advances and retreats of the margin (Marren, 2002b; Marren and Toomath, 2013; 2014). For instance, the ongoing retreat of Skaftafellsjökull has resulted in the abandonment of the western branch of the Skaftafellsá such that the eastern channel is the only active river draining the ice margin (Figure 5.1).
110 Figure 5.1. The Skaftafellsjökull foreland fieldsite (Vatnajökull National Park, 2007).
Significant drainage changes have taken place since the image was taken. The main ones are the expansion and merging of the ice-contact lakes and drainage diversion causing the drying of the western branch of the Skaftafellsá. The black box shows the approximate location of Figure 3.11, which shows the dry branch of the Skaftafellsá.
5.2.2. Glacial deposits
Till is a sediment that has been entrained and deposited by glacial ice, with little or no sorting by water (e.g. Shaw, 1985). Till deposits are complex and can be highly
heterogeneous, containing deposits with varied hydraulic properties (e.g. Anderson, 1989; Meriano and Eyles, 2009). The hydraulic conductivity of till can vary over seven orders of magnitude (Freeze and Cherry, 1979). The origin and secondary processes which impact till are important determinants of its hydraulic conductivity (e.g. Hendry, 1982; Stephenson
et al., 1988). Till deposits can also contain extensive layers of fine grained material, which
can form extensive aquitards (e.g. Meriano and Eyles, 2009). The dominant glacial
landform at the Skaftafellsjökull foreland is moraines, which surround many of the lakes at the Northern and Southern Oasis. The internal hydrology of moraines is complex, and can include layers of low permeability (e.g. Langston et al., 2013), which can impact
111 groundwater flow and lake formation. Therefore, it is hypothesised that the groundwater flow through moraines will be highly variable. Field observations suggest that the main till deposits at the Skaftafellsjökull foreland are found in the Northern Oasis (Figure 5.2).
Figure 5.2. Fine-grained deposits at the Northern Oasis.
A. Sequence of fine-grained clay layers near Lake Lupin (Figure 5.1). B. An attempt to emplace piezometers at the Northern Oasis lakes. Notice the poor sorting of the deposits, which consist of fine sediments and boulders. The pit is located approximately 50 m from the lakeshore of Lake Lupin.
5.2.3. Lacustrine processes
Lacustrine deposits are generally fine grained and have low permeability (e.g. Domenico and Schwartz, 1998). These layers of fine sediment are likely to significantly impact the hydraulic conductivity of the lakes and impact lake-aquifer exchange (e.g. Blume et al., 2013). Conversely, significant groundwater-lake exchange has been observed in lakeshores with coarse deposits, such as talus slopes (e.g. Hood et al., 2006; Roy and Hayashi, 2008).
112 Lakes and lake basins of various sizes are common features of glacier forelands,
including the Skaftafellsjökull foreland (Figure 3.13). Confined topographic basins in which lakes can form can be created by a number of glacial processes including the erosion of overdeepenings (e.g. Marren and Toomath, 2013) and the melt-out of buried ice. They can also form with inter-moraine basins (e.g. Robinson et al., 2008). Glacial lakes connected with proglacial meltwater systems with high sediments loads are associated with relatively rapid sedimentation rates. Seasonal variations in meltwater discharge and sediment delivery to glacial lake basins can result in the formation of varves that are characteristic of glaciolacustrine sequences (e.g. Marren, 2002a). These typically
comprise alternating layers of clay and silt-sand representing annual cycles. Lake basins that are not connected to surface meltwater rivers and that are in contrast fed by rainfall and groundwater are characterised by much lower turbidity and sedimentation rates.
Lakeshores which are underlain by fine-grained deposits at the Skaftafellsjökull foreland are located in the Northern Oasis (Figure 5.1), in drying lakebeds at the western part of the Southern Oasis, and at the eastern shore of the Instrumented Lake (IL). Conversely, the western lakeshore of the IL is underlain by coarse glaciofluvial deposits. These differences suggest high variability in the hydraulic conductivity of lakeshores at the Skaftafellsjökull margin. In addition to the lakes in the Southern and Northern Oasis, the foreland of Skaftafellsjökull is also dominated by the ice-contact lake, which has been expanding substantially following the retreat of Skaftafellsjökull into an overdeepened basin (Marren and Toomath, 2013).
5.2.4. Summary of depositional environments at the
Skaftafellsjökull foreland
The retreating of the Skaftafellsjökull foreland generates complex glaciofluvial, glacial, and lacustrine processes (Marren, 2002a). In contrast to Skeiðarársandur, the dominant fluvial regime at the Skaftafellsjökull foreland is generally controlled by ablation. Furthermore,
113 the Skaftafellsjökull foreland is not impacted by jökulhlaups and glacial surges. Most of the deposits at the Skaftafellsjökull foreland originate from glaciofluvial processes, till
deposition, and lacustrine deposits (Marren and Toomath, 2013). This high variability in proglacial geomorphology is therefore expected to lead to a high variability in
hydrogeological parameters.
This overview provides a good understanding for the geomorphic processes and their associated environments which impact the Skaftafellsjökull foreland and the related deposits which are characteristic to these environments. In addition to the description of the dominant depositional and geomorphic processes the current study also took field and laboratory hydrogeological measurements to determine small-scale aquifer parameters. These results are important for the hydrogeological classification of the field site (objective ii) and for the understanding of groundwater exchange with rivers (objective iv) and lakes (objective v).