Characteristics and controls on stream channel heads in paraglacial regions—a field study from northern Fennoscandia

The location of stream channel heads on hillslopes is an important metric to determine cumulative channel length and remotely map channels. However, identification of channel heads can present a challenge in any landscape, where seasonal variations in flow may affect the location of perennial flow, which may not necessarily coincide with the location of a channel with definable banks. The location at which a channel starts usually reflects a point where concentrated water flow exceeds a shear stress threshold that erodes sediment. However, in areas with subsurface flow, these relationships between slope and contributing area at the channel head are much more variable. Furthermore, most studies of channel head locations have been conducted in mountain environments or other landscapes where alluvial processes dominate. Paraglacial regions, especially those formed after continental glaciation, contain depositional glacial landforms with variable till deposits that may not reflect current landscape equilibrium conditions. Abundant peatlands, even high in catchments, also typify paraglacial regions in northern Fennoscandia.

The objective of this study is to determine characteristics of and controls on channel head locations in a paraglacial region that has undergone Pleistocene continental glaciation with mainly depositional glacial landforms and abundant till. We focused on a region in northern Sweden encompassing ca. 500 km² over an elevation range of >1000m, around the city of Kiruna. We divided the study area into three elevation regions: low (~250-500 m), mid (~500-800 m) and high (~800-1500 m). Channel head identification was conducted remotely for those originating in a peatland, after field-verification of a subset. Thirty-four channel heads originating in unconsolidated sediment, mostly till, were field-identified in the three elevation regions: two at the low elevation, 13 at the mid elevation and 19 at the high elevation. At each channel head, locations were taken using an RTK-GPS and surveys of the longitudinal profile and several channel geometry measurements were taken along a 100-m reach. Nearly double the number of channels originate in peatlands than in unconsolidated sediment.

For the channels originating in unconsolidated sediment, the cumulative area required for channel head formation ranged over three orders of magnitude, ranging from <0.001 to nearly 0.1 km² at the low elevation sites (median: 0.0075 km²), <0.001 km² to nearly 1.0 km² at the mid (median: 0.08 km²) and high elevation sites (median: 0.07 km²). The gradients of the longitudinal profiles showed a weak positive trend with elevation, but the slopes of the high-elevation sites varied considerably from 6 to 40%. Sinuosities ranged from 1 to 1.2. Preliminary results also show highly variable cumulative areas for peatland-initiated channel heads, indicating a strong role of subsurface flow. We also present controls on channel head formation in terms of upstream hillslope gradients, surficial geology type and glacial depositional feature. However, these large and variable ranges of values for contributing areas of channel heads in paraglacial regions highlight challenges for remote prediction of channels heads and thus channel length, which can have broad implications, including for hydrological and biogeochemical modelling.