Unravelling fluvial versus glacial legacy controls on boulder‐bed river geomorphology for semi‐alluvial rivers in Fennoscandia

Abstract

River management is founded on predictable self‐organisation between river form and catchment controls in alluvial rivers. However, a substantial proportion of rivers are not fully alluvial. In previously glaciated landscapes, boulder‐rich glacial till influences river channel form and process. Increasing interest in nature‐ and process‐based river restoration requires knowledge of pre‐disturbance natural processes, which does not exist for semi‐ and non‐alluvial rivers in Fennoscandia. We aimed to determine the role of Pleistocene glaciation and subsequent deglaciation versus Holocene fluvial processes in controlling channel form of boulder‐bed rivers in Fennoscandia. We quantified morphological characteristics of northern Swedish boulder‐bed rivers, in which channel morphology was minimally impacted by humans, and used the degree of alluvial signatures to infer fluvial and legacy glacial controls. We conducted surveys of reach‐scale channel geometry, boulder and wood distributions and catchment characteristics for 20 reference reaches (drainage area: 11–114 km2). Reaches ranged in slope from 1% to 8% and were extremely diverse in channel geometry. Rivers showed little self‐organisation at the reach scale; no association exists between channel width and channel slope or bed sediment size. Boulders were rarely clustered into bedforms (e.g., step‐pools) typical of boulder‐bed mountain rivers. Drainage area was positively correlated with channel capacity but not channel width, slope or sediment size. Channel boulder density was best predicted by surveys of terrestrial boulders. Consequently, channel geometry, boulder size and the distribution of boulders were primarily controlled by legacy glacial conditioning rather than current fluvial processes, with some alluvial adjustment of smaller particles within the boulder template. Therefore, restoration of semi‐alluvial rivers should take into account local sediment and geomorphic conditions rather than use management principles built for fully alluvial rivers.

How big is a boulder? The importance of boulder definition choice in earth science research and river management

“…This study extends the analysis of field data collected to understand the controls on boulder-bed channel morphology in previously glaciated catchments in northern Sweden (Mason & Polvi, 2023). In these catchments, glacial legacies influence both valley geometry and reachscale channel morphology via glacial legacy sediments in the form of large boulders (Figure 1).…”

“…They found limited evidence of alluvial processes and concluded that channel geometry and boulder characteristics were derived from legacy glacial processes, with little modification by contemporary fluvial processes (Mason & Polvi, 2023). We repeat this analysis with different definitions of boulder size.…”

“…In these catchments, glacial legacies influence both valley geometry and reachscale channel morphology via glacial legacy sediments in the form of large boulders (Figure 1). Mason & Polvi (2023) aimed to understand the influence of these boulders, deposited during Pleistocene glaciation (and de-glaciation) versus contemporary fluvial processes on channel morphology.…”

“…Valley confinement estimates the degree of connectivity between hillslopes and the river channel, important in some systems as a source of boulders. Therefore, Mason & Polvi (2023) examine associations between boulder characteristics, reach geometry and catchment characteristics to understand the role of fluvial forcing versus glacial legacy boulders on river morphology.…”

“…To do this, we analysed data from 20 rivers and more than 4,700 boulders, collected by Mason & Polvi (2023). They used a fixed definition (>1 m) and we re-analyse their data according to a second definition; where boulders are defined relative to the surface sediment distribution (> D 84 ).…”