Controls on sediment transport in semi-alluvial boulder-bed streams—implications for restoration of spawning gravel

Many streams in northern Fennoscandia are considered semi-alluvial in that they contain abundant coarse sediment (cobbles and boulders) deposited during continental glaciation in the form of moraines, eskers, and erratics. These streams contain sensitive trout and salmon populations, and restoration efforts (after channelization from over a century of timber-floating) strive to re-create essential habitat and spawning beds. However, little is known about controls on sediment transport processes in boulder-bed streams that can affect both channel evolution and restoration of spawning gravel. Prior flume and field research of boulder-bed channels in mountainous areas show that boulder protrusion and proximity can alter critical shear stress; however, in contrast to mountain streams, boulder-bed channels in northern Fennoscandia have relatively low bed slopes (S₀: 0.5-6%) and low-magnitude flow regimes (buffered by upstream lakes).

In order to determine controls on gravel and cobble sediment transport in semi-alluvial boulder-bed streams, a sediment tracer experiment was conducted in the Vindel River catchment in northern Sweden. Approximately 1500 tracer clasts (b-axis: 2.5-15 cm), with imbedded RFID tags, were placed in five stream reaches (121-556 per reach) with a range of channel slopes (S₀: ~2-6%), boulder densities, and degrees of protrusion. The geometry of each tracer clast (a-, b-, and c-axes) was quantified, and the location of each tracer clast was surveyed with a total station in the summers of 2017, 2018, and 2019. Both the morphologic setting (e.g., step, pool, riffle, backwater, directly above/below boulder) and constrainment class (e.g., unconstrained, shielded, imbricated, buried) were classified for each tracer at each survey occasion. There was a 80-90% recovery rate of tracer clasts; despite several reaches experiencing >Q₅₀ snowmelt flood, the median transport distance for D₁₀- to D₅₀ clasts was ~0.1 m. Preliminary analyses showed large variation in particle-size thresholds for entrainment and relationships with transport distance within and among reaches. There was no clear relationship between local bed slope or calculated bankfull shear stress and transport distance. Differences in entrainment and transport distances among reaches was controlled by boulder density and protrusion, which likely increase grain resistance and thus critical shear stress, reducing sediment transport (as shown by previous studies in boulder-bed mountain streams). Factors negatively affecting sediment transport include shielding, proximity to boulders, and certain morphologic settings (e.g., backwaters and pools). Variability was too high to allow confident prediction of entrainment of individual grains; however, based on these results and observations, some general guidelines for stream restoration of spawning gravel in semi-alluvial boulder-bed channels are presented.