In northern Fennoscandia, semi-alluvial boulder-bed channels with coarse glacial legacy sediment are abundant, and due to widespread anthropogenic manipulation during timber-floating, unimpacted reference reaches are rare. The landscape context of these semi-alluvial rapids— with numerous mainstem lakes that buffer high flows and sediment connectivity in addition to a regional low sediment yield— contribute to low amounts of fine sediment and incompetent flows to transport boulders. To determine the morphodynamics of semi-alluvial rapids and potential self-organization of sediment with multiple high flows, a flume experiment was designed and carried out to mimic conditions in semi-alluvial rapids in northern Fennoscandia. Two slope setups (2% and 5%) were used to model a range of flows (Q1 (summer high flow), Q2, Q10 & Q50) in a 8 x 1.1 m flume with a sediment distribution analogous to field conditions; bed topography was measured using structure-from-motion photogrammetry after each flow to obtain DEMs. No classic steep coarse-bed channel bedforms (e.g., step-pools) developed. However, similarly to boulder-bed channels with low relative submergence, at Q10 and Q50 flows, sediment deposited upstream of boulders and scoured downstream. Because the Q50 flow was not able to re-work the channel by disrupting grain-interlocking from preceding lower flows, transporting boulders, or forming channel-spanning boulders, the channel-forming discharge is larger than the Q50. These results have implications for restoration of gravel spawning beds in northern Fennoscandia and highlight the importance of large grains in understanding channel morphodynamics.
Ecological and geomorphic theory assume longitudinal connectivity; we test whether these concepts apply in a naturally disconnected stream network with mainstem lakes and coarse glacial legacy sediment. We determined downstream hydraulic geometry relationships for channel width and inventoried riparian vegetation in each new process-domain (rapids, slow-flowing, lakes) along a continuous ~10 km segment in two catchments in northern Sweden. Hydraulic geometry relationships for width were very weak, indicating that although channel width does increase in the downstream direction, there is very large local variation in width, within and among process domains. Riparian vegetation richness did not increase markedly downstream as expected in a connected stream network, and there are very weak relationships between riparian vegetation composition similarity among reaches and distance between reaches, indicating that hydrochory plays a minor role plays in metacommunity organization. Formerly continentally-glaciated catchments are thus highly fragmented and local factors steer geomorphic form and biotic organization.