Author(s): Lina E. Polvi; Lovisa Lind
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Abstract: An important component of stream restoration projects is evaluation of restoration success and determining which factors impact how a stream recovers. Beyond physical manipulation of a channel and planting of riparian vegetation, passive recolonization occurs through hydrochory (seed dispersal by water) in a connected catchment, and geomorphic adjustment can occur based on the current flow regime in alluvial channels. These biotic and geomorphic adjustment processes are partially based on the idea of predictable longitudinal changes in a stream network, as described by the concepts of downstream hydraulic geometry and the river continuum concept. Here, we test whether these concepts are applicable in a naturally disconnected stream network with numerous mainstem lakes and coarse glacial legacy sediment. In two medium-sized catchments in northern Sweden, which contain three process domain types (rapids, slow-flowing reaches and lakes), we determined downstream hydraulic geometry relationships for channel width and we inventoried riparian vegetation in each new process domain reach along a continuous～10 km segment. We found that hydraulic geometry relationships (power relationships with drainage area and channel width) were very weak and with low exponents (b-values) even when lakes were excluded, indicating that channel width does not increase predictably or as quickly as expected, longitudinally. Riparian vegetation richness did not increase markedly downstream as would be expected in a connected stream network, and there were very weak relationships between similarity in riparian vegetation composition between reaches and distance between reaches, indicating that hydrochory plays a small role in metacommunity organization. These biotic and geomorphic results show that these catchments are highly fragmented and local factors, such as glacial deposits and local seed banks, likely steer geomorphic form and biotic organization. With weak downstream hydraulic geometry relationships, traditional stream ecology concepts that assume increasing width as drainage area increases may not apply. Stream restoration success in northern Sweden must therefore be evaluated based on local conditions and passive restoration may not be enough for recovery of stream geomorphology or riparian zone communities.
Year: 2018