Short-term river response and restoration of biological diversity following slit construction

• Bank scour or unconsolidated sediment failures occurred even during normal discharge.
• Velocity and channel geomorphic unit diversity increased after the slit construction.
• Species diversity decreased from 2.33 to 2.12 because of the rapid river response with the debris flow.

This study examined short-term temporal river restoration following slit construction using yearly surveys conducted from 2009 to 2011. The temporal changes caused by river restoration were monitored with regards to the river response, velocity diversity, channel geomorphic unit diversity and species diversity.The temporal change indicated a rapid increase in the hydraulic and channel geomorphic unit diversity by the river response, whereas the species diversity decreases by the rapid river response with the debris flow. The channel pattern changes were explained by an excess of shear stress, which eroded the bank toe. Bank scour or sediment failures then occurred during normal discharge. This process was the main mechanism of river widening in the Wasada stream.We suggested methods to assess the velocity and geomorphic diversity based on the Shannon diversity index for river conditions. The velocity and channel geomorphic unit diversity increased after the slit construction, with 1.31 in 2009, 1.68 in 2010, and 1.93 in 2011 for the velocity diversity and 1.05, 1.45, and 1.66 for the channel geomorphic unit diversity. Both diversities responded immediately to the slit construction, after which the response slowed. However, the species diversity remained lower than the pre-condition levels after the physical environment recovered. The reasons for the diversity decrease were the species evenness and the decrease in taxa richness. In the results, the species diversity varied as 2.33 (2009) to 2.38 (2010), and 2.12 (2011), while the species evenness decreased continuously: 0.79 (2009) to 0.74 (2010), and 0.73 (2011). The latter trend was caused by a rapid river response by debris flow that disturbed both the species population and species diversity. Species density and diversity decrease when the river response is very active in the early stage of river restoration.