In fishway design, the combination of fish swimming behaviors and suitable fishway hydraulic characteristics increases the fish passage efficiency. In this study, the most representative grass carp among the “four major Chinese carps” was selected. Under conditions similar to the time period for feeding migration, juvenile grass carps were targeted to study the swimming characteristic indicators (i.e., critical and burst swimming speeds) and swimming behaviors that were closely associated with fishway hydraulic design using the incremental water velocity method in a homemade test water tank. (1) The study results reveal that both the absolute critical (U_crit) and burst (U_burst) swimming speeds increased linearly with increasing body length and both the relative critical (U’_crit) and burst (U’burst) swimming speeds decreased linearly with increasing body length. There existed a quantitative relationship between U_burst and U_crit, which could facilitate the fishway hydraulic design. (2) This study analyzed the effects of water velocity changes on fish swimming behaviors and proposed a classification method for four fish swimming behaviors—swimming freely, staying, dashing at a long distance, and dashing at a short distance—of tested fish during the process of adapting to water velocity changes interspersed with one another. The entire swimming process under the incremental water velocity was divided into four stages. (3) This study suggests that the maximum water velocity of the mainstream in a fishway using grass carp as the major passage fish should not exceed 52–60% U_burst at stage 1. For the high-water velocity areas of a fishway, such as vertical slots and orifices, the optimal water velocity should not be higher than 76–79% U_burst at stage 2 and should absolutely not exceed 90–96% U_burst at stage 3.

The successful fish upstream movement through a dam/gate is closely associated with the hydraulic conditions of a fishway. To improve the passage efficiency, this study investigated the upstream swimming behaviors of juvenile grass carp, a representative fish of four major Chinese carps, under characteristic hydraulic conditions of a designed vertical slot fishway model. The impacts of different discharges and baffle lead angles on the successful movement of test fish were analyzed, and the selection of the movement trajectory was studied through overlay of their upstream swimming trajectories on the water flow field resulting from numerical modeling. We found that under the same discharge, the percentage of successful test fish movement with a lead angle of 45° was higher than 60° and 30°. Within a fixed lead angle, the higher the discharge, the lower the percentage of successful movement. During upstream movement, the test fish had a preferred water velocity of 0.01–0.45 m/s in the pool, and avoided areas where the turbulence kinetic energy (TKE) was greater than 0.012 m²/s². These results provide a basis for the hydraulic design of vertical slot fishways and a reference for studying swimming behaviors of other fish species.

The decrease in periodic scouring of pulsed flows in regulated rivers can result in algal communities dominated by filamentous algae, not available as food sources for fish and macroinvertebrates. To study the pulsed flow velocity required to scour benthic algae from natural river beds, the removal effects on the algal biomass and resistances of different species were tested in a laboratory flume at different velocities of 0.8, 1.1, 1.4, 1.7, and 2.0 m/s. The removal of total algal biomass showed a significant positive relationship with increasing velocities, which reached 22% at 2.0 m/s. The biomass removal of green algae and diatoms was higher than that of blue–green algae. The flow velocity at 1.4 m/s had a clear removal effect on filamentous algae. The velocity higher than 1.7 m/s caused a significant increase in the removal percentage of total biomass dominated by diatoms and blue–green algae. To reduce the filamentous algae and retain the diatoms and blue–green algae, a range of near bed flow velocity was suggested to be 1.4–1.7 m/s. This range could serve as a reference for required pulsed flow velocity to reduce the growth of excessive or nuisance periphyton.