Flow-electrode capacitive mixing (F-CapMix) utilizes ion adsorption/desorption on active materials in the flow-electrode to harvest salinity gradient power. In order to facilitate flow-electrode flow in the F-CapMix unit cell, the current collectors should include a meandering flow channel for the cell assembly and should facilitate efficient electron transport between them and the flow-electrode active materials. This study evaluates the F-CapMix performance variation and pumping power of the flow-electrode and feed water depending on flow channel depth. It was found that both parameters increased as the cross-sectional area of the flow channel narrowed. This result was due to the increasing linear velocity of the flow-electrode, which itself was associated with the narrowing cross-sectional area. The maximum gross power densities of the F-CapMix increased in a parabolic shape with respect to the flow-electrode linear velocity increase. (0.68 → 0.83 → 0.86 → 1.03 → 1.14 W/m²) On the contrary, pumping power densities of the flow-electrode increased linearly (2.22 → 2.98 → 3.51 → 5.93 → 19.67 W/m²) with linear velocity increment of the flow-electrode. Further, the F-CapMix net power density was negatively affected by the increasing linear velocity of the flow-electrode (net power density variation from −2.06 to −19.10 W/m²), likely owing to the higher pumping power density required due to the narrower flow channel. This study verifies the importance of securing an appropriate cross-sectional area of the meandering flow channel to maintain positive net power of the F-CapMix.