One of more important elements affecting the efficiency of fish passes is a 3D spatial structure of the flow and the flow velocity. The paper presents an analysis pertaining to the deformations of these elements caused by the modification of the fishway’s partition geometry. It has a crucial meaning for the ability of fish to migrate through the fish pass. The research has a key practical significance as it proves the way the apparent modification of the partition geometry affects the hydraulic conditions within the entire the channel of the fishway. It may have very serious consequences for the design assumptions. The presented results of the experimental research were obtained as a consequence of the site and laboratory tests carried outusing a physical model. The measurements were performedfor 3 components of flow’s instantaneous speed within the indicated measurement sections. The results were developed using Matlab software. It was established that the number of whirls produced in the fishway’s pool depends on the quantity of spillway slots and – what seems to have the greatest impact on the flow structure – on the location of slots. The size of whirls is determined by the geometry of spillway slots. The presence of additional spillway slots in the pool, except for the huge main migration slot, contributes to the generation of more whirl structures within the recirculation zone. The more whirls produced in each of the whirl zones, the smaller their diameter. Thisshould create more favourable conditions for the fish migration.