An important consideration in the development of a plan for technical upstream fish passage installation operation and enhancement at a hydroelectric dam is to ensure that an adequate zone of passage exists in the channel downstream of the fishway entrance that will guide fish and facilitate safe, timely and effective fish movement up to the fishway entrance during project operations. In addition, project operations should ensure adequate attraction flows from the fishway entrance that allow migratory fish to locate and enter the fish way for passage upstream. The objective of this presentation is to discuss the use of computational fluid dynamics (CFD) modeling (2D and 3D) to examine potential flow patterns and dynamics in the tailrace and powerhouse discharge of the approved generation expansion of the Holtwood Hydroelectric Project on the Susquehanna River. The CFD modeling was used to assess if velocity barriers created by the proposed powerhouse discharge and associated channel excavation scheme could impede the upstream migration of fish. The CFD model was also used to assess if an area of low velocity flow would exist along the shore leading to the fishway and in the spillway channel, facilitating adequate zones of passage which fish could negotiate up to the fishway entrances. The model was also used to examine potential false attraction or guidance to areas of the dam where no fish passage existed and fish could become stranded or delayed.