Hydropower operation is evolving in the changing power grid with increased renewable energy capacity. Much of the renewable capacity is from variable renewable energy (VRE) sources such as wind and solar power technologies. In future, the ability to operate hydropower reservoirs flexibly can enable hydropower to provide multiple grid services. Since most large reservoirs are operated for multiple purposes, understanding the operational limitations of reservoirs for hydropower production is important. Specifically, it is critical to understand whether and how downstream biota are impacted by diurnal fluctuations in flow as reservoirs are operated in a high VRE future grid. This study compares the critical periods at seasonal and diurnal scales from the perspective of both the power grid and biodiversity. Two primary models are used in this study to understand temporal patterns of stress to the power grid and to a species of game fish using Yadkin Pee Dee (YPD) cascade in the southern U.S. We used PLEXOS, a production cost model, to better understand the operations and economic implications of the YPD cascade hydropower plants on critical grid periods. The sub-daily version of the Quantus model simulates the effects of hydro- and thermo-peaking regimes on the most-vulnerable, sessile life stage of fishes with different reproductive life histories. Results indicate that dispatch from hydropower plants follow the netload pattern, which has morning and evening peaks during colder months and evening peaks during warmer months. In addition, hydropower dispatches to compensate the VRE variability in generation. Furthermore, hydro plants are operated at maximum and minimum capacity levels with higher ramping between different capacity levels in future high VRE scenarios. These operating patterns are directly related to water release patterns and impact the downstream fish population. Results show that ramping restrictions during nighttime hours results in lower fish mortality under both current and future grid scenarios. Exploring multiple water release scenarios indicate the possibility of finding water release patterns that are informed by both power grid and biodiversity constraints.