A hydrologist is not necessarily a civil engineer, but the two fields are closely related. Hydrologists study the movement and distribution of water, including precipitation, evaporation, and groundwater, as well as the impacts of human activities on water resources and ecosystems. Civil engineers, on the other hand, design and construct infrastructure such as buildings, roads, bridges, and water-related infrastructure such as dams, canals, and wells.
However, many civil engineers specialize in water resources engineering, which is a subfield of civil engineering that focuses on the planning, design, and management of water-related infrastructure. In this subfield, civil engineers use their knowledge of hydrology, as well as other disciplines such as geology and meteorology, to design and construct infrastructure that can effectively manage and utilize water resources.
So, a hydrologist and a civil engineer who specializes in water resources engineering may have a lot of overlap in their knowledge and skills. However, a hydrologist would have more focus on understanding the water cycle, its interactions with different environments, and the impacts of human activities on water resources and ecosystems, whereas a civil engineer would have more focus on designing and building infrastructure to manage and utilize water resources.