The two correct answers that helped hydrologists predict floods are snowpack and memorable land area (which I interpret as referring to land characteristics such as topography, land use, or other memorable or significant environmental features that can affect runoff and drainage).
Water temperature, nitrate levels, and dissolved oxygen are more related to water quality rather than directly impacting flood predictions. Snowpack is crucial as it serves as a significant water source that can lead to flooding when it melts, and land characteristics can influence how water flows during rainfall or melting conditions.