Mississippi is a state with a humid subtropical climate, characterized by hot summers and mild winters. Major surface flows in the state include the Mississippi River, which forms the western border of the state, and its major tributaries, such as the Yazoo River and the Big Black River. There are also several large reservoirs and dams in Mississippi, including the Ross Barnett Reservoir and the Sardis Lake. The state's hydrology is influenced by its topography, which is characterized by low-lying coastal plains in the south and higher elevation regions in the north. Snowpack is generally not a significant factor in Mississippi's hydrology, but heavy rain events and tropical storms can cause flooding in low-lying areas.