Complex sentence: Because of its great depth, the lake as a whole never freezes in winter, but an ice sheet covering between 10% and 90% of the lake area typically develops, depending on the severity of the winter.
Split into simpler sentences: Because of its great depth, the lake as a whole rarely freezes in winter. During the winter months, an ice sheet covering between 10% and 90% of the lake area typically develops, depending on the severity of the winter.