Complex sentence: Much of the lake is flooded marshland created by the damming of the Fox River downstream in Menasha in the 19th century, which explains the lake's flat and shallow character (maximum depth is 15 ft).
Split into simpler sentences: Much of the lake is flooded marshland, which was created by the damming of the Fox River downstream in Menasha in the 19th century. This has resulted in the lake's flat and shallow character (maximum depth is 15 ft).