Complex sentence: The 2011 Souris River flood is a record hundred-year flooding event on the Souris, a tributary of the Assiniboine River, which it meets near Treesbank, Manitoba.
Split into simpler sentences: The 2011 Souris River flood was greater than the hundred-year flooding event for the Souris. The Souris River is a tributary of the Assiniboine River, which it meets near Treesbank, Manitoba.