Complex sentence: Labrador is an irregular shape: the western part of its border with Quebec is drainage divide for the Labrador Peninsula i.e. land drained by rivers that flow into the Atlantic Ocean are part of Labrador, the rest belongs to Quebec.
Split into simpler sentences: Labrador is an irregular shape: the western part of its border with Quebec is drainage divide for the Labrador Peninsula. Lands drained by rivers that flow into the Atlantic Ocean are part of Labrador, the rest belongs to Quebec.