Complex sentence: The drainage was not however continuous, as Lake Agassiz periodically had other outlets as the Laurentide ice sheet retreated and advanced with climatic variations, which in turn contributed to isostatic adjustments in the level of the land over which the watercourses ran.
Split into simpler sentences: The drainage was not however continuous, as Lake Agassiz periodically had other outlets. The Laurentide ice sheet retreated and advanced with climatic variations and these changes in the ice sheet in turn contributed to isostatic adjustments in the level of the land over which the watercourses ran.