Complex sentence: The land was heavily forested and swampy due to its proximity to the Mississippi River, only 5 miles to the east, which made the clearing and draining necessary to build on the site a difficult and slow-going task.
Split into simpler sentences: The land was heavily forested and swampy due to its proximity to the Mississippi River 5 miles to the east. Extensive clearing and draining was necessary, making construction at the site a difficult and slow-going task.