Complex sentence: Prior to 600 AD, the lake Mälaren was connected to the sea, but due to land elevation, the lake was slowly cut off and boats had to be ported over land to and from the lake.
Split into simpler sentences: Prior to 600 AD, the lake Mälaren was connected to the sea. Due to land elevation, the lake was slowly cut off, and boats had to be carried over land to and from the lake.