Complex sentence: It stretches roughly three miles and was formerly a dense area of mature oak and ash trees, it was flooded in 1954 to facilitate the building of two hydro-electric dams in Inniscarra, of which it forms part of the upper reservoir.
Split into simpler sentences: It stretches roughly three miles and was formerly densly populated with mature oak and ash trees. However it was flooded in 1954 to facilitate the building of two hydro-electric dams in Inniscarra, of which it forms part of the upper reservoir.