Complex sentence: In 1960, they built the Thomaston Dam; in 1965, the Northfield Brook Dam; and in 1969, the Colebrook Dam.
Split into simpler sentences: In 1960, they built the Thomaston Dam. Following the building of the Thomaston Dam; they build the Northfield Brook Dam in 1965; and in 1969, the Colebrook Dam.