Complex sentence: In 1957 construction began on the first dam to close off a natural sea bay, the Shek Pik Reservoir, using it to store freshwater that had previously been "lost to the sea" during the rainy season.
Split into simpler sentences: In 1957 construction began on the first dam that would close off a natural sea bay and create the Shek Pik Reservoir. The reservoir was built to store freshwater that previously had been "lost to the sea" during the rainy season.