Complex sentence: The organizers of the New York Public Library, wanting an imposing main branch, chose a central site available at the two-block section of Fifth Avenue between 40th and 42nd streets, then occupied by the no-longer-needed Croton Reservoir.
Split into simpler sentences: The organizers of the New York Public Library, wanting an imposing main branch, chose a central site available at the two-block section of Fifth Avenue between 40th and 42nd streets. It was occupied by the no-longer-needed Croton Reservoir.