Complex sentence: From this confluence the creek flows down the south side of Thornhill until Thornhill Elementary School were it then undergrounds and flows down the remainder of Thornhilll and then down Mountain Blvd. to Broadway Terrace, before draining into Lake Temescal.
Split into simpler sentences: From this confluence the creek flows down the south side Thornhill until Thornhill Elementary School were it then undergrounds. Then it flows underground down the remainder of Thornhilll and then down Mountain Blvd. to Broadway Terrace, before draining into Lake Temescal.