Complex sentence: Estimated to be in length, the levee was completed circa 1453; the levee kept the spring-fed fresh water in the waters around Tenochtitlan and kept the brackish waters beyond the dike, to the east.
Split into simpler sentences: Estimated to be in length, the levee was completed circa 1453. The levee kept fresh spring-fed water in the waters around Tenochtitlan and kept the brackish waters beyond the dike, to the east.