Complex sentence: He made a careful survey of the coast, ascended the Rio del Tizon or Rio de Buena Guia (Colorado River) for 85 Spanish miles, and was thus able on his return to New Spain in 1541 to construct an excellent map of California.
Split into simpler sentences: He made a careful survey of the coast, ascended the Rio del Tizon or Rio de Buena Guia (Colorado River) for 85 Spanish miles. Upon his return to New Spain in 1541 to construct an excellent map of California.