Complex sentence: However, due to their vantage point, neither Fages nor any of his men saw the Sacramento clearly, instead assuming that the San Joaquin, coming from the south, was the largest of the merging rivers they saw.
Split into simpler sentences: However, due to their vantage point, neither Fages nor any of his men saw the Sacramento clearly. They assumed that the San Joaquin, coming from the south, was the largest of the merging rivers they saw.