This was Grand River.

At last they had reached the place where these two streams unite, thirteen hundred feet below the surrounding country; the mysterious Junction which, so far as the records go, Macomb and all white men before had failed to find.
Therefore when Powell and his band floated down till the waters of the Green mingled with those of the Grand they were perhaps the first white men ever to arrive at the spot.

The Colorado proper was now before them.
It was the mystery of mysteries..