"And it came to pass that he called the name of the river, Laman, and it emptied onto the Red Sea; and the valley was in the borders near the mouth thereof."
As was the tradition when entering a new ground they named the area. These names may not have been permanent, but they helped recorded the journeys taken and gave a place of distinction for the traveling group. With all right they could have named it Lehi because he was leading them. But showing great respect for his eldest son he named the river after Laman. This was a great honor I believe as well as Lehi showed his love for his son, Laman.