"And when my father saw that the waters of the river emptied into the fountain of the Red Sea, he spake unto Laman, saying: O that thou mightest be like unto this river, continually running into the fountain of all righteousness!"
Lehi, finding a great gospel connection to the river, speaks to Laman. he states his desire for Laman to always be fountain of righteousness. always striving to follow the word of God and being, I guess you could say, a source of or an example of one who would follow the commandments of the Lord. Laman being the oldest would normally have this responsibility.