בראשית ב:י פירוש: ספורנו, רד״ק, The Midrash וRav Hirsch

ונהר יוצא מעדן, the Torah explains that Adam’s work did not entail anything strenuous, such as carrying pails of water, or such as getting wet in the rain.

ונהר יוצא מעדן להשקות את הגן, Adam, who had been placed in the garden, had also been instructed to tend it and protect it from intruders, and without water he could not perform the work he had been instructed to perform. However, the Torah attests that Adam did not also have to irrigate the soil; this was accomplished by an external; source of water originating in Eden and flowing through the garden.