The function of the first couplet: (1) Describe the scenery. The river is covered with swaying cattail and has uneven rhombic leaves; (2) Point out the season of "early autumn on the river"; (3) Rendering the atmosphere lays a sad emotional tone for the whole poem, and also paves the way for the following story of wandering life.
The function of couplets: (1) borrowing scenery to express emotion (2) using allusions.