You may hear a lot about rivers in ballads about love. Usually a poet uses the symbolism of a river to reflect upon flow of life or time. The speaker seems to be doing the same thing in this poem with the romantic imagery of grass growing on the weirs of the river. The speaker is looking to speed things up, but the girl wants to sit and watch the water flow lazily through the weirs.
Lines 9-10: By the second stanza the speaker is standing with his love by the river. The romantic imagery here is common to the ballad form, and reminds us of the way life works (or is supposed to work) in a natural sort of way.
Lines 13-14: The girl tells the speaker to take life easy here like the grass growing on the weirs. This simile is about appreciating the view, taking time to stop and smell the… river, or at least enjoy the view of it moving along its way. The speaker's got no time for that, though, and he pays the price.