Plane jets in shallow water are often encountered in geophysical flows and engineering applications. The present investigation shows that depending on the distance from the origin scaled by depth, the jets can be divided into three fields with distinct behaviour. A model is then presented allowing discussion of the conditions related to the observed increase or decrease of momentum flux in the vicinity of the orifice. The influence of skin-friction on spreading and entrainment is studied on the basis of existing experimental evidence including the one in the present work.