The dynamics of buoyant water entering a rotating basin are studied using a series of laboratory experiments designed to elucidate the alongshore transport mechanisms in river plumes. Inflowing water, which is discharged perpendicular to the tank wall, is observed to form a growing anticyclonic bulge and a coastal current downstream of the bulge. Detailed simultaneous measurements of the velocity and buoyancy fields in the plume confirm that the bulge momentum is in a gradient–wind balance and the coastal current is geostrophic. The growth of the bulge and accumulation of fluid within it coincides with a reduction in coastal current transport to approximately 50% of the inflow discharge. The bulge is characterized by a depth scale, $h$, which is proportional to the geostrophic depth, $h_{g}$, and two time-dependent horizontal length scales, $y_{c}$, the displacement of the bulge centre from the wall, and $r_{b}$, the effective radius of the bulge. These two length scales are proportional to the inertial radius, $L_{i}$, and the local Rossby radius, $L_{b}$, respectively. When $r_{b}\gg y_{c}$, the bulge is held tightly to the wall, and a relatively large fraction of the inflow discharge is forced into the coastal current. For plumes with $y_{c}$ approaching $r_{b}$, the bulge is further from the wall, and the coastal current flux is reduced. Once ${y_{c}}/{r_{b}}\,{>}\,0.7$, the bulge separates from the wall causing flow into the coastal current to cease and the bulge to become unstable. In this state, the bulge periodically detaches from and re-attaches to the wall, resulting in pulsing transport in the coastal current. Scaling of the bulge growth based on $h_{g}$, $L_{i}$ and $L_{b}$ predicts that it will increase as $\hbox{\it Ro}^{1/4}$, where $\hbox{\it Ro}$ is the inflow Rossby number. The bulge growth, inferred from direct measurements of the coastal current transport, is proportional to $\hbox{\it Ro}^{0.32}$ and agrees with the predicted dependence within the experimental error.