There might be even an option without a plug-in: If the river bottom is implemented as a minimum-head constraint for the transfer BC, in the lastest FEFLOW versions (from patch 8 for FEFLOW 7.0), the transfer BC is internally turned off in case that the BC value (river water level) is below the minimum-head constraint (river bed elevation). With this, you can simulate a varying shoreline by simply using time-varying conditions for the water level in connection with a constraint representing the river bottom.