Bathymetry on continental hydrographic networks is up to now limited to small areas. This is highly limiting hydrological or ecological studies on hydrographic networks. To enlarge rivers bathymetry surveys, Bathymetric LiDAR looks an interesting tool. But, if this technique has been widely used for coastal surveys, very few studies focus on water depth measurement quality and limits, in particular for rivers. We aimed here to study the theorical limit of water depth minimum measurement using a green LiDAR full waveform model. To do so, we developed first a model introducing specificities for rivers. Those, we tried to assess from simulated full waveforms this inferior limit and its sensitivity to river parameters. The results exhibit an inferior limit of 10 centimeters in best cases, i.e. when water surface is rough. This allows us to conclude on the utility of bathymetric LiDAR for gravel-beds rivers usually having a mean depth around 40 centimeters.