River mouths are potentially abundant locations for the exploitation of the clean and renewable salinity gradient energy (SGE) as here perpetually fresh water mixes with saline seawater. However, the practical yield of SGE depends on the spatiotemporal variability of the salinity structure. Here we show this relationship for exemplary river mouths. Depending on characteristics of the salinity structure, SGE resources can be reduced to only 0.2% of the theoretical potential. We derive a descriptor for a quick general assessment of the site specific potential and propose a classification of river mouths according to their suitability for SGE generation. It is shown that the tidal range is the most limiting factor for the harnessing of SGE at river mouths. Systems with a tidal range greater than 1.2 m seem not to be suitable locations.