Construction of isopleth plots of net velocity, material concentration, or material flux in cross-sections of tidal estuaries is not a trivial matter. To construct a flux-preserving isopleth plot requires that each instantaneous measure is weighted by the subarea for which the measure is representative. This area-weighted averaging procedure is outlined. Without area-weighting, net isopleth plots typically yield misleading results in tidal estuaries. In our example, net fluxes of total nitrogen are over-estimated without area-weighting.