The structure and function of intertidal communities are intimately linked with hydrologic regime, which itself is defined by elevation gradients relative to sea level. Water surfaces have long been used as a reference plane for determining topographic profiles of the underlying ground surface. Although relatively straightforward in static systems such as lakes or ponds, this method is more complicated in the intertidal zone where the water surface is in vertical motion. Here we evaluate a technique using water marks to indicate maximum tide heights from which ground elevations relative to a tidal datum can be calculated. Comparisons of data using this technique to optical-leveling surveys in several salt marshes of Connecticut and Massachusetts showed that the water surface conformed to a horizontal plane at high, slack tide and that watermarks could be used as a reliable reference plane for determining ground elevations from flooding depth.