The characteristics of a given stream or river are linked to the material that makes up its channel bed. Usually, a vertical stratification by particle size can be recognized. The presence a coarser surface layer is considered to be one of the most important features of a gravel bed stream. Since this surface layer consists of a distinct population of material, it is necessary to be able to separate it from the underlying material and quantify it distinctly. This is done through surface sampling. Two of the most common adhesive areal sampling techniques, and the subject of the present work, are known as clay and wax sampling. If the material obtained in an areal sample is analyzed as a frequency distribution by weight, it has been shown that the size distribution is biased in favor of the larger particles when compared to the results of a bulk sample.

The present research shows that this bias is dependent not only upon the sampling method used to remove the material but also upon the size distribution of the sample itself. Not only are the raw results of areal samples not comparable with volumetric samples, but they are not comparable with other areal samples. Before any comparisons are made among areal samples, it is recommended that the size distribution of each areal sample be first converted into the size distribution that would have resulted from an equivalent volumetric sample. The features and limitations of the gravel simulation model that is used to obtain the necessary conversion formula is also the subject of the present work. In addition, the conversion of both matrix and framework supported gravel mixtures that has been areally sampled with either clay or wax, is addressed. Finally, criteria for approximating the minimum depth required for a volumetric sample is presented.