The variation of the hydraulic units flow depth and flow velocity measured in a river reach have shown to be essential for in-stream biodiversity. In this research 19 river reaches with different bedforms classified into: straight rivers, meandering rivers, rivers with alternate bars and braided rivers were analyzed with the hydromorphological index of diversity (HMID). The measurements were taken along multiple cross-sections with equal spacing. Subsampling broke down the river reaches in smaller sub-reaches of different sizes. The HMIDs from the sub-reaches were compared with the HMID of the whole reach to point out the minimum reach length necessary. For straight rivers, rivers with alternate bars and braided rivers, already data from two cross-sections give a good idea about the reach HMID. Meandering rives show a special behavior. There at least four cross-sections should be taken into account for the HMID computation. The minimum length is half a wavelength. Analyses also showed that a larger spacing and a reach length of two wavelength result in a more solid result than the same amount of cross-sections in halv a wavelength. The results from this study should help applicants of the HMID in restoration and classification process and give them an idea about the sufficiency of their data and the error their HMID underlies.