The RHS database allows sites of a similar nature to be grouped together for comparative purposes. Channel slope, distance from source, height of source and site altitude are used to cluster RHS sample sites for so-called “context analysis” based on Principal Components Analysis (PCA) plots (Jeffers, 1998). The RHS database allows detailed investigation of the relationships between physical variables (e.g. gradient, geology), channel modifications and habitat features at spot-check and 500m site level. These investigations can make use of available water chemistry and hydrological data, plus survey results of benthic macroinvertebrates, aquatic macrophytes, fish and breeding water birds where biological sampling has been done in or near RHS sites.

Assessment of habitat quality and extent of channel modification can be derived from RHS data, and these indices used as a basis for setting physical quality objectives for rivers. For example, Habitat Quality Assessment (HQA) is a broad indication of overall habitat diversity provided by natural features in the channel and river corridor. Points are scored for the presence of features such as point, side and mid-channel bars, eroding cliffs, large woody debris, waterfalls, backwaters and floodplain wetlands. Additional points reflect the variety of channel substrata, flow-types, in-channel vegetation, and also the distribution of bank-side trees and the extent of nearnatural land-use adjacent to the river. Points are added together to provide the HQA score.

In contrast to HMS (described below), higher HQA scores represent more diverse sites. The character and pattern of features in a site is influenced by natural variation and also the extent of human intervention both in the channel and adjacent land. The RHS database allows HQA scores to be compared using sites with similar physical characteristics (e.g. gradient, distance from source) and geology. Features determining habitat suitability for individual species such as European river otter Lutra lutra and dipper Cinclus cinclus can also be used as attributes, thereby providing a more sophisticated species or community-based context for comparing sites.

Carrying out RHS and aquatic macrophyte surveys in reaches of known good or high quality has provided calibration of HQA across a wide range of river types. Between 1994 and 2009, this ‘benchmarking’ exercise involved 181 RHS sites on 82 rivers in Britain and Ireland. Specially targeted ‘benchmark’ surveys have been extended to mainland Europe, including rivers in Finland, Norway, Slovenia, Bavaria, the Tyrolean Alps, the Cévennes in south-eastern France, Poland, the Picos de Europa of northern Spain and the Mediterranean area of southern Portugal.

Habitat Modification Score (HMS) is an indication of artificial modification to river channel morphology. To calculate the HMS for a site, points are allocated for the presence and extent of artificial features such as culverts and weirs and also modifications caused by the re-profiling and reinforcement of banks (Habitat Modification Score Rules 2003). Greater and more severe modifications result in a higher score. The cumulative points total provides the Habitat Modification Score (HMS). A Habitat Modification Class (HMC) protocol has been developed which allocates the condition of the channel in a site to one of five modification classes, based on the total score (1 = near-natural; 5 = severely modified). In contrast to HQA, higher HMS scores reflect more artificial intervention and modification of the river channel within a site.

RHS made an important contribution to development of the CEN guidance standard for assessing the hydromorphological features of rivers and is a recommended method for the agreed protocol for field survey and recording of morphological features. RHS was also used to help develop and test the associated CEN guidance standard on determining the degree of modification on river hydromorphology. In the UK, RHS has been used for several Water Framework Directive (WFD) purposes, helping to identify water bodies in ‘reference condition’, those classified as ‘heavily modified’ and also assessing morphological pressures affecting river catchments.

The STAR (STAndardisation of River Classifications) project was a research initiative funded by the European Commission and was completed in 2005. The main aim was to provide standard biological assessment methods compatible with WFD requirements. It also set out to develop a standard for determining the class boundaries of ‘ecological status’ and another one for inter-calibrating existing methods. In Austria, The Czech Republic, Denmark, Germany and Italy ‘core’ RHS sites were chosen to reflect a gradient in habitat and morphology degradation. Results from the STAR project were published in a special issue of Hydrobiologia in 2006.