The River Habitat Survey (RHS) method was developed to broadly describe the physical character of freshwater streams and small rivers in the British Isles, using a standardised framework (Naura 2012).
The survey criteria are set out in detail within River Habitat Survey in Britain & Ireland: Field Survey Guidance Manual: 2003 version (Environment Agency 2003). However, in summary, River Habitat Survey is performed along a 500 m length of a watercourse, where features of the channel (both in-stream and banks) and adjacent corridor are recorded. Ten equally spaced (ideally one every 50 m) spot-checks are chosen from which to record:

The channel substrate
The flow-type
The habitat features
The aquatic and bankside vegetation and structure
Any artificial modifications to the channel or bank; and
The surrounding land-use

These are recorded using a two-letter abbreviation for each feature. In addition, a ‘sweep-up’ checklist is performed at the end of the survey to ensure that all features are recorded. This involves a count of the overall number of riffles, pools and point bars present along the 500 m section of river. Measurements of water and bank-full width, bank height and water depth are also recorded at one location, as well as valley form (Naura 2012). Following River Habitat Survey, the watercourses are scored for comparison using two assessment frameworks, comprising:

The Habitat Quality Assessment (HQA)
The Habitat Modification Score (HMS)

The Habitat Quality Assessment score is a broad measure of the diversity and ‘naturalness’ of the physical (habitat) structure of a watercourse, including both the channel and river corridor. Habitat quality is determined according to the occurrence and diversity of habitat features of known value for wildlife, and is derived from information recorded at each spot-check. Features such as flow-type, waterfalls, point/side/mid-channel bars, large woody debris and bank-side tree distribution score points which total to provide the HQA score.
As river habitat quality is significantly influenced by artificial modification, the Habitat Modification Score assigns points for various different types of modifications including bank reinforcements, realignments and weirs. Pristine channels with no modifications will score 0 and those which are the most heavily modified may score 45 or more. The table below shows the HMS categories for describing the physical state of the river channel and are taken from:

Raven P J et al. 1998. River Habitat Quality – the physical character of rivers and streams in the UK and Isle of Man. Environment Agency, Bristol. Habitat Modification Score (HMS) categories for describing the physical state of the river channel at RHS sites (taken from Raven et al. 1998).
0 Pristine
0-2 Semi-natural
3-8 Predominantly unmodified
9-20 Obviously modified
21-44 Significantly modified
45 or more Severely modified