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The appendices collate information on RHS indices developed so far to help with diagnosing issues.  The existing indices are summarised below and more information on their calculations and interpretation can be found in the appendices and online on the RHS website resource section:

RHS indices:

Map of CSI using geostatistical techniques at every 500m across the 1/50,000 river network on a gradient from bedrock/boulder (blue) to gravel-pebble (green) and silt-sand-clay (brown).


RHS index



Habitat quality indices

Habitat Modification Score (HMS)

0 to 6000+

Quantifies the extent, potential impact and persistence of engineering structures

HMS class

1 (semi-natural) to 5 (severely modified)

Classification of HMS score into 5 categories using set boundaries

Habitat Quality Assessment score (HQA)

0 to 100

Quantifies the diversity and naturalness of habitat features

HQA class

1 (very low) to 5 (very high)

Classification of HQA score by comparing to sites of similar types using a context analysis.

River Habitat Quality (RHQ) index

I (excellent) to V (very poor)

Classification combining HMS and HQA classes and representing overall habitat quality and conservation value

Riparian Quality Index (RQI)

1 (very low) to 5 (very high)

Quantifies the complexity, continuity and naturalness of the riparian vegetation

Hydromorphological indices

Channel Substrate Index (CSI)

-2 (silt) to 1 (boulders)

Represents the average substrate size

Flow Regime Index (FRI)

-1 (glide) to 2 (waterfall)

Represents the average flow-type

Channel Vegetation Index (CVI)

-2 (free floating vegetation) to 1 (mosses and lichens)

Represents the dominant vegetation types

Geomorphic Activity Index (GAI)

-1 (no activity sign) to 1.6 (high activity)

Represents the level of geomorphic activity (i.e. erosion and deposition)

Hydromorphological Impact Ratio (HIR)

1 (low impact) to 5 (very high impact)

Quantifies the level of departure from natural state of CSI, FRI, CVI and GAI combined

Channel Resectioning Index (CRI)

0 (no resectioning) to 30 (all resectioned)

Quantifies the occurrence of bank and channel modification by resectioning (i.e. dredging of the bed and reprofiling of the banks) along the channels