The IHG index for hydromorphological quality assessment of rivers and streams: updated version

The IHG index for hydromorphological quality assessment of rivers and streams: updated version An updated version of the IHG index is presented. The index is based on three appraisal parameters: 1) the functional quality of the fluvial system, including a) flow regime naturalness, b) sediment supply and mobility, and c) floodplain functionality; 2) the channel quality, including a) channel morphology and planform naturalness, b) riverbed continuity and naturalness of the longitudinal and vertical processes, and c) riverbank naturalness and lateral mobility; and 3) the riparian corridor quality, including a) longitudinal continuity, b) riparian corridor width, and c) structure, naturalness and cross-sectional connectivity.


INTRODUCTION
Hydrogeomorphological river dynamics is the key factor in fluvial systems. It is important not only in functional terms but also in terms of the ecological, landscape and environmental value of the systems (Malavoi and Bravard, 2010). The IHG hydrogeomorphological assessment index is used to implement the 2000/60/EU Directive to reduce the deterioration of fluvial systems, to identify, understand and solve or mitigate the environmental problems of these systems, to improve and conserve their functionality and naturalness, to recognise their hydrogeomorphological values, to train managers and students and to raise awareness in society.
The index was first presented in Barcelona in April 2006 at a workshop on tools for hydromorphological quality assessment in rivers organised by the Water Agency of Catalonia. A first version was published in the journals Geographicalia (Ollero et al., 2007) and Limnetica (Ollero et al., 2008), and a user guide is available on the website of the Ebro Basin Water Authority . The IHG index has been applied to more than 400 river and stream reaches (Gonzalo, 2009;Díaz and Ibisate, 2009;Gimeno, 2009;Acín et al., 2009;Ballarín and Mora, 2010). It has also been considered and applied by other research groups: Raven et al. (2010), Álvarez-Cabría et al. (2010), and Rinaldi et al. (2010).
The experiences obtained from these applications have led the authors to propose some methodological changes to the index. These changes include the assessment of more hu-man impacts and the modifications of some scores. The evaluation of riparian corridor quality has also been restructured. In this short communication, we present the updated version of the IHG index in English, integrating all the changes mentioned above.

UPDATED VERSION
The IHG evaluates nine parameters arranged in three groups: fluvial system functional quality, channel quality and riparian corridor quality. Each parameter has an initial score of 10, corresponding to the natural state and functionality of the system. However, after the impacts and pressures are assessed, points are deducted from this initial value according to different criteria. The full IHG hydrogeomorphological assessment of each river reach is performed by adding the nine values obtained. The highest possible score is 90 points. If the score is between 75 to 90 points, the hydrogeomorphological quality is considered very good. Scores from 60 to 74 points are considered good, scores from 42 to 59 are considered moderate, scores from 21 to 41 points are considered poor and scores from 0 to 20 points are considered very bad. However, the index could also be used to assess the quality of the system based on a single group of parameters: the functionality, the riverbed quality, or the quality of the riparian corridor. In such cases, only the values of the 3 parameters within each of these groups will be added, with a maximum value of 30 points (Table 1). Moreover, before the application of

Sediment supply and mobility £
The sediment discharge arrives at the functional reach without any retention of human origin and the fluvial system carries out the functions of sediment entrainment and transport without any restrictions 10 There are dams or weirs with the ability to retain sediments in the watershed and further upstream if more than 75 % of the watershed area upstream the reach presents sediment retention -5 if between 50 % and 75 % of the watershed area upstream the reach presents sediment retention -4 if between 25 % and 50 % of the watershed area upstream the reachpresents sediment retention -3 if there are dams or weirs that retain sediments, although these effect less than 25 % of the watershed area upstream the reach -2 In the reach there are gravel and/or sand extractions and/or dredging which limit sediment supply and mobility remarkable and frequent -2 minor -1 In the reach there are symptoms or signs of difficulties in the sediment mobility (armouring, embeddedness, alterations of the specific stream power, growth of certain plants…) which can be attributed to human factors remarkable -2 minor -1 The drainage network and the small tributaries that flow into the reach have human alterations that affect the sediment mobility or their connection with the valley, the floodplain or the riverbed is not continuous very important alterations and/or disconnections -3 significant alterations and/or disconnections -2 minor alterations and/or disconnections -1

Floodplain functionality £
The floodplain can exert, without human restrictions, its energy dissipation functions in flood processes, dispersal of peak flows due to sediment overflow and sediments deposition 10 The floodplain has dikes that restrict the natural functions of peak flow reduction, decantation and energy dissipation The floodplain presents land uses that reduce its natural functionality or it has been kept away from the channel due to dredging or channelisation if the raised land or the land impervious to water exceeds 50 % of the surface -3 if the raised land or the land impervious to water constitute between 15 % and 50 % of its surface -2 if there are raised land or land impervious to water, although it constitutes less than 15 % of its surface -1 Retrospective and progressive changes can be seen in the reach in the channel plan-form morphology due to human activities in the basin or to the effect of infrastructures remarkable -2 slight -1

Riverbed continuity and naturalness of the longitudinal and vertical processes £
The channel is natural and continuous and its hydromorphological longitudinal and vertical processes are functional, natural and coincident with the basin and valley characteristics, the substrate, the slope and the hydrological behaviour The topography of the riverbed, the bedform sequences, the granulometry-morphometry of the materials or the riverbed aquatic or pioneer vegetation show symptoms of having been altered by dredging, extractions, floorings or clearances in more than 25% of the reach length -3 in between 5 and 25% of the reach length -2 in odd cases -1

Riverbank naturalness and lateral mobility £
The channel is natural and has the ability to move laterally without restrictions, since its natural banks pre-sent a morphology according to its hydrogeomorphological processes of erosion and sedimentation CHANNEL QUALITY £ the IHG index, the river course to be evaluated must be divided longitudinally in reaches. These river reaches should differ according to hydrogeomorphological criteria, such as discharge, valley slope,valley confinement and channel morphology. The previous version of IHG has been improved in several respects. The updated version includes some improvements in the assessments of sediment supply and mobility and of riparian quality. The assessment of sediment supply and mobility now includes an evaluation of the impact of gravel and sand extractions and dredging. Moreover, the score and calculation of the longitudinal continuity of the riparian corridor have been changed by increasing the score discount for different types of ruptures. The riparian quality assessment has been reorganised to enhance the importance of the riparian corridor width, which has now been included as an independent second parameter within the riparian quality assessment. Additionally, the structure and naturalness and cross-sectional connectivity have been combined to form the third parameter within the riparian quality assessment.

Functional quality assessment of the fluvial system
The functional quality of the fluvial system is evaluated by adding the assessments of the following three parameters (Table 2): a) Flow regime naturalness. This parameter is assessed in relation to the natural state. This standard of comparison implies that the river currently has a natural discharge regime with seasonal flow changes and floods.
b) Sediment supply and mobility. This parameter is assessed by examining how dams, dredging and extractions alter and reduce sediment flows. Importance is also given to the lateral inputs of sediment through mass failure processes and the contribution of tributaries.
c) Floodplain functionality. This parameter is assessed by considering how the presence of human activities in a floodplain could seriously modify its functionality.

Assessment of the channel quality
The channel quality assessment is obtained from the sum of the scores for the following parameters (Table 3): a) Channel morphology and planform naturalness. Changes in the channel planform are evaluated by considering whether they are direct (channel realignment) or indirect (regulation, deforestation) human alterations. b) Riverbed continuity and naturalness of the longitudinal and vertical processes. This parameter is estimated by considering the impact from dams and weirs (barrier effect, breaking longitudinal continuity, triggering incision processes downstream), and also from other types of human alterations in channels (dredging, gravel extractions, floorings, and vegetation clearcutting).

c) Riverbank naturalness and lateral mobility.
This parameter considers pressures that confine the lateral mobility of the channel or alter the erosion and sedimentation processes (especially bank defences).

Assessment of the riparian corridor quality
The riparian corridor is the space (vegetated or not) in which the movement of the channel has occurred historically. In this section, the hydrogeomorphological function of the riparian corridor is assessed (Table 4) through the following key features: a) Longitudinal continuity. This parameter is assessed according to the number of discontinuities in the riparian corridor resulting from human occupancy. b) Riparian corridor width. The current width is assessed relative to the optimal width in the past or in a reference scenario. c) Structure, naturalness and cross-sectional connectivity of the riparian corridor. Riparian patches and the internal quality of the riparian zone are estimated by evaluating disturbances and ruptures in the connectivity of the corridor. if riparian zone is entirely removed -10 -10 -10 If the length of the discontinuities are more than 85 % of the riverbank's length -10 -9 -8 If the length of the discontinuities are between 75 % and 85 % of the riverbank's length -9 -8 -7 If the length of the discontinuities are between 65 % and 75 % of the riverbank's length -8 -7 -6 If the length of the discontinuities are between 55 % and 65 % of the riverbank's length -7 -6 -5 If the length of the discontinuities are between 45 % and 55 % of the riverbank's length -6 -5 -4 If the length of the discontinuities are between 35 % and 45 % of the riverbank's length -5 -4 -3 If the length of the discontinuities are between 25 % and 35 % of the riverbank's length -4 -3 -2 If the length of the discontinuities are between 15 % and 25 % of the riverbank's length -3 -2 -1 If the length of the discontinuities are less than 15 % -2 -1 -1

Riparian corridor width £
The surviving riparian corridor keep all their potential width, so that they play perfectly their role in the hydrogeomorphological system 10 The width of the surviving riparian corridor has been reduced due to anthropic occupation if the average width of the current riparian corridor is less than 50 % of thepotential one -3 if the average width of the current riparian corridor is between 50 % and 75 % of the potential one -2 if the average width of the current riparian corridor has been reduced but it remains over 75 % of the potential width -1 if the Longitudinal continuity has resulted 0 (totally eliminated riparian corridor) -10 After applying these scores, if the final result is negative, assess 0 if the Longitudinal continuity has resulted 1 -2 if the Longitudinal continuity has resulted 2 or 3 -1

Structure, naturalness and cross-sectional connectivity of the riparian corridor £
In the surviving riparian corridor the natural stages of vegetation, the complexity of the habitats, the naturalness of the species, and all the transversal diversity is maintained, not existing any internal human obstacle that separates or disconnects the different habitats or environments 10 There are human pressures in the riparian zone (grazing, clearing of vegetation, logging, fires, aquifer exploitation, dead wood picking, oxbow lake filling, garbage, builder's rubble, recreational use…), that cause alterations in its structure; or that cause the riparian zone to became scrubland due to the disconnection of water The reach is laterally constricted, generally longitudinally or diagonally, by linear structures such as roads, defences, ditches, trails, paths. These structures alter the transversal connectivity of the riparian corridor if the discontinuities are distributed throughout the whole sector and the addition of their lengths exceeds 150 % of the reachlength -4 if the addition of the lengths of the discontinuities gives a value between 100 % and 150% of the reach length -3 if the addition of the lengths of the discontinuities gives a value between 50 % and 100% of the reach length -2 if the addition of the lengths of the discontinuities is less than 50 % of the reach length -1 if the Longitudinal continuity has resulted 0 (totally eliminated riparian corridor) -10 After applying these scores, if the final result is negative, assess 0 if the Longitudinal continuity has resulted 1 -2 if the Longitudinal continuity has resulted 2 or 3 -1