This is a bibliographic review concerning the effects of HYMO pressures on hydromorphological processes and variables resulting from both degradation and restoration. It further provides a tool to identify gaps in present HYMO knowledge, which is needed to improve our understanding of the mechanisms that control degradation-restoration processes. The following groups of HYMO pressures are addressed: hydrological regime alteration, river fragmentation and morphological alteration.

This report is a bibliographic review concerning the effects of HYMO pressures on hydromorphological processes and variables resulting from both degradation and restoration. Based on this review, we aim to identify the most significant HYMO pressures as well as relevant hydromorphological effects of the different pressure types on fluvial systems across spatial and temporal scales and in particular those that have a significant impact on aquatic biological elements.

This review further provides a tool to identify gaps in present HYMO knowledge, which is needed to improve our understanding of the mechanisms that control degradation-restoration processes. To illustrate relevant gaps conceptual schemes have been developed of the interactions between HYMO pressures, the main processes affected and the resulting quantified changes on HYMO variables. Referenced citation frequencies were used to relate the different elements of each scheme.

Hydromorphological pressures were grouped into the following types:

1.  Hydrological regime pressures, including water abstraction and flow regulation

2.  River fragmentation pressures

3.  Morphological alteration pressures

The pressure effects were analyzed separately for each hydromorphological pressure by developing a diagram showing its direct effects on the processes and on the state variables, but in turn also the induced process changes with respect to HYMO variables.The following main HYMO processes were considered:

·      Water flow dynamics

·      Sediment dynamics (sediment entrainment, transport, deposition, armouring)

·      Bank dynamics (bank erosion & failure, stabilization)

·      Vegetation dynamics (vegetation encroachment, uprooting, recruitment)

·      Large wood dynamics (entrainment, transport, deposition)

·      Aquifer dynamics (aquifer recharge, discharge)

The findings, however, did not only allow identifying the main pressures and most important processes, they also pose major challenges on identifying key variables and variables’ changes which significantly affect the biotic response. Major knowledge gaps comprise the interplay between synchronously and asynchronously responding variables and the assessment of the finally resulting status of the hydromorphological variables in different river types. Closely related to that, assessing the resulting potential effects on biota as well as differential responses of different taxa to various variable changes and variable states provide additional challenges.