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Geomorphic unit - Area containing a landform created by erosion and/or deposition inside (instream geomorphic unit) or outside (floodplain geomorphic unit) the river channel. Geomorphic units can be sedimentary units located within the channel (bed and mid-channel features), along the channel edges (marginal and bank features) or on the floodplain, and include secondary aquatic habitats within the floodplain. Some geomorphic features (biogeomorphic units) are formed in association with living and dead (e.g. large wood) vegetation as well as sediment.

Hydraulic unit: Spatially distinct patches of relatively homogeneous water surface flow and substrate character. A single geomorphic unit can include from one to several hydraulic units.

River element: Relatively small element of a river environment that includes individuals and patches of sediment, plants, wood, etc. A single geomorphic unit can include many river elements. For example a bar may contain a continuum of surface elements or patches of different particle size.

Delineation criteria

These three spatial units underpin the habitat mosaic that is present in and around a river. They do not require ‘delineation’ at this stage of the analysis, but in later analytical stages we emphasise the identification of geomorphic units as key characteristics of river reaches. Hydraulic units and river elements are usually closely related to geomorphic units. Since identification of hydraulic units is affected by river stage and river elements form small components of geomorphic and hydraulic units, in the remainder of this report we focus on geomorphic units as the key smaller spatial unit than the reach. Geomorphic units are sufficiently large and prominent that they can often be recognised from remotely sensed sources and are the most straightforward to consistently recognise in the field

Methods and data sources

Geomorphological field survey

Habitat survey (e.g. UK River Habitat Survey)