Stage 1: River Character and Behaviour

Stage 1: Catchment-Wide Baseline Survey of River Character and Behaviour

Stage One of the River Styles framework provides a baseline survey of river character and behaviour. In this analysis we are asking the questions,

How does the character of River 1 differ from the character of River 2?

What are the processes that operate along Rivers 1 and 2?

River 1: Confined valley with occasional floodplain pockets River Style

The channel abuts bedrock valley margin along more than 90 % of its length and occasional floodplain pockets occur on the insides of bends.

River 2: Wandering gravel bed River Style

Multiple channels are separated by mid-channel bars. Floodplain surfaces have palaeochannels and channels regularly shift over the valley floor.

Each River Style is characterised by a distinctive set of attributes, analysed in terms of channel planform, the geomorphic units that make up a reach, and bed material texture. The identification and interpretation of geomorphic units allows interpretation of processes that reflect the range of behaviour of a River Style. River Styles, and their downstream patterns, are then appraised in terms of their landscape setting and the spatial and temporal linkages of geomorphic processes.

Stage 1 Step 1: Assessment of regional and catchment setting controls

Review background information and available literature on the catchment, including, academic papers and reports.

 Derive catchment-framed maps including streamlines, geology, rainfall distribution, land use etc. and analyse catchment morphometric parameters, using GIS where available.

 Designate landscape units and produce longitudinal profiles with contributing area plots.

 Analyse subcatchment morphometric parameters.

 Analyse discharge data and hydrological regimes.

 Summarize regional setting information.

In the River Styles framework, landscape units are readily identifiable topographic features with a characteristic pattern of landforms. Identification and mapping of landscape units is undertaken on the basis of physiographic character, landscape position, geology and relief. Examples of landscape units include: tablelands, uplands, mountains, escarpment, rounded foothills, low lying hillslopes and lowland plain. A map showing the distribution of landscape units in the catchment is produced. Elevation, longitudinal valley slope and valley width are tabulated to characterise each landscape unit. These attributes represent fundamental controls on river character and behaviour.

Longitudinal profiles record the downstream changes in elevation, and hence slope, along a river course. Overlaying longitudinal profiles from different subcatchments can be used to compare downstream changes in slope and assess the importance of tributary-trunk relationships as determinants of the downstream patterns of River Styles (Stage One Step Three). In the River Styles framework, contributing area is superimposed onto the longitudinal profiles. The area draining into each section of the river course presents a fundamental control on downstream changes in discharge. It also defines the relative contributions of area from different parts of the catchment, and provides a quick, visual overview of changes in catchment area (and hence discharge) at tributary confluences. It is often instructive to note (and explain) whether the character and behaviour of the trunk stream changes downstream of tributaries.

The timing and frequency of flows dictate the capacity of a river to adjust its morphology, while the sequencing of floods affects the geomorphic effectiveness of the flow (i.e. the capacity of a given flood to perform geomorphic work, that is, transport sediment). The hydrological analyses undertaken in the River Styles framework are used to gain an appreciation of what scale of event is the dominant control on river morphology, and how frequently that type of flood occurs.

Stage 1 Step 2: Definition and mapping of River Styles across the catchment

Analyse the catchment-wide distribution of river character and behaviour.

 Designate River Styles using air photographs and produce a River Styles tree.

 Produce a catchment-specific River Styles tree based on the valley-setting in which a river operates.

 Select representative reaches of each River Style, planform maps using air photograph interpretation, and draft proformas.

 Ratify River Styles boundaries in the field.

 Complete field analyses to finalise proformas and amend planform maps for each representative field reach.

 Characterise and explain river behaviour for each River Style through interpretation of reach-scale assemblages of geomorphic units.

The definition and interpretation of River Styles is initially undertaken through air photograph interpretation. Fieldwork is then undertaken to collect relevant information on river character and behaviour for each River Style in the catchment and ratify boundaries between River Styles. The primary trunk streams in each subcatchment are systematically analysed, identifying the distribution of floodplains along river courses to determine the range and pattern of valley-settings. Within each valley-setting, River Styles are identified on the basis of river planform and the assemblage of geomorphic units. Bed material texture provides a finer level differentiation that is completed in the field. Dependent on whether the reach falls into a confined, partly confined or laterally-unconfined valley setting, differing sets of procedures are used to identify the River Style (Figure 1). The importance of each parameter for assessing river character and behaviour depends on the valley-setting. A River Styles tree is constructed that outlines the specific identification criteria for each River Style. Each River Style is given a diagnostic name and a draft catchment wide map is produced showing the distribution of River Styles. Geomorphic planform maps are produced for representative reaches of each River Style.

Figure 1: Procedures used to identify River Styles in different valley settings.

Representative examples of each River Style are examined in the field. Valley-scale cross sections are surveyed and sedimentological and vegetation analyses performed for differing geomorphic units. Representative photographs are also taken. The presented River Styles proformas contain a summary of the character and behaviour of each River Style across the catchment. This entails synthesis of the range of geomorphic conditions found in each River Style.

Stage 1 Step 3: Interpretation of the controls on character, behaviour and downstream patterns of River Styles

Determine downstream patterns of River Styles. Group the tributaries and trunk stream according to the patterns identified.

Determine the imposed boundary condition controls on river character and behaviour along longitudinal profiles.

Determine the flux boundary condition controls on river character and behaviour along longitudinal profiles.

For representative examples of each downstream pattern of River Styles, produce a summary longitudinal profile that synthesises the relationship between patterns of River Styles and relevant controlling parameters.

Produce a summary table that synthesises catchment-wide understanding of controls on river character and behaviour.

One of the key components of the River Styles framework is the need to understand how and why each reach looks and behaves in the manner that it does. To further this understanding a summary assessment of controls on the distribution of River Styles is developed. To assess controls on the character and behaviour of each River Style, river courses are grouped according to their downstream pattern of River Styles.

Critical controls on river behaviour may vary from reach to reach. Initial insights into the array of controls on any given reach may be gained by plotting downstream patterns of River Styles onto longitudinal profiles. Analyses of slope and contributing area are combined with catchment area-discharge relationships to estimate gross stream power, from which stream power ranges are determined for each River Style. The critical role of downstream changes in valley confinement is explained, generally in terms of the geological imprint (structure and lithology) along with interpretations of long-term landscape history. Landscape unit boundaries placed on the longitudinal profile-contributing area plots and an interpretation of the contemporary process zones (source, transfer, accumulation) and sediment transport regime (bedload, mixed load, suspended load) of the river course are made at this stage.