Stage 2: Geomorphic River Condition

Stage 2: Catchment-Framed Assessment of River Evolution and Geomorphic River Condition

Stage Two of the River Styles framework assesses and explains river condition throughout a catchment. In this analysis we are asking the question,

For reaches of the same River Style, is the geomorphic condition of Reach 1 better or worse than Reach 2?

Example: Confined valley with occasional floodplain pockets River Style

Reach 1: Good geomorphic condition
A good condition reach is characterized by significant heterogeneity in the geomorphic structure of the channel. Geomorphic units include bedrock pools, runs, cascades. An extensive riparian corridor is evident.
Reach 2: Poor geomorphic condition
A poor condition reach is characterized by a homogenous sand sheet that infills pools and smothers the channel bed. A scattered or non-existent riparian corridor is evident.
We are asking questions about,
How do different River Styles adjust within their valley-settings under contemporary flow and sediment regimes?
Are the adjustments observed along any given reach expected for that river type?
If processes are accelerated, or anomalous for that River Style, the reach is considered to be in poor geomorphic condition. If the character and behaviour of the reach is as expected for that River Style, then the reach is considered in good geomorphic condition.

Stage 2 Step 1: Determine the capacity for adjustment of the River Style

Identify the timeframe over which environmental conditions in the catchment/region have been relatively uniform.

Construct an evolutionary sequence for each River Style in the catchment.

Draw planform maps and schematic cross-sections for each timeslice in the evolutionary sequence.

Determine whether irreversible geomorphic change has occurred for each reach.

Derive desirability criteria for river character and behaviour based on relevant geoindicators for each River Style.

Identify reference conditions for each River Style.

Select the most appropriate reference reach using the ‘relevant geoindicators’ and asking the question: What would be the expected character and behaviour of the river given its position in the catchment, and the prevailing water, sediment and vegetation regimes?

To determine the capacity for adjustment of each River Style in a catchment, the question is asked: In what ways can this type of river adjust within its valley-setting under the prevailing set of flow and sediment characteristics? Ways in which the channel adjusts its position on the valley floor, and how the reach delivers sediment downstream should be considered. In asking this question, three degrees of freedom are considered in the River Styles framework, namely, channel attributes, channel planform and bed character.

Within each degree of freedom, a series of geoindicators is measured or assessed to determine the ability of each reach to adjust within its valley-setting (see Brierley and Fryirs, 2005). In this framework, channel attributes are assessed in terms of the size and shape of a channel, bank morphology and vegetation and loading of wood. Channel planform is assessed in terms of the number, sinuosity and lateral stability of channels, the assemblage of instream and floodplain geomorphic units, and riparian vegetation structure and composition. Bed character is assessed in terms of grain size, bed configuration (i.e. sorting, bed stability, hydraulic diversity) and sediment regime (i.e. whether the reach acts as a sediment source, transfer or accumulation zone). Only those geoindicators that provide direct insight into how that river adjusts are assessed for each River Style. This type of analysis is used as a simplified measure of the sensitivity of a River Style to change. Those reaches with significant adjustment potential are considered sensitive to change, while those with localized adjustment potential are considered resilient to change.

Relevant geoindicators are then used to construct a set of ‘desirability criteria’ for each River Style. Based on these criteria, and the evolutionary sequence, a reference condition is identified for each River Style. This is then used as a benchmark against which to assess the geomorphic condition of each reach.

Stage 2 Step 1: Determine the capacity for adjustment of the River Style

Identify the timeframe over which environmental conditions in the catchment/region have been relatively uniform.

Construct an evolutionary sequence for each River Style in the catchment.

Draw planform maps and schematic cross-sections for each timeslice in the evolutionary sequence.

Determine whether irreversible geomorphic change has occurred for each reach.

Derive desirability criteria for river character and behaviour based on relevant geoindicators for each River Style.

Identify reference conditions for each River Style.

Select the most appropriate reference reach using the ‘relevant geoindicators’ and asking the question: What would be the expected character and behaviour of the river given its position in the catchment, and the prevailing water, sediment and vegetation regimes?

To determine the capacity for adjustment of each River Style in a catchment, the question is asked: In what ways can this type of river adjust within its valley-setting under the prevailing set of flow and sediment characteristics? Ways in which the channel adjusts its position on the valley floor, and how the reach delivers sediment downstream should be considered. In asking this question, three degrees of freedom are considered in the River Styles framework, namely, channel attributes, channel planform and bed character.

Within each degree of freedom, a series of geoindicators is measured or assessed to determine the ability of each reach to adjust within its valley-setting (see Brierley and Fryirs, 2005). In this framework, channel attributes are assessed in terms of the size and shape of a channel, bank morphology and vegetation and loading of wood. Channel planform is assessed in terms of the number, sinuosity and lateral stability of channels, the assemblage of instream and floodplain geomorphic units, and riparian vegetation structure and composition. Bed character is assessed in terms of grain size, bed configuration (i.e. sorting, bed stability, hydraulic diversity) and sediment regime (i.e. whether the reach acts as a sediment source, transfer or accumulation zone). Only those geoindicators that provide direct insight into how that river adjusts are assessed for each River Style. This type of analysis is used as a simplified measure of the sensitivity of a River Style to change. Those reaches with significant adjustment potential are considered sensitive to change, while those with localized adjustment potential are considered resilient to change.

Relevant geoindicators are then used to construct a set of ‘desirability criteria’ for each River Style. Based on these criteria, and the evolutionary sequence, a reference condition is identified for each River Style. This is then used as a benchmark against which to assess the geomorphic condition of each reach.

Stage 2 Step 2: Assess river evolution as a basis for identifying irreversible geomorphic change and a suitable reference condition

Interpret how each River Style can adjust under the prevailing flux boundary conditions (i.e. sediment, flow and vegetation characteristics).

Assess the ability of each degree of freedom to adjust for each River Style by determining which geoindicators within each degree of freedom are relevant for each River Style.

Construct a table noting relevant geoindicators that will be used to identify a reference condition and assess the geomorphic condition of reaches.

Stages of geomorphic river evolution are appraised for each River Style over timeframes in which boundary conditions have not changed. This will, in most cases extend into the Holocene record. Detailed analyses are conducted along one representative reach of a River Style using a range of historical information. Timeslices are constrained by the dates of air photographs, historical information, anecdotal, archival and dating data sources.

In those settings that retain little evidence of evolutionary change, ergodic reasoning can be used to complete an evolutionary sequence. Reaches used for ergodic reasoning must be of the same River Style, operate under similar catchment boundary conditions, and sit at a similar position in the catchment. In this way, future scenarios are added where examples existed. Schematic representations of channel cross section and planform are produced for each timeslice. From this, each reach in the catchment can be placed onto the relevant evolutionary sequence for the River Style.

The evolutionary sequence of each reach is used to identify if, how and when irreversible geomorphic change occurred and identify a reference condition. The ability to identify whether a reach has experienced irreversible geomorphic change requires evidence of major shifts in the catchment boundary conditions under which the river operates and associated irreversible responses in the structure and function of the river.

If a reach has experienced irreversible geomorphic change, it’s condition must be assessed in terms of the River Style to which it has shifted. In this instance, comparing the contemporary reach with a pre-disturbance reference condition is irrelevant in setting realistic management goals. If a river still operates within the process regime of its pre-disturbance River Style, and reversible change has occurred, the condition of that reach is assessed in light of that River Style. These insights are used to frame subsequent management actions.

Four types of natural reference conditions can be identified, based on the irreversibility/permanency of geomorphic change, and the prevailing catchment boundary conditions (Brierley and Fryirs, 2005):

Remnant reaches that have been minimally disturbed by humans, such that geomorphic changes to river character and behaviour remain reversible.
Reaches where human-disturbance has occurred, but geomorphic changes to river character and behaviour remain reversible.
Reaches where change has been induced by indirect human-disturbance and irreversible change has resulted.
Reaches where change has been induced by direct human-disturbance and irreversible geomorphic change has resulted.
A reference reach of a River Style is identified by asking the question: What would be the expected character and behaviour of the river given its position in the catchment, and the prevailing water, sediment and vegetation regimes? A reference reach should fit the desirability criteria constructed for each River Style. These reaches are either available in the catchment and can be used as direct analogues or are identified from the evolutionary sequence for the River Style under examination.

Stage 2 Step 3: Determine and explain the geomorphic condition of the reach

Determine the geomorphic condition of each reach using the degrees of freedom and good, moderate and poor condition matrix.

Interpret and explain the geomorphic condition of the reach.

Stages of geomorphic river evolution are appraised for each River Style over timeframes in which boundary conditions have not changed. This will, in most cases extend into the Holocene record. Detailed analyses are conducted along one representative reach of a River Style using a range of historical information. Timeslices are constrained by the dates of air photographs, historical information, anecdotal, archival and dating data sources.

In those settings that retain little evidence of evolutionary change, ergodic reasoning can be used to complete an evolutionary sequence. Reaches used for ergodic reasoning must be of the same River Style, operate under similar catchment boundary conditions, and sit at a similar position in the catchment. In this way, future scenarios are added where examples existed. Schematic representations of channel cross section and planform are produced for each timeslice. From this, each reach in the catchment can be placed onto the relevant evolutionary sequence for the River Style.

The evolutionary sequence of each reach is used to identify if, how and when irreversible geomorphic change occurred and identify a reference condition. The ability to identify whether a reach has experienced irreversible geomorphic change requires evidence of major shifts in the catchment boundary conditions under which the river operates and associated irreversible responses in the structure and function of the river.

If a reach has experienced irreversible geomorphic change, it’s condition must be assessed in terms of the River Style to which it has shifted. In this instance, comparing the contemporary reach with a pre-disturbance reference condition is irrelevant in setting realistic management goals. If a river still operates within the process regime of its pre-disturbance River Style, and reversible change has occurred, the condition of that reach is assessed in light of that River Style. These insights are used to frame subsequent management actions.

Four types of natural reference conditions can be identified, based on the irreversibility/permanency of geomorphic change, and the prevailing catchment boundary conditions (Brierley and Fryirs, 2005):

Remnant reaches that have been minimally disturbed by humans, such that geomorphic changes to river character and behaviour remain reversible.
Reaches where human-disturbance has occurred, but geomorphic changes to river character and behaviour remain reversible.
Reaches where change has been induced by indirect human-disturbance and irreversible change has resulted.
Reaches where change has been induced by direct human-disturbance and irreversible geomorphic change has resulted.
A reference reach of a River Style is identified by asking the question: What would be the expected character and behaviour of the river given its position in the catchment, and the prevailing water, sediment and vegetation regimes? A reference reach should fit the desirability criteria constructed for each River Style. These reaches are either available in the catchment and can be used as direct analogues or are identified from the evolutionary sequence for the River Style under examination.