Stage 3: Assessment of the Future Trajectory of Change and Geomorphic River Recovery Potential
Stage Three of the River Styles framework examines the future trajectory of change a reach is likely to take and the potential for that reach to recover along that trajectory. We are asking the question, will a reach of a certain River Style adjust from example 1 to 2 to 3 to 4 and therefore be on a pathway of recovery, or will the condition of the reach deteriorate in the future and adjust from example 4 to 3 to 2 to 1 and therefore be degrading. We then ask the question,
Example: Wandering gravel bed River Style
Stage 3 Step 1: Determine the trajectory of river change
Position each reach on its evolutionary sequence for the River Style.
Translate each timeslice of the evolutionary sequence of the River Style onto the recovery diagram using the decision tree.
Determine the trajectory of change of each reach in the catchment (i.e. either degradation, restoration or creation).
Assessment of limiting factors and pressures that constrain the recovery of landforms and ecosystems enables the adoption of appropriate measures to minimise the impacts of these constraints. Application of this kind of thinking enables management strategies to address underlying causes rather than the symptoms of problems relating to factors such as sediment exhaustion, flow management; nutrient fluxes, weed dispersion, depleted seed sources etc. These considerations must be assessed on a catchment-by-catchment and reach-by-reach basis. In the River Styles framework, within-catchment linkages of physical processes are examined, such that disturbances in one part of the catchment can be related to river responses elsewhere. In this way, both off-site impacts and lag effects can be appraised.
The trajectory of change a reach will take is determined by placing each reach in the catchment on the recovery diagram (Brierley and Fryirs, 2005). This diagram comprises three pathways, the degradation scale on the left and two recovery pathways, restoration and creation on the right (Figure 1). Assessment of the trajectory of change of a reach is based initially on analysis of the evolutionary sequences and explanation of geomorphic condition undertaken in Stage Two of the River Styles framework. Based on this analysis, stages of adjustment for each reach (i.e. the trajectory of change) are identified and interpreted.
Figure 1 The recovery diagram used in the River Styles framework (from Brierley and Fryirs, 2005).
Stage 3 Step 2: Assess river recovery potential: Place reaches in their catchment context and assess factors limiting recovery
Determine the sensitivity of the reach (completed in Stage Two, Step One).
Note the geomorphic condition of the reach (completed in Stage Two, Step Three).
Assess limiting factors and pressures in the catchment.
Place each reach in relation to ongoing (and prospective) catchment dynamics.
Determine the recovery potential of each reach using the decision tree.
Given that rivers are evolving, adjusting entities, a reach can sit at any position on the degradation pathway and, given favourable conditions, may move onto a recovery pathway at any point. However, even though a reach may sit on a recovery trajectory and show signs of recovering towards a restored or created condition, the river may not have the “potential” to recover. To assess the likelihood of geomorphic recovery for each reach of each River Style, limiting factors and pressures to recovery must be examined. Factors considered in these assessments include human-imposed constraints on water and sediment transfer, vegetation patterns (including the loading of wood), and other pressures that may constrain river recovery. Each reach must be placed within its catchment context and appraised in terms of physical linkages between reaches. In this context, the recovery potential of each reach is assessed individually, based on a combined assessment of its sensitivity to change, its geomorphic condition, its position in the catchment and the limiting factors and pressures operating upstream and downstream of it.