Recognition of morphological diversity and understanding of river processes (dynamics) are essential for effective river management (Brierley and Fryirs, 2005, Gurnell et al., 2015, Rinaldi et al., 2016, Hohensinner et al., 2018). Differences between rivers are often contextualised at the reach scale (10− 1-101 km; Belletti et al., 2017) where geomorphic structure and function are approximately uniform, determined by a set of boundary conditions within which the river operates (Frissell et al., 1986; Brierley and Fryirs, 2005; Wyrick and Pasternack, 2014). Imposed boundary conditions (e.g. geology, tectonics, and climate) influence the valley setting and topography of the landscape, while flux boundary conditions such as the interaction of water discharge and sediment transport induce reach-scale variability in morphodynamics (Brierley and Fryirs, 2005). Alterations to flux boundary conditions, including human-induced pressures and disruption to water and sediment flows, may irreversibly damage fluvial systems (Rhoads, 2020). Integrating principles from hydrology, geomorphology, and ecology strengthens the potential of river management applications (Brierley and Fryirs, 2008; Brierley et al., 2019), this requires detailed information of river morphological diversity across multiple spatiotemporal scales (Gurnell et al., 2016).
A range of classification schemes have been developed to assess river morphological diversity. Generally, these schemes seek to categorise reaches by grouping similar process and form characteristics. Classification schemes vary in their approach, the environment for which they were developed, and the spatiotemporal scales over which they are applied (Kondolf et al., 2003; Buffington and Montgomery, 2013). However, Kasprak et al. (2016) demonstrate that the underlying principles and premises of such geomorphological analyses are inherently consistent, with differing approaches generating similar outputs (i.e. maps). Classification based on hydrology and river geomorphology (hydromorphology) often provides a first step in the analysis of river systems (Fuller et al., 2013), and is a fundamental starting-point when integrating interdisciplinary components of analysis (e.g. Sear et al. 1995; Gilvear 1999; Kondolf et al. 2003; Downs and Gregory 2004; Brierley and Fryirs 2005, 2008; Meitzen et al. 2013; Tadaki et al. 2014; Rinaldi et al., 2016, Dallaire et al., 2019). Many river classification schemes were designed to improve scientific understanding, rather than with an explicit focus on river management (Rinaldi et al., 2016).
Spatially-hierarchical frameworks that support river management strategies, include those to maintain ecosystem functions (Dollar et al., 2007; Beechie et al., 2010), mitigate the effects of flood hazards (Rinaldi et al., 2013; 2015) and restore degraded rivers (Beechie et al., 2010). These frameworks use a nested spatial hierarchy to organise and structure complex river systems, wherein large-scale features (i.e. a river) are subdivided into sequentially smaller features (e.g. segment to river reach to geomorphic unit/s to microhabitat subsystems; Frissell et al., 1986). The River Styles Framework developed by Brierley and Fryirs (2005), incorporates spatially-hierarchical geomorphic analyses within a catchment-based approach to river management. Knowledge of the catchment (including what is happening both upstream and downstream of a site) is essential to contextualise local adjustment (Brierley and Fryirs, 2009), especially as disturbances may occur anytime and anywhere in a catchment (Gurnell et al., 2016). The River Styles Framework provides a set of consistent and generic procedures and guidelines for river assessment that can be locally or regionally adjusted to different situations. The framework has four stages. Stage One involves identifying and characterising River Styles. Stage Two uses geomorphic principles to assess evolution, and river condition, with recovery potential assessed in Stage Three. Target conditions and priorities are set in Stage Four to realise effective river management (Brierley and Fryirs, 2005).
Rivers in the Philippines are particularly dynamic, with fluctuating sediment supply driven by monsoon and typhoon related landslides, earthquakes, and volcanoes (Gran et al., 2011; Catane et al., 2012; Gob et al., 2016; Dingle et al., 2019). Additional pressures from anthropogenic activities include: flow alteration for fishing (Fig. 1e); dam construction, artificial alignment and confinement (Fig. 1a, h, i); gravel extraction (Fig. 1f); and floodplain use for agriculture (Fig. 1d) and recreational purposes (Fig. 1b, c). A major aspect of river management is in the containment of water and sediment by engineered structures (Fig. 1h). Water bodies (including rivers) in the country are classified based on water quality standards set by the Department of Environment and Natural Resources coupled with its intended use (DENR Administrative Order 2016-08). River basin management plans exist but only for the 18 major catchments (drainage area > 3000 km2). Morphological attributes of rivers are infrequently addressed in these plans. Analyses of physiography, climate, geology, and land use lack fluvial geomorphological detail (i.e. stream network characteristics, geomorphic units, bed material, sediment, and flow regime). Consequently, gaps in hydromorphological understanding have resulted in local, reactive, and often incoherent management interventions such as misplaced dikes and river training measures that are expensive to build and maintain. Such structural interventions (e.g. Figure 1h) that impose a particular width and/or alignment on a channel not only modify water flow and sediment fluxes, they also restrict the capacity for adjustment, effectively ‘fighting’ against the prevailing river behaviour (Brierley and Fryirs, 2009). In light of the increasing magnitude and variability of river flows (Tolentino et al., 2016) and growing pressures on water supply from climate change and floodplain land use from agricultural and urban development (Eccles et al., 2019), such management responses increase problems for the future. Hence, geomorphologically-informed approaches to river management in the Philippines are critical.