**3. Conclusions and research recommendations**

Fluvial-riparian ecosystems are hierarchically and dynamically influenced by physical and biotic processes that vary spatially among reaches, over stream order and time within the watershed, approaching but rarely achieving equilibrium in channel geometry, fluid and matter transport, biotic composition, and ecosystem energy dynamics and structure. A wide array of conceptual physical and ecological models has described aspects of FRE ecology and responses to natural and anthropogenic perturbations. However, most models have focused on single or a reduced suite of variables at site-specific, within-reach, alluvial or constrained channels, other watershed scales, and most often on anthropogenically-altered streams. The WCM emphasizes the importance of understanding temporal and spatial scaling across the entire basin in natural systems to provide guidance for improving FRE stewardship.

Despite much progress, a wide array of important ecohydrological processes, questions, and issues remain to be addressed or more fully integrated. Not presented in prioritized order, this list of additional research topics includes but is not limited to: (1) corresponding convergence toward dynamic equilibrium of physical and

biological processes; (2) extent of self-similarity among physical and biological processes across reach and stream order, space, and time; (3) groundwater-surface water interactions and connectivity under changing climates; (4) the significance, extent, and roles of groundwater and headwater springs as zero order streams in FRE ecology [120, 271]; (5) natural inter-relationships among lentic and downstream lotic reaches; (6) ephemeral stream ecosystem ecology; (7) the limiting effects of photosynthetically-active radiation in canyon-bound streams; (8) stream microclimate ecology; (9) the interactive effects of flooding, ice, and glacial effects in boreal and high elevation streams; (10) multidirectional subsidy and gene exchange in dendritic pathways; (11) the role of plant physiology in riparian vegetation zonation; (12) the significance of corridors, barriers/filter, and refugial biogeographic effects in dendritic river ecosystems [80]; (13) stream order-driven and cross-sectional spatial impacts on biodiversity; (14) population and successional models among FRE biota across trophic levels; (15) FRE ecosystem genetics and the evolution of endemism across latitude, longitude, and among tectonic landscapes; and (16) the role of noise on FRE faunal assemblages. Adequately examining and incorporating these and other topics will more fully expand the WCM model through future research and will enhance collaborative discussion among hydrogeological, ecological, and socio-cultural disciplines [272]. Such data and integration efforts will improve understanding, modeling, and stewardship of FREs at local, regional, and global spatial and temporal scales.

FREs are complex, continually changing, and vital to life on Earth. Although informative and elegant, all FRE models remain incomplete, and even the most comprehensive FRE conceptual syntheses fall short of adequately representing these remarkable, important, and dynamic ecosystems. Furthermore, discipline-based or regional specificity has often limited the applicability of some models (e.g., [273–278]). Here we present and illustrate a synthesis of FRE knowledge through the WCM, and suggest topics for further investigation. However, FREs cannot be readily, adequately, or usefully reduced to a single suite of equations or simple illustrations. For example, non-Judeo-Christian-Islamic cultures commonly view rivers as living entities, supporting divine spirits, and essential to cultural well-being. Integrating indigenous traditional ecological concepts and knowledge into improved stewardship has rarely been achieved. We suggest that improved comprehension of FREs may require consideration of other socio-cultural dimensions. Enhanced understanding of the complex, multidimensional inter-relationships among Physical, biological, cultural, and socio-economic elements and processes within watersheds is essential to improving FRE stewardship and sustaining ecological functions vital to nature, human life, and societal integrity.
