**1. Introduction**

Fluvial-riparian ecosystems (FREs) are watershed- (catchment-, drainage area-) based riverine landscape systems that integrate aquatic, riparian, and

upland domains within watersheds, linking physical, biological, and culturaleconomic processes [1–3]. From the context of a system FREs consist of "… *a structured set of objects and/or attributes… (,) components or variables …that exhibit discernible relationships with one another and operate together as a complex whole …*" ([4], 1–2; [5–7]). FREs include all sources of water that contribute to the basin's riverine ecosystem, including springs, surface runoff, lakes, and atmospheric sources such as humidity and fog. Only an average of 2120 km3 (0.0002 percent) of the world's water exists in river systems at any given time [8] (**Figure 1**). But while rivers process only a tiny fraction of the Earth's fresh water and occupy only a minute proportion of the Earth's terrestrial surface, FREs are highly productive and ecologically interactive, often supporting complex landforms and diverse, densely packed biotic assemblages that change across fine to coarse spatial and

### **Figure 1.**

*Surface hydrological cycle and fluvial-riparian landscape within the watershed. Numbers represent the percent of freshwater storage 6 (redrawn from [1]).*

*Characteristics and Process Interactions in Natural Fluvial–Riparian Ecosystems: A Synopsis… DOI: http://dx.doi.org/10.5772/intechopen.107232*

temporal scales [9, 10] and burgeoning human populations. FRE physical and biological characteristics and processes among aquatic and riparian domains step, intergrade, and may interact through reaches within the watershed, from the headwaters to the terminus in an endorheic basin or the sea, and can extend far out into the submarine environment (e.g., [11]; **Figure 1**). Physically, FREs are "*complex adaptive process–response system(s) with …morphological system(s) of channels, floodplains, hillslopes, deltas, … and cascading system(s) of …water and sediment*" [12].

Within FREs, the riparian domain is a zone of "*transition between the aquatic ecosystem and the adjacent terrestrial ecosystem* [13]. Riparian zones function as filters that reduce the impacts of flooding and surface runoff, as habitats that support vegetation, fish, and wildlife populations and habitat, and often provide critically important ecosystem goods and services [13–19]. Elevated FRE biodiversity is linked to, and influenced by factors including tectonics, geology, climate, hydrology, geomorphology, and latitude, in the context of shifting habitat mosaic heterogeneity and ecotonal dynamics [20–23]. Human reliance on FREs, and our species' evolutionary history and modern demography clearly demonstrate that reliance. As human domination of the Earth has progressed, rivers have been subjected to a host of anthropogenic alterations, including resource extraction, groundwater withdrawal, flow diversion and regulation, water quality degradation, and introduction of non-native species. The natural dimensions and human impacts on FREs have stimulated deep interest, concern, and much basic and applied research, generating a vast literature and prompting development of a suite of interrelated, but not necessarily integrated ecohydrogeological models. Focus on particular aspects of FRE channel development, geomorphology, ecology, or sociology has sometimes diminished wholistic integration. Also, graphic representation of FRE ecology can be improved to enhance conceptualization, and improve educational outreach.

Here, we provide an overview description and illustrated summary of the watershed-continuum model (WCM) [1], which couples interdisciplinary physical and ecological conceptualization of FRE ecology. The WCM links conceptual models of fluvial spatio-temporal development and geomorphology across stream order and reaches FRE ecology, trophic energy and matter dynamics, biodiversity, and evolutionary interactions from the river's source to its mouth. We provide a chronological analysis of major concepts in FRE ecohydrogeology (**Table 1**) and illustrate the WCM with an improved spatio-temporal, reach-linked conceptual diagram that integrates "bottom-up" physical factors, including geology, hydrology, geochemistry, geomorphology, sedimentology, and fluvial climate, with aquatic and riparian biotic assemblages and ecosystem structure within the watershed, and the potential for trophic cascade effects [26, 27, 36, 37, 99–102]. Due to the brevity of this manuscript, we emphasize here integration of physical and ecological conceptual elements and processes in natural, unmanipulated FREs, recognizing that such an integration is needed as a basis to improve watershed stewardship.

We reserve more detailed discussions on the details of riparian and aquatic community ecology related to the WCM for subsequent summaries [1] but focus on Integration and clear depiction of FRE domain interactions among reaches and over time within the basin. We discuss understudied issues and opportunities, the resolution of which will help advance FRE ecology in the future. Such an objective is essential for sustainable management of rivers.


*Characteristics and Process Interactions in Natural Fluvial–Riparian Ecosystems: A Synopsis… DOI: http://dx.doi.org/10.5772/intechopen.107232*


### **Table 1.**

*Concepts in physical and biological FRE ecology, presented in approximate chronological order of publication. Rows in gray indicate concepts that are primarily focused on physical processes, whereas unshaded rows indicate concepts that are more strongly focused on eco-biological issues.*
