Functional Process Zones and the River Continuum Concept by

Research conducted over the last three decades has demonstrated that the geomorphology and ecology of a river system are inextricably linked. Fundamentally, the physical variables used to describe a river network control the ecology of that network as well. One of the goals of river and stream ecology is to understand both structure and function of a river network. In order to do so, classification of both the physical and biotic variables that control ecological structure and function must be accomplished. If river networks are classified successfully, predictions about ecological patterns may be made. This chapter discusses the recent riverine ecosystem synthesis by Thorp et al. (2006), its proposed conceptual model for river ecosystem classification, and how this new tool can be used in terms of the classic predictive model, the River Continuum Concept. INTRODUCTION & BACKGROUND Over the past three decades, there have been many attempts to understand and describe the ecological complexity of rivers and streams, as well as identify patterns in riverine networks. Numerous conceptual models have been proposed, some complementary to previous and contemporary work. Other conceptual models contradicted previous work as the state of understanding for these complex networks evolved rapidly during that time. Thorp et al. (2006) have recently proposed a new conceptual framework in the riverine ecosystem synthesis that encapsulates work conducted since Vannote et al.’s (1980) seminal paper on river ecology. Their framework, intended to provide insight into both longitudinal and lateral patterns along river networks and ecological patterns across various temporal and spatial scales, draws heavily upon ecogeomorphology (Thoms and Parsons 2002), an emerging field of river research that emphasizes geomorphological impacts on fluvial ecology (Thorp et al. 2006). E.L. Hestir April 30, 2007 Page 2 of 13 The River Continuum Concept First presented in 1980, the River Continuum Concept (RCC) proposed by Vannote et al. has dominated approaches to river and stream ecological research. This concept was predicated in large part on earlier work by Vannote recognizing that structural and functional characteristics of stream communities are controlled by the physical state of the stream. The River Continuum Concept posits that there is a continuous gradient of physical conditions from the headwaters to the mouth of any river. The conditions listed include width, depth, velocity, flow volume, and entropy gain. The RCC predicts that these physical gradients regulate biotic processes within the river, specifically nutrient dynamics. Although many exceptions to the RCC have been published since its original presentation (e.g. gradients are not generally viewed as continuous), the overarching concept of longitudinal change along a river continues to dominate alternative hypotheses to the RCC. Furthermore, this concept seems to adequately describe observations made in the field. The ultimate goal of this project is to understand the Wallowa & Grande Ronde Rivers, Oregon, USA, in terms of the River Continuum Concept by testing whether any of the predictions made by Vannote et al. are realized through simple data collection procedures and analysis as we move from the headwaters to the mouth of the Wallowa and Grande Ronde Rivers. Classification of Rivers and Streams A fundamental goal of river and stream ecology is to understand both physical and ecological structure and function of a river network. However, this is challenging as these networks are open systems with high temporal and spatial variability in their physical structure (Thorp et al. 2006). To further this goal, Hawkes (1975) attempted to divide river ecosystems into discrete, longitudinally ordered zones. In 1980 Vannote et al. challenged Hawkes’ zoning scheme when they presented the River Continuum Concept, which conceptualized river ecosystems as continua rather than separate, discrete zones. The conceptual model predicted that these ecological continua are strategically adapted to longitudinal (headwaters to mouth) energy efficiency. E.L. Hestir April 30, 2007 Page 3 of 13 The River Continuum Concept Considered one of the most influential papers of the twentieth century, Vannote et al.’s model is still widely accepted for headwaters through medium rivers. The RCC makes two predictions: 1) a continuous gradient of physical conditions observable from the headwaters to a river’s mouth; and 2) this longitudinal gradient of physical conditions will control biotic responses. Hence a continuum of organic matter loading, transport, utilization, and storage should also be observed along the river (Vannote et al. 1980, Thorp et al 2006). This model of longitudinally continuous ecosystems contrasted with earlier conceptualizations of isolated zones predictably distributed along the longitudinal dimension of a river (i.e. Hawkes 1975). Figure 1. Vannote et al.’s longitudinal relationship between stream size and ecological structure and function (reproduced from Vannote et al. 1980) Many predictions stemming from the RCC have been criticized. The concept of ecological continua is not realized when applied to real rivers. In terms of stream hydraulics, E.L. Hestir April 30, 2007 Page 4 of 13 instead of observable gradients, there are often abrupt discontinuities in water flow that result in abrupt changes in substrate size. This in turn results in abrupt changes in species assemblages (Statzner and Higler 1986). Perry and Schaeffer (1987) did not observe the predicted continuum in species assemblages; they were able to demonstrate only a minor downstream gradient in bottom-dwelling river species. They characterized species distributions in a river as punctuated gradients, rather than ecological continua. Benda et al. (2004) identified tributary junctions at biological hotspots in the network dynamics hypothesis (NDH), further refuting the concept of an ecological continuum, and highlighting the fact that rivers are better viewed as networks. Junk et al. 1989, stated that Vannote et al.’s concept that downstream foodwebs were highly dependent on organic matter leakage from upstream (see Fig. 1) was contradicted by the flood pulse concept in lateral floodplains. Thorp and Delong 1994, 2002, criticized the RCC because it did not take into account autochthonous production (within system productivity). In an attempt to reconcile the RCC with subsequent river research, Thorp et al. acknowledged these criticisms of the RCC, and highlighted work done by Townsend (1989), Poole (2002), and Montgomery (1999), that proposed alternate explanations of patterns in river networks. Rather than being continuous gradients of energy resources, sensu Vannote et al., rivers are composed of patchy discontinuities in which communities are more likely to respond to local landscape features rather than any sort of longitudinal gradient. That is, an ecological community within a stream segment may be as equally differentiated from neighboring communities as from up or downstream communities, based on local processes (Poole, 2002). This presented the concept of ecological patchiness within a river as an alternate to ecological continua in river ecology. Montgomery (1999) concluded that the RCC was valid only for low-relief watersheds with relatively constant climate and simple geology, descriptors that are decidedly not applicable to the Wallowa and Grande Ronde Rivers. An alternate concept known as “process domains” was proposed. This concept is centered on the importance of local geomorphic conditions and landscape-scale disturbances. Furthermore, it can be applied to regions of high relief, variable climates, and complex geology, such as the Grande Ronde River basin. Montgomery’s process domains focus on the spatial variability in geomorphic process that governs temporal patterns of disturbances that, in turn, influence ecosystems (Montgomery 1999). E.L. Hestir April 30, 2007 Page 5 of 13 Figure 2: An example of coarse-scale (landscape-scale) riverine process domains for Pacific Northwest drainage basins (reproduced from Montgomery 1999). HYDROGEOMORPHIC PATCHES & FUNCTIONAL PROCESS ZONES In spite of the criticisms of the River Continuum Concept, it did, and to a large extent still does, inform many empirical observations of ecological patterns in river networks. This is because it is one of the few conceptual models that provide predictive power. River ecologists continue to ask the question, “How predictable are rivers’ ecological communities along longitudinal dimensions of river networks? ” Thorp et al. caution that this answer is complex due to dependency on scale. Nevertheless, it should be answerable. In support of this declaration, they cited Thoms and Parsons (2003), who used 230 hydrological and geomorphic variables to describe rivers. Their study supported the concept of spatial zonation along longitudinal gradients, at least in terms of hydrological characteristics. This implies that patterns along the longitudinal dimension of a river network, although not continuous, do exist. They should be observable, and perhaps the RCC should not be disregarded in its entirety. E.L. Hestir April 30, 2007 Page 6 of 13 Hydrogeomorphic Patches Thorp et al. propose an alternative to the concept of continuous, longitudinal gradients of physical conditions. Rivers can instead be viewed as downstream “arrays” or networks (Benda et al. 2004) of large “hydrogeomorphic patches” formed by catchment-scale geomorphology and flow (Thorp et al. 2006). These patches are defined by shifts in hydrological and geomorphic conditions. These physical boundaries (shifts) may be distinct, or indistinguishable by field observation, but can be delineated using standard geomorphological techniques and terminology (Thorp et al. 2006). For example, an area of river with a constricted flow channel would be considered a hydrogeomorphic patch, a