Chapter 13 Stream and Riparian Ecology
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Chapter 3 Stream and Riparian Ecology John S. Richardson and R.D. (Dan) Moore INTRODUCTION Water is a valuable resource for humans (direct con- Geomorphology: Fluvial Forms, Processes, and sumption, power, irrigation, industry) and provides Forest Management Effects”). essential habitat for many organisms, including The purpose of this chapter is to provide an highly valued fish species such as salmonids. Aquatic overview of stream and riparian ecology in Brit- habitat is influenced by processes active not only ish Columbia. It describes the ecological structure in the near-stream (riparian) zone (e.g., provision and function of streams and the associated riparian of shade) but also over the entire watershed (e.g., zones, and the interactions between them. Several hillslope hydrologic processes that control the supply topics in this chapter are integrative; therefore, some of water or the generation of landslides). Processes at topics are addressed only briefly but references are both of these scales, and others, can be profoundly provided for more in-depth coverage. For additional influenced by forestry activities, as described in information on stream and riparian ecology, see more detail in Chapter 7 (“The Effects of Forest Giller and Malmqvist (998), Naiman and Bilby (edi- Disturbance on Hydrologic Processes and Watershed tors, 998), Naiman et al. (2005), Allan and Castillo Response”), Chapter 9 (“Forest Management Effects (2007), and Richardson and Danehy (2007). on Hillslope Processes”), and Chapter 0 (“Channel STREAMS Definitions hence, the term stream is used to mean all running waters. Stream channels include the active chan- The word stream”“ is a generic term used to de- nel (or open channel), as well as relict channels that scribe watercourses of all sizes that have channels of may be disconnected from the active channel or running water and which show evidence of fluvial are connected only at high flow. The active chan- processes, even if it is only the erosion of organic nel is characterized by open water interacting with materials during peak runoff events. The term its bed materials (mineral substrate, wood). Below “river” is generally used to describe a large stream and adjacent to the active channel is the hyporheic (e.g., Richardson and Milner 2005). No specific scale zone—a zone of saturated sediments where infiltra- distinguishes a river from a stream, creek, or brook; tion of stream water via the streambed and banks 44 has a strong influence on water quality. It is typically stream in the central interior of British Columbia. the interface between stream water and groundwater The use of various terms to define streams can and, as such, is often characterized as an ecotone cause confusion. For example, “headwaters” is spanning the boundaries of the surface and subsur- sometimes used to refer to the sources of any stream, face environments. The riparian zone is the terrestri- regardless of its size (Gomi et al. 2002; Moore and al area adjacent to the stream. It is influenced by the Richardson 2003). In this chapter, headwater streams water in the stream and (or) has an influence on the are defined as having no perennially flowing tribu- aquatic system (see below). A more comprehensive taries (Gomi et al. 2002; Moore and Richardson discussion of the classification of streams, channel 2003; Richardson and Danehy 2007). This narrow types, and morphology, and the processes that shape definition distinguishes headwaters from networks stream environments is provided in Chapter 0 of streams (Gomi et al. 2002), placing the focus (“Channel Geomorphology: Fluvial Forms, Process- instead on the different processes governing head- es, and Forest Management Effects”). waters versus non-headwaters and on the differ- The duration of surface flow throughout the year ent management strategies available for these two in a given stream will vary from year to year, de- channel types (see Gomi et al. 2002; Richardson and pending on weather patterns. It will also vary among Danehy 2007). streams, and along streams, with smaller streams more likely to have lower flow duration than bigger Influence of Channel Type on Habitat streams. In perennial streams, flow over the stream- bed is usually maintained throughout the year, Channel components such as pools, riffles, steps, although the entire active channel does not have to cascades, and plane beds provide important habitat be inundated (Feminella 996). In general, peren- for many organisms (Chapter 0, “Channel Geo- nial streams are considered distinct from intermit- morphology: Fluvial Forms, Processes, and Forest tent and ephemeral streams along a continuum of Management Effects”). Variations in the hydraulic flow duration (Feminella 996; Halwas et al. 2005). characteristics of different reaches and channel Intermittent and ephemeral channels differ in the sub-units create distinctive habitat characteristics duration of surface flow. Intermittent channels carry that influence the spatial distributions of aquatic water through an extended portion of the year and organisms (Hawkins et al. 993). For example, riffles, may support populations of some benthic inverte- a common channel feature, are characterized by brates with adaptations to this environment. Ephem- relatively high flow velocities. This channel feature is eral channels show evidence of fluvial processes but favoured habitat for a diversity of aquatic organisms have flows only during and shortly after precipitation that filter fine particles of organic matter or prey events (Halwas et al. 2005). from the water column and that require sufficient During extended periods of low flow, the water flow to bring these materials to them. Larval black- table may drop below the streambed because of a flies and net-spinning caddisflies, for example, can combination of reduced runoff generation from filter large masses of particles from passing water upslope areas and transpiration by riparian vegeta- (Wallace and Merritt 980), removing this load of tion. Under these conditions, sections of streams organic material from water supplies and thus pro- dry up wherever stream discharge is insufficient to viding an important ecosystem service. Fish often maintain continuous surface flow and to satisfy wa- require particular channel features during different ter losses through the streambed and banks. Stream life cycle stages. Adult salmonids (Pacific salmon, drying may occur frequently in the upper portions trout, char) generally use riffles for spawning redds, of the channel network, which can interrupt con- whereas juvenile salmonids usually inhabit pools. nectivity (Hunter et al. 2005). For example, Story et Within these pools, young salmon take advantage of al. (2003) found that dewatering of an intermediate cover behind boulders or logs, finding refuge from segment of stream channel effectively decoupled a the full hydraulic force of the channel flow but still lower reach from the warming effects of harvesting having ready access to prey carried along in the cur- and road construction on an upper reach of a small rent (i.e., “drift”). 442 HYPORHEIC ZONES Hydrology important for salmon redds, where the through-flow of surface water maintains high dissolved oxygen. Hyporheic exchange flow refers to the transfer of This topic is covered in more detail in Chapter 4 stream water through the streambed and banks (“Salmonids and the Hydrologic and Geomorphic (recharge), the down-valley transport of this water Features of Their Spawning Streams in British Co- within the saturated zone surrounding the stream lumbia”). channel, and the subsequent discharge from the sub- Hyporheic exchange flow is controlled primarily surface back into the channel (Figure 3.). Hyporhe- by the morphology of the valley floor and stream ic exchange flow occurs in response to variations in channel (Harvey and Bencala 993; Wroblicky et al. hydraulic potential along the streambed and banks 998; Kasahara and Wondzell 2003; Gooseff et al. and takes place at a range of spatial and temporal 2006; Wondzell 2006). In steep headwater streams, scales. For example, at a small scale, water pressure hyporheic exchange flow can be controlled by is often higher on the upstream side of bedforms boulder or log steps in the longitudinal profile of the than on the downstream side; the resulting hydrau- stream (Kasahara and Wondzell 2003; Anderson et lic gradient induces infiltration of stream water on al. 2005). The volume of sediment stored above these the upstream side of the bedform and discharge on steps largely determines the extent of the hyporheic the downstream side. This type of exchange flow is zone. Downwelling stream water seeps into the FIGURE 3. A stream reach showing many of the elements and processes that link streams and riparian areas (E. Leinberger, University of British Columbia). 443 streambed upstream of the steps and returns to the zone. Because of the contributions of channel and stream channel below the steps, where it upwells hillslope water, hyporheic zones are biogeochemi- through the streambed (Harvey and Bencala 993; cally distinct from groundwater. For example, the Moore et al. 2005b; Scordo and Moore 2009). hyporheic zone tends to have enhanced dissolved In streams with riffle-pool morphology, channel oxygen, particularly near the recharge zones where water tends to flow into the streambed and banks at oxygenated channel water infiltrates the stream- the riffle heads and discharge back into the stream bed and banks (Ward et al. 998). The contrasts in the downstream pools (Harvey and Bencala in biogeochemistry between groundwater and the 993; Kasahara and Wondzell 2003; Anderson et hyporheic zone are particularly important in rela- al. 2005). In larger, unconstrained streams with tion to nitrogen processing. Information on nitro- relatively wide and flat floodplains, exchange flows gen cycling in terrestrial and aquatic ecosystems is occur through mid-channel bars. Stream water flows provided in Chapter 2 (“Water Quality and Forest into the bars through the streambed and bank on Management”). the upstream side, and returns to the stream chan- The hyporheic zone is often thermally interme- nel on the downstream side (Vervier and Naiman diate between surface water and groundwater.