2.10 Hyporheic Exchange: Links to Physical and Ecological Systems
Hyporheic exchange occurs as stream water circulates into and out of the stream channel, bed, and banks, to mix with the adjacent groundwater system. Excellent overviews and specifics regarding the physical and ecological role of hyporheic systems in stream and groundwater settings are provided in many works (e.g., Winter et al., 1998; Buss et al., 2009; Boano et al., 2014; Cardenas, 2015; Ward, 2016; Woessner, 2017; Hauer and Lamberti, 2017; and Conant et al., 2019).
The hyporheic zone encompasses that portion of the groundwater system where a mixture of surface water and groundwater occur as shown in Figure 29 and Figure 30 (Woessner, 2017). Conant and others (2019) refer to the hyporheic zone as a transition zone between surface water and groundwater systems in which various biogeochemical processes occur. In addition to circulating and mixing waters, hyporheic systems also create habitat and refuge for macroinvertebrates, microbes, and fish. These aquatic ecotones are both influenced by water chemistry and stream biota (Figure 31). The circulating waters process carbon, nutrients, and solutes, while fueling ecosystem metabolism (Woessner, 2017). Hyporheic exchange is viewed by aquatic ecologists as an ecotone between groundwater and river ecosystems characterized by hydrologic, zoologic, chemical and metabolic features (e.g., Burke and Gonser, 1997; Ward, 2015; Hauer and Lamberti, 2017).
The hyporheic exchange process is enhanced in the presence of channel complexity (e.g., braided and meandering channels), bars, variations in channel topography, the distribution and magnitude of permeable bed sediments, in-channel and bank vegetation, and variations in flow regimes (e.g., Harvey and Bencala, 1993; Carling et al., 1999; Woessner, 2000; Malcolm et al., 2005; Storey et al., 2003; Buffington et al., 2004; Anderson et al., 2005; Gooseff et al., 2006; Worman et al., 2007; Cardenas and Wilson 2007abc; Greig et al., 2007; Tonina and Buffington, 2007; Cardenas, 2008ab; Arrigoni et al., 2008; Cardenas, 2009; Bean et al., 2013; Boana et al., 2014). In contrast, stream modifications that reduce channel complexity (e.g., channelizing, and damming) often result in degrading hyporheic exchange areas, locations and rates.
The hyporheic zone can form a temporary or permanent refuge and habitat for aquatic organisms including fish and invertebrates (e.g., Boulton, 2007; Datry and Larned, 2008; Stubbington et al., 2009); Buss et al., 2009; Ward, 2016) including zoobentos in various stages of life histories (e.g., Hauer and Lamberti, 2017; Lamberti and Hauer, 2017). It also cycles solutes, including nitrates and phosphorus, and organic matter between the river and hyporheic zone (e.g., Fisher et al., 1998; Boulton, 2007). Hyporheic zones can act to modify surface water chemistry and focus biological production at locations where hyporheic water is discharging back to the stream known as hot spots (e.g., Valett et al.,1990, 1994; Coleman and Dahm, 1990; Pepin and Hauer, 2002; Boulton, 2007). In some settings, contaminated groundwater discharging to a stream or contaminated surface water circulating in the hyporheic zone may be altered by processes operating in the hyporheic exchange system (e.g., Conant, 2004; Conant et al. 2019).
Hyporheic zones are studied by developing conceptual models based on the exchange literature, modeling and field data. Initial conceptual models are tested and revised after undertaking a site characterization program based on specifics of the physical setting and results of biogeochemical sampling and analyses (e.g., LaBaugh and Rosenberry, 2008; Boano et al, 2014; Buss et al., 2009; Cardenas, 2015; Woessner, 2017; Weight and Woessner, 2019).