2.9 Exchange at the Channel Scale

The conceptual models presented in Figures 15 through 21 suggest that groundwater can enter or leave a stream channel though its bed and banks at multiple locations along a channel (Figure 28). At the channel scale, in addition to the groundwater exchanges previously described as effluent, influent, flow-through and zero-exchange, river water also circulates into the bed and bank sediments and associated floodplain (Figure 29). When head differences between the stream stage and head in the bed, banks and floodplain contrast, stream water can flow into shallow groundwater and circulate back into the stream as described above. Stream water that leaves the stream channel bed and banks and then returns to the channel at a downstream location is defined as hyporheic water (Figure 30). Stohedahl et al. (2013) view this hyporheic exchange as driven by changes in channel geomorphology (e.g., meander- and bar-driven); variations in the channel profile (i.e., pool- and riffle-driven), and by the heterogeneous bed surface (bottom-driven). Channel segments can be dominated by hyporheic exchange, groundwater exchange or both depending on the surface water and groundwater hydraulics operating in the segment. Exchange investigations at the channel scale are mostly focused on hyporheic exchange and areas of groundwater discharge (Figure 30).

Researchers also recognize that the extent and magnitude of hyporheic exchange can be physically difficult to document. Measuring and mapping exchanges in stream channels increases in complexity as study sites become smaller (e.g., Woessner, 2000). Often studies rely on in-channel instruments, floodplain monitoring well networks, chemical analyses of stream water and hyporheic water, along with numerical modeling of groundwater and surface-water dynamics. Instrumentation typically includes mini-piezometers, seepage meters, and temperature monitoring and modeling (e.g., LaBaugh and Rosenberry, 2008; Woessner, 2017; Weight and Woessner, 2019). Differences between surface water and regional groundwater, including temperature, chemistry, natural and environmental isotopes, and radon 222, are often used to identify the hyporheic waters illustrated in Figure 30 (e.g., Healy et al., 2007; Boana et al., 2014) and discussed in Section 5 of this book. In some cases, transition zones occur where discharging regional groundwater appears to fully or partially mix with infiltrating stream water. When groundwater discharge to the channel occurs at a sufficiently high rate, all water in the channel bed and bank sediments may be dominated by the groundwater chemistry thus signaling the absence of hyporheic flow (e.g., Cardenas and Wilson, 2006). However, in local losing-channel settings (e.g., riffles), bed and bank water and adjacent floodplain water will be dominated by river water characteristics (Figure 30). Simulations of hyporheic flow have been used to assess the likely extent of hyporheic zones and subsurface flow complexity (e.g., Woessner, 2000; Cardenas and Wilson, 2006; Tonina and Buffington, 2007; Boano et al., 2014).