2.7  Exchange at the Valley Segment/River Corridor Scale

William W. Woessner

2.7 Exchange at the Valley Segment/River Corridor Scale

The valley-segment scale includes exchange within the entire valley corridor over a few to 10’s of kilometers of stream channels and associated riparian zones, active and inactive floodplains, and older river terraces. Woessner (2000) described this multiple featured area as the fluvial plain (Figure 24). Except for bedrock lined channel settings, generally, the fluvial-plain sediments include deposits of anisotropic and heterogeneous materials.

Figure showing valley segment/river corridor and the fluvial plain — **Figure 24 –** Valley segment/river corridor and the fluvial plain. a) The river corridor includes the channel and surrounding lowlands (black rectangle) and represents kilometers of stream channel length. b) The landscape making up the river corridor is referred to as the fluvial plain. It is usually composed of unconsolidated fluvial sediments and includes the river channel, present floodplain, and older river terraces. It is bounded by the adjacent uplands that can be composed of unconsolidated or consolidated earth materials (gray) (modified from USEPA, 2019).

Exchange at this scale is most commonly quantified as water-balance-computed changes in streamflow between two stream gauging stations or sites. In the simplest form, an upgradient and downgradient stream gauging site are selected, the discharge is determined at both sites and a water budget is computed. The results determine if the river corridor section is gaining, losing or showing no change in flow (Figure 25). Multiple types of stream channel exchanges may be occurring over the selected river segments; however, the streamflow analyses will yield the net change for a given segment without identifying if exchange processes in some sections of the segment differ from the net exchange. Water budgets are more complex when additional sources or losses of water occur within the study section (Figure 26). It is necessary to quantify flow measurement errors to determine if measured flow differences are significant (e.g., Healy et al., 2007) as discussed in Section 5 of this book.

**Figure 25 –** Map views of a valley segment and two stream gauging locations, Q1 upstream and Q2 downstream (triangles). a) A gaining stream corridor where the measured upstream flow rate, Q1, is less than the downstream flow rate, Q2 (indicated by the gray-blue channel). b) A losing stream corridor where the measured upstream flow rate, Q1, is greater than the downstream flow rate, Q2 (indicated by the red stream channel). c) An example of a gaining stream corridor in which Q1<Q2; however, exchange within the segment is complex (Woessner, 2020).

Figure showing a 3 km stream corridor that has an inflow of tributary water and a loss of water by an irrigation canal diversion — **Figure 26 –** Example of a 3 km stream corridor that has an inflow of tributary water at gauge 2, and a loss of water by an irrigation canal diversion at gauge 3. The dominant exchange process is computed by the equation shown in the figure. This assumes that no other significant inflows and outflows occur over the time period when flow is measured (e.g., evaporation, transpiration). The photo image is from Google Earth in 2015 (Woessner, 2020).

Characterizing exchange at the valley-segment/river-corridor scale lumps conditions into the two streamflow discharge measurements. Exchange may vary as streamflow changes over time. If river corridors are dominated by losing streamflow, stream water recharge will enter the surrounding fluvial and upland geologic materials. Some of this water may be recirculated in the fluvial plain and discharge back to the stream channel in a downstream section of the stream. In other settings, water may enter a larger regional groundwater system and not return to the river corridor.

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