How Hydrologic Connectivity Regulates Water Quality in River Corridors

Abstract

Research Impact Statement: We quantify river connectivity as the balance between downstream flow and the exchange of water with the bed, banks, and floodplains of rivers, and demonstrate the impact on downstream water quality.ABSTRACT: Downstream flow in rivers is repeatedly delayed by hydrologic exchange with off-channel storage zones where biogeochemical processing occurs. We present a dimensionless metric that quantifies river connectivity as the balance between downstream flow and the exchange of water with the bed, banks, and floodplains. The degree of connectivity directly influences downstream water qualitytoo little connectivity limits the amount of river water exchanged and leads to biogeochemically inactive water storage, while too much connectivity limits the contact time with sediments for reactions to proceed. Using a metric of reaction significance based on river connectivity, we provide evidence that intermediate levels of connectivity, rather than the highest or lowest levels, are the most efficient in removing nitrogen from Northeastern United States' rivers. Intermediate connectivity balances the frequency, residence time, and contact volume with reactive sediments, which can maximize the reactive processing of dissolved contaminants and the protection of downstream water quality. Our simulations suggest denitrification dominantly occurs in riverbed hyporheic zones of streams and small rivers, whereas vertical turbulent mixing in contact with sediments dominates in mid-size to large rivers. The metrics of connectivity and reaction significance presented here can facilitate scientifically based prioritizations of river management strategies to protect the values and functions of river corridors. (KEYWORDS: hydrologic connectivity; river corridor; hyporheic flow; Clean Water Rule.) Paper No. JAWRA-18-0029-P of the Journal of the American Water Resources Association (JAWRA).

Is the Hyporheic Zone Relevant beyond the Scientific Community?

“…Current river management measures focus not only on restoring flora and fauna but also aim to improve N removal by increasing hyporheic connectivity and optimizing hyporheic residence times [133]. To maximize the reaction yield, hyporheic water must interact with reactive sediments and biofilms for a period comparable with the relevant reaction timescale(s).…”

Solute Transport and Transformation in an Intermittent, Headwater Mountain Stream with Diurnal Discharge Fluctuations

“…Discharge is often considered a master variable in river corridors because it directly controls residence times of river water, and thus, many chemical reaction processes [1,2]. However, there are a wide range of interactions between transport and transformation, including changes in the relative importance of downstream transport and hydrologic exchange flows [3,4], changes in timescales of contact with reactive storage zones [5,6], and changes in reactivity as streams wet and dry [7]. Despite the recognized role of discharge as a primary control on both transport and transformation processes individually, few studies have considered the joint responses of transport and transformation to dynamic hydrologic forcing.…”

“…Overall, we expect the relative role of storage to be maximized during periods of lowest discharge, resulting in extended transit times through the study reach. For example, during low flow conditions, we expect the contact between stream water and the streambed to be maximized due to: (1) the largest bed area per unit discharge [54][55][56]; (2) exchange flux representing the largest fraction of streamflow [4,36,57]; and (3) less light attenuation with depth in the water column, as would be expected during higher flows [58][59][60]. Intermittent flow ensures that neither water nor solutes are able to bypass the reactive streambed and hyporheic zone, eliminating the potential shunting of unreacted solutes through the study reach [61].…”

Small Ponds in Headwater Catchments Are a Dominant Influence on Regional Nutrient and Sediment Budgets

“…Conversely, small ponds can eventually reach their capacity and shift to become sources (Villarreal et al, ), or in the event of a dam removal, a pulse of nutrient‐rich fine sediment can clog the streambed (Stanley & Doyle, ), reducing the amount of denitrification and assimilation of reactive phosphorus (Hall et al, ). Future efforts could evaluate and compare small pond function with other river corridor components, such as hyporheic zones, floodplains, and wetlands, to more fully understand dominant water quality processes (Harvey et al, ; Wohl et al, ).…”