No abstract

The exchange of water between the surface and subsurface environments plays a crucial role in hydrological, biogeochemical, and ecological processes. The exchange of water is driven by the local morphology of the streambed (hyporheic exchange) and the regional forcing of a large-scale hydraulic gradient, which results in losing or gaining flow conditions. We measured the effects of losing and gaining flow conditions on hyporheic exchange fluxes in a sandy streambed using a novel laboratory flume system (640 cm long and 30 cm wide) under a combination of average overlying velocities and losing/gaining fluxes. Hyporheic exchange fluxes were analyzed based on a new conceptual framework. This combination of experimental observations and modeling revealed that hyporheic exchange fluxes under losing and gaining flow conditions are similar. Because interfacial transport increases proportionally to the square of the overlying velocity and linearly with increasing fluxes of losing and gaining conditions in the sand bed, the hyporheic exchange flux becomes smaller when the losing or gaining flux increases. Thus, losing and gaining flow conditions become the dominant mechanisms of water exchange at a certain flux, which depends on the competitive interaction between the overlying velocity in the stream and the losing/ gaining fluxes.

Perennial rivers and streams make a disproportionate contribution to global carbon (C)cycling. However, the contribution of intermittent rivers and ephemeral streams, which

Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors.

Fine particle deposition and streambed clogging affect many ecological and biogeochemical processes, but little is known about the effects of groundwater flow into and out of rivers on clogging. We evaluated the effects of losing and gaining flow on the deposition of suspended kaolinite clay particles in a sand streambed and the resulting changes in rates and patterns of hyporheic exchange flux (HEF). Observations of clay deposition from the water column, clay accumulation in the streambed sediments, and water exchange with the bed demonstrated that clay deposition in the bed substantially reduced both HEF and the size of the hyporheic zone. Clay deposition and HEF were strongly coupled, leading to rapid clogging in areas of water and clay influx into the bed. Local clogging diverted exchanged water laterally, producing clay deposit layers that reduced vertical hyporheic flow and favored horizontal flow. Under gaining conditions, HEF was spatially constrained by upwelling water, which focused clay deposition in a small region on the upstream side of each bed form. Because the area of inflow into the bed was smallest under gaining conditions, local clogging required less clay mass under gaining conditions than neutral or losing conditions. These results indicate that losing and gaining flow conditions need to be considered in assessments of hyporheic exchange, fine particle dynamics in streams, and streambed clogging and restoration. Plain Language Summary Deposition and accumulation of excessive amounts of clay and silt isone of the common causes of degradation of river ecosystems. We conducted experiments to evaluate the effects of flow from the stream into the groundwater (losing stream) and from the groundwater into the stream (gaining stream) on the deposition of clay particles in a sand bed and the resulting changes in water exchange between the stream and the subsurface. We found that clay deposition substantially reduced water exchange due to clogging. Computer simulations of this process revealed that the locations of clogging are closely related to locations where water exchange occurred, but these patterns differed in losing and gaining streams. This type of clay accumulation influences water budgets in streams and reduces connectivity between streams, floodplains, and the underlying aquifers. Such a reduction in connectivity may negatively affect water resources, ecosystem functions, and river resilience. These results indicate that losing and gaining flow conditions in streams need to be considered in assessments of streambed clogging and river restoration.FOX ET AL. 4077