Variability of Water Exchange in the Hyporheic Zone of a Lowland River in Poland Based on Gradientometric Studies

Abstract The Moszczenica is a lowland river, which is a third-order river with a length of 55 km; it flows into the Bzura River in central Poland. The objective of this study was to evaluate two important factors in the exchange between surface water and groundwater in the hyporheic zone: a considerable change in water flow conditions and various origins of riverbed sections, natural and artificial. To identify the spatial variation of the hydraulic gradient in the hyporheic zone of the river, a gradientmeter was applied. The measurements show that at low water stages, upwelling was dominant, with an evidently inactive zone, whereas downwelling was inconsiderable. However, the morphology of the riverbed changed during the flood flow, and downwelling clearly dominated. Upwelling zones retained their activity despite a major change in hydrological conditions. Present studies on the artificially dug Moszczenica Canal have documented outflow of water from an artificial medieval canal to a naturally formed drainage base. This means that despite the passage of hundreds of years, the natural drainage base of the Moszczenica River is still active. Studies have demonstrated the applicability of the gradientmeter for evaluating the interaction between surface and groundwaters in the hyporheic zone.