In semi-arid coastal wetlands, there is limited information on how the water table responds to the artificial, anthropogenic opening of sandbars (SB) at their mouths. We hypothesize that water restrictions, combined with a prolonged closure of the SB, degrade stream quality. Conversely, opening the barrier should lead to a flushing effect, causing streams to resemble seawater in composition. We studied the Tongoy estuary in the Coquimbo Region, Chile, where six station points were surveyed over 20 months, encompassing several SB openings and closings. Water pH, turbidity and redox potential tended to decrease following SB openings, while dissolved oxygen saturation, electrical conductivity, salinity, and density increased as seawater entered the estuary. Water temperature generally followed seasonal air temperature trends, whereas dissolved oxygen concentration showed a negative relationship with temperature. Nitrate and phosphorus levels were less influenced by SB openings. Although several variables exhibited trends consistent with SB status, in the long term, they tended to align with seawater conditions. Our results suggest that SB openings drive a flushing process and a directional shift towards marine conditions. This transition from freshwater to saline environments follows a predictable cycle, with potential implications for habitat creation and loss, as well as for biological communities. When respect to ecological management, determining an "ideal" state for the Tongoy stream is not straightforward. Establishing protocols for the opening and closing of the SB is crucial and requires studies on species composition dynamics and physicochemical variables to balance human interests, nature conservation and ecosystem function.