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Estuarine salt intrusion greatly threatens freshwater supply in surrounding lands. Physical barriers, which reduce the estuary length (L), are widely constructed to control salt intrusion. Yet, the role of Lin salt intrusion remains unknown. Using a process‐based, idealized, semi‐analytical three‐dimensional model, we systematically investigate for the first time this unknown for tide‐dominated, periodically weakly stratified estuaries. Results show decreasing Lsignificantly reduces salinities for short estuaries (L< Lw/4, with Lwthe dominant tidal wavelength), but not for long estuaries. Tidal pumping remains a key salt importer in most estuaries, regardless of L. However, substantial decreases in Lrelative to Lw/4 can change the dominant landward salt importer from tidal pumping to horizontal diffusion. The latter, together with gravitational circulation, weakens responses of salt intrusion to changes in tidal and river forcing in short estuaries. This study highlights the importance of considering Lto understanding and mitigating salt intrusion. Salty seawater can enter rivers through estuaries, where rivers meet the sea, thus affecting the surrounding freshwater supply and the healthy functioning of estuarine ecosystems. To mitigate the adverse impact of salt intrusion under aggravating climate change and human activities, expensive salinity barriers are commonly built around the world. These barriers result in an abrupt change in the estuary length and can substantially affect estuarine salt intrusion. This study uses a simplified computer model to understand the dependency of salt intrusion on estuary length in estuaries with small top‐to‐bottom salinity differences. Our results show that estuary length substantially influences the salinity distribution and estuaries of different lengths can respond to changes in tides and river flow contrastingly. Our study highlights the importance of considering the impact of estuary length to effectively mitigate estuarine salt intrusion. Estuary length strongly affects tidal propagation, residual circulation, and salt intrusion in periodically weakly stratified estuariesThe dominant salt balance may change when substantially reducing estuary lengthSteady lateral shear dispersion can reduce responses of salt intrusion to forcing changes in short estuaries with weak stratification Estuary length strongly affects tidal propagation, residual circulation, and salt intrusion in periodically weakly stratified estuaries The dominant salt balance may change when substantially reducing estuary length Steady lateral shear dispersion can reduce responses of salt intrusion to forcing changes in short estuaries with weak stratification

Salt intrusion in surface waters endangers freshwater availability, influences water quality, and affects estuarine ecosystem services with high economic and social values. Salt transport and the resulting salinity distributions result from the non‐linear interactions between salt and water dynamics. Estuaries are often considered under (quasi)‐steady assumption or by focusing on specific timescales. Our understanding of their temporal multiscale response to transient forcing is limited, which hinders the implementation of effective mitigation strategies. We apply wavelet analyses to quantify the variability of salt intrusion from hourly to seasonal timescales and unravel the temporal variability of its response across scales. We focus on an estuary that undergoes significant transient forcing, the Modaomen estuary in the Pearl River Delta, and apply the wavelet analyses to year‐long data generated by a coastal ocean numerical model. Our results show that this estuary responds to changes in tidal and riverine forcing throughout the year over interwoven timescales. Our results highlight the temporal variability of the salt intrusion response time both within a given regime and for the transition between regimes. They also suggest that tides control the response time more strongly than river discharge, even though river discharge determines the magnitude of the salt intrusion, and thus modulates the evolution of the salt intrusion response time. We propose a broadly applicable framework to calculate response times with simple data. These results can provide a first‐order guidance for design and implementation of estuarine management strategies and mitigation measures that ensure water access and facilitate sustainable development. With more and more people living by coast, more water resources are needed, and the fragile equilibrium of estuaries and coastal ecosystems is at risk. In estuaries and deltas, the salty water from the ocean meets the fresh water from the river and can propagate upstream. We call this salt intrusion. One of the solutions used to control salt intrusion is releasing water to push the salt intrusion back, but for this to be effective it needs to be well timed. We use a computer model to understand how the changes in river and ocean affect how far and when the salty water reaches upstream. We show that the river determines how far it reaches but it is mostly the tide that determines when this will happen. Since the ocean moves at different scales, the salt intrusion also responds at multiple scales with different time delays, and it is difficult to distinguish one from another because they overlap. The time delays are not constant and evolve throughout the year. We need to consider the multiple overlapping scales and the evolving time response to ensure the sustainable management of estuaries. We analyze the multiscale temporal response of salt intrusion to transient river and ocean forcing during dry and wet seasonThe salt intrusion response time evolves over time throughout the year both within regimes and across regimesThis response time is mainly controlled by tidal forcing and indirectly modulated by river forcing We analyze the multiscale temporal response of salt intrusion to transient river and ocean forcing during dry and wet season The salt intrusion response time evolves over time throughout the year both within regimes and across regimes This response time is mainly controlled by tidal forcing and indirectly modulated by river forcing