This paper demonstrates that the multivariate monitoring methods are capable to underpin the systematic investigation of thehysteretic behaviour occurring during gradually-varied flows. For this purpose, we present simultaneous measurements of stage,index velocity and free-surface slope acquired continuously with high-frequency sampling instruments deployed at several rivergaging sites exposed to different storm magnitudes. The experimental evidence reveals intrinsic features of unsteady open-channel flow mechanics that are hinted by pertinent governing equations but rarely substantiated with in situ measurements.The illustrations are intentionally made for fluvial waves propagating in lowland rivers where the relationships among flowvariables are most likely displaying hysteretic phasing in the progression of the hydraulic variables and loops in their relation-ships. The presented measurements highlight that: (a) the hysteretic behaviour is apparent in both time-independent and time-dependent graphical representations of any two of the hydraulic variables; (b) the severity of the hysteresis is commensurate withthe geomorphic, hydraulic and hydrological characteristics of the measurement site; and (c) there are flow monitoring paradigmsthat can more accurately track changes of the flow variables during gradually-varied flows than those currently used in practice.Also discussed are research needs for advancing the understanding of the mechanisms underlying the movement and storageof water in the lowland river environments as well as for increasing the accuracy of streamflow monitoring, modelling andforecasting.1 | IntroductionThe highly dynamic conditions associated with repeated wet-dry cycles in lowland and plain rivers lead to gradual movementand/or storage of water in the channel network that inher-ently produce hysteresis (e.g., Henderson 1966; Fenton 2001).Hysteresis is a property of a physical system to produce differentprocess outcomes depending on the trajectory of the changesThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided theoriginal work is properly cited and is not used for commercial purposes.© 2025 The Author(s). Hydrological Processes published by John Wiley & Sons Ltd.